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• W2789167483 abstract "In 2002, the Kidney Disease Outcomes Quality Initiative guidelines for identifying and treating CKD recommended that clinical laboratories report estimated glomerular filtration rate (eGFR) with every creatinine result to assist clinical practitioners to identify people with early-stage CKD. At that time, the original Modification of Diet in Renal Disease (MDRD) Study equation based on serum creatinine measurements was recommended for calculating eGFR. Because the MDRD Study equation was developed using a nonstandardized creatinine method, a Laboratory Working Group of the National Kidney Disease Education program was formed and implemented standardized calibration traceability for all creatinine methods from global manufacturers by approximately 2010. A modified MDRD Study equation for use with standardized creatinine was developed. The Chronic Kidney Disease Epidemiology Collaboration developed a new equation in 2009 that was more accurate than the MDRD Study equation at values above 60 mL/min/1.73 m2. As of 2017, reporting eGFR with creatinine is almost universal in many countries. A reference system for cystatin C became available in 2010, and manufacturers are in the process to standardize cystatin C assays. Equations for eGFR based on standardized cystatin C alone and with creatinine are now available from the Chronic Kidney Disease Epidemiology Collaboration and other groups. In 2002, the Kidney Disease Outcomes Quality Initiative guidelines for identifying and treating CKD recommended that clinical laboratories report estimated glomerular filtration rate (eGFR) with every creatinine result to assist clinical practitioners to identify people with early-stage CKD. At that time, the original Modification of Diet in Renal Disease (MDRD) Study equation based on serum creatinine measurements was recommended for calculating eGFR. Because the MDRD Study equation was developed using a nonstandardized creatinine method, a Laboratory Working Group of the National Kidney Disease Education program was formed and implemented standardized calibration traceability for all creatinine methods from global manufacturers by approximately 2010. A modified MDRD Study equation for use with standardized creatinine was developed. The Chronic Kidney Disease Epidemiology Collaboration developed a new equation in 2009 that was more accurate than the MDRD Study equation at values above 60 mL/min/1.73 m2. As of 2017, reporting eGFR with creatinine is almost universal in many countries. A reference system for cystatin C became available in 2010, and manufacturers are in the process to standardize cystatin C assays. Equations for eGFR based on standardized cystatin C alone and with creatinine are now available from the Chronic Kidney Disease Epidemiology Collaboration and other groups. Clinical Summary•Glomerular filtration rate (GFR) is the most useful overall measure of kidney function and an important tool for monitoring patients with CKD.•GFR estimating equations have developed over time, and an equation based on creatinine and/or cystatin C measurements with calibration traceable to the international reference system should be used.•Laboratory reporting of estimated GFR is almost universal for adults in many countries. •Glomerular filtration rate (GFR) is the most useful overall measure of kidney function and an important tool for monitoring patients with CKD.•GFR estimating equations have developed over time, and an equation based on creatinine and/or cystatin C measurements with calibration traceable to the international reference system should be used.•Laboratory reporting of estimated GFR is almost universal for adults in many countries. Glomerular filtration rate (GFR) is considered the most useful overall measure of kidney function and forms part of the criteria for diagnosis and classification of kidney disease as well as playing an important role in drug dosing decisions for many therapeutic agents.1KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease.Kidney Int Suppl. 2013; 3: 1-150Abstract Full Text Full Text PDF Scopus (1594) Google Scholar, 2Matzke G.R. Aronoff G.R. Atkinson Jr., A.J. et al.Drug dosing consideration in patients with acute and chronic kidney disease-a clinical update from Kidney Disease: Improving Global Outcomes (KDIGO).Kidney Int. 2011; 80: 1122-1137Abstract Full Text Full Text PDF PubMed Scopus (302) Google Scholar Direct measurement of GFR is performed using exogenous filtration markers such as inulin, iothalamate, iohexol, and others and is considered the reference method both for research into other ways of estimating GFR as well as for patient care when GFR estimation may be unreliable. Note that there are differences in measured GFR values depending on the procedure used, and these differences contribute to the uncertainty of estimated glomerular filtration rate (eGFR) calculated using any equation.3Seegmiller J.C. Burns B.E. Schinstock C.A. et al.Discordance between iothalamate and iohexol urinary clearances.et al.Am J Kidney Dis. 2016; 67: 49-55Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar, 4Soveri I. Berg U.B. Bjork J. et al.Measuring GFR: a systematic review.Am J Kidney Dis. 2014; 64: 411-424Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar Because measurement of GFR is a complex task, a range of surrogate estimates have been developed over time. Blood, serum, or plasma creatinine is the most commonly used laboratory test for the assessment of kidney function because the assay is inexpensive, available in all routine chemistry laboratories and frequently included in biochemical profiles ordered for patients with a wide range of presenting conditions. Creatinine measurements, and calculations including the creatinine results, are useful to identify people with kidney disease, to assess the severity of the disease, and to monitor progression of the disease. The concentration of creatinine in the circulation is a balance of production and removal. When using creatinine as a tool to assess GFR, a major limitation relates to its production from muscle metabolism that varies with the muscle mass of an individual and can lead to significant differences in blood creatinine concentration for the same approximate kidney function. In order to assess creatinine filtration, rather than creatinine concentration, the initial approach was to measure the creatinine clearance which is calculated using the basic formula for clearance (urine concentration × urine volume × collection time/serum concentration) and typically used a 24-hour urine collection and a concurrent serum sample. Experience showed that collecting a 24-hour urine specimen was a challenge for patients and the variable amount of urine missed during the collection interval, or overcollections due to misunderstanding the process, contributed to uncertainty in interpreting a measured creatinine clearance. Furthermore, due to tubular secretion, creatinine is cleared in the kidneys at a higher rate than the GFR, which can be seen when creatinine clearance values are compared with measured GFR.5Levey A.S. Bosch J.P. Lewis J.B. Greene T. Rogers N. Roth D. Modification of Diet in Renal Disease Study GroupA more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation.Ann Intern Med. 1999; 130: 461-470Crossref PubMed Scopus (13028) Google Scholar Despite these limitations, creatinine clearance is relatively easy to perform and was one of the main tests of kidney function for many decades. To overcome the need for a 24-hour urine collection in routine patient care, prediction equations were developed to estimate the creatinine clearance from only a blood creatinine measurement and other indicators of muscle mass and/or body size. One of the most prevalent estimating equations for creatinine clearance in adults was published by Cockcroft and Gault.6Cockcroft D.W. Gault M.H. Prediction of creatinine clearance from serum creatinine.Nephron. 1976; 16: 31-41Crossref PubMed Scopus (13125) Google Scholar Despite the equation being based on results for only 249 males and an arbitrary adjustment for the average lower muscle mass of women, it has been widely adopted and remains used to this day particularly for adjusting the dose of drugs that have nephrotoxic side effects or have narrow therapeutic/toxic concentration intervals and are cleared by the kidneys. The Cockcroft–Gault equation was developed, and also assessed for validity, at a time when creatinine assays were not standardized. The introduction of accurate serum creatinine assays, which generally give lower results than older, nonstandardized assays, has meant that results for the Cockcroft–Gault equation today are higher than in earlier times, producing a greater overestimation relative to actual GFR. There is no revised version of the equation for use with standardized creatinine assays (see below). The US National Kidney Foundation recommends against using the Cockcroft–Gault estimate of creatinine clearance for drug dose adjustment and recommends an equation that estimates GFR from creatinine methods with calibration traceable to an isotope dilution mass spectrometry (IDMS) reference measurement procedure (RMP).7National Kidney FoundationWhy isn't the Cockcroft-Gault formula recommended for clinical use? Available at:.https://www.kidney.org/professionals/KDOQI/gfr_calculatorCocDate accessed: April 27, 2017Google Scholar Equations to estimate GFR, rather than creatinine clearance, from blood creatinine have been developed for both adults and children. The Modification of Diet in Renal Disease (MDRD) Study equation for adults was published in 1999 and provided a reasonably accurate eGFR compared to measured GFR.5Levey A.S. Bosch J.P. Lewis J.B. Greene T. Rogers N. Roth D. Modification of Diet in Renal Disease Study GroupA more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation.Ann Intern Med. 1999; 130: 461-470Crossref PubMed Scopus (13028) Google Scholar The original 6-variable MDRD Study equation enabled eGFR to be calculated from blood creatinine, urea and albumin, plus age, gender, and whether a person was of Caucasian or African-American descent without requiring information such as the height, weight, or body surface area of an individual. An important advance introduced with the MDRD Study equation was an estimate of GFR rather than of creatinine clearance, as well as a result normalized for body surface area, with results in mL/min/1.73 m2. A 4-variable MDRD Study equation that eliminated blood urea and albumin was later shown to be as accurate as the original 6-variable equation.8Levey A.S. Greene T. Kusek J.W. Beck G.J. A simplified equation to predict glomerular filtration rate from serum creatinine.J Am Soc Nephrol. 2000; 11 (abstract): A0828PubMed Google Scholar, 9National Kidney FoundationK/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification.Am J Kidney Dis. 2002; 39: S1-S266PubMed Google Scholar The MDRD Study equation had limitations because it was derived from individuals with GFR <90 mL/min to qualify for the study, and numeric values above 60 mL/min/1.73 m2 were not sufficiently accurate to be physiologically reliable. Nonetheless, the MDRD Study equation could be used by clinical laboratories to report an eGFR along with creatinine results because age and gender were typically available to a laboratory as part of the test ordering process. Race is less reliably available as part of a laboratory test order, and many patients have mixed ethnic background. Consequently, the practice to report eGFR values for both African-American and non–African-American ethnicity was suggested to allow the clinical practitioner to determine the most suitable value in locations where there was a need for both versions.10Myers G.L. Miller W.G. Coresh J. et al.Recommendations for improving serum creatinine measurement: a report from the laboratory working group of the national kidney disease education program.Clin Chem. 2006; 52: 5-18Crossref PubMed Scopus (1007) Google Scholar The 2002 Kidney Disease Outcomes Quality Initiative (KDOQI) guidelines for identifying and treating CKD recommended classifying CKD primarily on GFR.9National Kidney FoundationK/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification.Am J Kidney Dis. 2002; 39: S1-S266PubMed Google Scholar Furthermore, this guideline recommended that clinical laboratories report eGFR with every creatinine result to assist clinical practitioners to identify people with early-stage CKD that could be treated to slow progression of the disease. Providing an eGFR enabled clinical practitioners to relate creatinine to a physiologic parameter that more meaningfully related to kidney function. The eGFR allowed use of universal clinical decision points irrespective of the age and gender of the patient. GFR and creatinine are continuous parameters that change incrementally as kidney function declines. The laboratory practice to report “reference intervals” or “normal values” for creatinine forced a continuous measure of function into a dichotomous “normal” or “abnormal” interpretation. Consequently, many people with eGFR values near 60 mL/min/1.73 m2 were classified as being within the “normal range” when in fact they had lost approximately half of their kidney function.11Coresh J. Byrd-Holt D. Astor B.C. et al.Chronic kidney disease awareness, prevalence, and trends among U.S. adults, 1999 to 2000.J Am Soc Nephrol. 2005; 16: 180-188Crossref PubMed Scopus (691) Google Scholar, 12Dalton R.N. Serum creatinine and glomerular filtration rate: perception and reality.Clin Chem. 2010; 56: 687-689Crossref PubMed Scopus (53) Google Scholar People with a mild increase above a “normal range” value for creatinine were being missed as eligible for intervention to slow progression of their CKD. Reporting eGFR calculated from creatinine provided an opportunity to better identify people with early CKD. The National Kidney Disease Education Program (NKDEP) was created by the National Institutes of Health at the time of the KDOQI guidelines to educate physicians and the pubic to raise awareness of and to identify people at risk for CKD so they could receive treatment earlier in the course of the disease to prevent or at least slow progression of complications and end-stage kidney disease. A leadership group of the NKDEP met with representatives of the Chemistry Resource Committee of the College of American Pathologists (CAP) in 2002 to request assistance to educate laboratories to report eGFR along with creatinine and to develop a mechanism to standardize creatinine measurements. At the time of the KDOQI recommendations, creatinine measurement results were not standardized among different methods used for measuring creatinine. The CAP group had conducted surveys of laboratory performance that showed as much as 30% difference in creatinine results between different commonly used methods of measurement.13Ross J.W. Miller W.G. Myers G.L. Praestgaard J. The accuracy of laboratory measurements in clinical chemistry: a study of 11 routine chemistry analytes in the College of American Pathologists Chemistry Survey with fresh frozen serum, definitive methods, and reference methods.Arch Pathol Lab Med. 1998; 122: 587-608PubMed Google Scholar Figure 1 from a more extensive study in 2003 shows the variability in creatinine results from different measurement procedures in common use at that time.14Miller W.G. Myers G.L. Ashwood E.R. et al.Creatinine measurement: state of the art in accuracy and interlaboratory harmonization.Arch Pathol Lab Med. 2005; 129: 297-304PubMed Google Scholar Biases relative to an IDMS RMP varied from zero for some Roche methods to +30% for others with many methods having biases in the +6% to +20% range. This variability in creatinine results among different laboratories would introduce a commensurate variability in the calculated eGFR that would compromise its use to classify the stage of a person's kidney disease. The meeting concluded with an agreement to form a Laboratory Working Group (LWG) of the NKDEP with 2 primary objectives: to direct laboratories regarding reporting eGFR along with creatinine and to develop a standardization program for creatinine measurement procedures to reduce the variability in eGFR. Standardization in laboratory medicine means that the calibration of all measurement procedures (all methods for making a measurement) is traceable to a common reference such that equivalent results for patient samples are produced irrespective of the measurement procedure used. When the creatinine standardization program began in 2003, a pure creatinine reference material was available from the National Institute of Standards and Technology (NIST) and IDMS RMPs were available from a few reference laboratories. The reasons for the lack of agreement among results for patients’ samples for commercially available creatinine methods included calibration schemes established for the methods and cleared by the FDA before the recommended reference system was available and inappropriate use at that time of secondary reference materials for serum creatinine that were noncommutable with patient samples.13Ross J.W. Miller W.G. Myers G.L. Praestgaard J. The accuracy of laboratory measurements in clinical chemistry: a study of 11 routine chemistry analytes in the College of American Pathologists Chemistry Survey with fresh frozen serum, definitive methods, and reference methods.Arch Pathol Lab Med. 1998; 122: 587-608PubMed Google Scholar, 14Miller W.G. Myers G.L. Ashwood E.R. et al.Creatinine measurement: state of the art in accuracy and interlaboratory harmonization.Arch Pathol Lab Med. 2005; 129: 297-304PubMed Google Scholar, 16Franzini C. Ceriotti F. Impact of reference materials on accuracy in clinical chemistry.Clin Biochem. 1998; 31: 449-457Crossref PubMed Scopus (42) Google Scholar Noncommutable reference materials have a response in a measurement procedure to the actual amount of creatinine present that is different from the response for a patient sample that contains the same amount of creatinine. Consequently, if calibration is performed based on a noncommutable secondary reference material, then the values measured for patient samples are biased relative to the actual amount of creatinine present.17Miller W.G. Myers G.L. Rej R. Why commutability matters.Clin Chem. 2006; 52: 553-554Crossref PubMed Scopus (145) Google Scholar, 18Vesper H.W. Miller W.G. Myers G.L. Reference materials and commutability.Clin Biochem Rev. 2007; 28: 139-147PubMed Google Scholar, 19Miller W.G. Myers G.L. Commutability still matters.Clin Chem. 2013; 59: 1291-1293Crossref PubMed Scopus (56) Google Scholar The limitations and implications of noncommutable reference materials were not adequately appreciated in 2003. The recommendations of the LWG of the NKDEP to support the recently introduced KDOQI clinical practice guidelines were published in 2006 and addressed actions required by clinical laboratories, in vitro diagnostics (IVD) manufacturers, professional organizations, metrology institutes and reference laboratories, and proficiency testing or external quality assessment providers.10Myers G.L. Miller W.G. Coresh J. et al.Recommendations for improving serum creatinine measurement: a report from the laboratory working group of the national kidney disease education program.Clin Chem. 2006; 52: 5-18Crossref PubMed Scopus (1007) Google Scholar The recommendations of the NKDEP were adopted by many professional organizations around the world. Prior to publication of the recommendations, the LWG had initiated a collaboration with IVD manufacturers on the importance of standardizing creatinine measurement procedures to enable consistent use of eGFR calculated from creatinine. The LWG also collaborated with NIST to develop new commutable secondary reference materials for creatinine that could be used by IVD manufacturers and clinical laboratories to establish calibration traceability to the reference system for creatinine. The new NIST standard reference material, SRM 967 Creatinine in Frozen Human Serum, became available in 2007.20Dodder N.G. Tai S.S. Sniegoski L.T. Zhang N.F. Welch M.J. Certification of creatinine in a human serum reference material by GC-MS and LC-MS.Clin Chem. 2007; 53: 1694-1699Crossref PubMed Scopus (70) Google Scholar Following depletion of the stocks of SRM 967, a new material from NIST (967a) is now available. Additionally, the Joint Committee for Traceability in Laboratory Medicine database lists a number of pure and serum-based certified reference materials, RMPs, and reference laboratories, providing readily available resources for establishing and verifying calibration traceability.21Joint Committee for Traceability in Laboratory Medicine (JCTLM) database. Available at: http://www.bipm.org/jctlm/. Accessed April 27, 2017.Google Scholar To support the standardization program, the CAP introduced an accuracy-based proficiency testing survey in 2005 that used commutable serum materials to assess the performance of laboratories and the status of standardization among commercial measurement procedures. Figure 2 plots data from CAP proficiency testing surveys performed in 2003 and 2011 that demonstrate the successful standardization of creatinine measurement procedures by the most prevalent IVD manufacturers with global distribution of their measuring systems. Recent reports suggest that some assays based on the Jaffe alkaline picrate reaction, despite improvements to minimize the effects of so-called “noncreatinine chromogens,” do not meet the NKDEP recommendations for standardization and are susceptible to interferences that compromise their use with some patients.22Jassam N. Weykamp C. Thomas A. et al.Post-standardization of routine creatinine assays: are they suitable for clinical applications.Ann Clin Biochem. 2017; 54: 386-394Crossref PubMed Scopus (17) Google Scholar, 23Lee E. Collier C.P. White C.A. Interlaboratory variability in plasma creatinine measurement and the relation with estimated glomerular filtration rate and chronic kidney disease diagnosis.Clin J Am Soc Nephrol. 2017; 12: 29-37Crossref PubMed Scopus (29) Google Scholar, 24Boutten A. Bargnnoux A.S. Carlier M.C. et al.Enzymatic but not compensated Jaffe methods reach the desirable specifications of NKDEP at normal levels of creatinine. Results of the French multicentric evaluation.Clin Chim Acta. 2013; 419: 132-135Crossref PubMed Scopus (42) Google Scholar Assays based on enzymatic reactions, while susceptible to some interfering substances,25Greenberg N. Roberts W.L. Bachmann L.M. et al.Specificity characteristics of seven commercial creatinine measurement procedures using enzymatic and Jaffe method principles.Clin Chem. 2012; 58: 391-401Crossref PubMed Scopus (115) Google Scholar are superior to Jaffe methods and better suited for measuring creatinine. The supply of diagnostic reagents in much of the world is dominated by a small number of multinational manufacturers all of whom now supply serum creatinine assays traceable to appropriate international reference materials and RMPs. However, in some countries, assay reagents and calibrators are purchased from local or other suppliers where calibration standardization has not been adopted and results from different creatinine assays remain highly variable. For laboratories to provide creatinine results that have standardized calibration, as required by international guidelines, they must select suitable assays based on information provided by the manufacturer. A recent international survey of creatinine assay kits with English language product information has demonstrated that for over half of the 79 creatinine assay kits reviewed, there was either insufficient information provided to assess the assay calibration traceability or the assay calibration traceability could not be assessed due to using calibrators not designed for use with a reagent kit.26Biljak V.R. Honović L. Matica J. Krešić B. Vojak S.S. The role of laboratory testing in detection and classification of chronic kidney disease: national recommendations.Biochem Med. 2017; 27: 153-176Crossref Scopus (21) Google Scholar The KDOQI guidelines and NKDEP recommendations influenced professional organizations in many countries to adopt the recommendations including to have laboratories report eGFR along with creatinine. In parallel with the laboratory effort to standardize creatinine measurement, laboratories in many areas of the world began to report eGFR calculated from creatinine. The original MDRD Study equation was used because it was the best available at that time. The equation was developed prior to creatinine standardization using creatinine results measured by a central laboratory that used a creatinine measurement procedure that was later shown to have an approximate bias of positive 5% relative to the IDMS RMP. The original MDRD Study equation was available in 4-parameter (creatinine, age, gender, and race) and 6-parameter (creatinine, urea, albumin, age, gender, and race) versions with very similar accuracy compared to measured GFR. Both versions of the MDRD Study equation were used in the early years of reporting eGFR despite the fact that NKDEP recommended to use the 4-parameter version primarily because serum albumin was also not standardized and it was technically not possible for serum albumin to be standardized at that time. Creatinine methods standardized to an IDMS RMP were introduced by most international manufacturers during 2006–2010.15Killeen A.A. Ashwood E.R. Venture C.B. Styer P. Recent trends in performance and current state of creatinine assays.Arch Pathol Lab Med. 2013; 137: 496-502Crossref PubMed Scopus (29) Google Scholar Consequently, a 4-parameter MDRD Study equation re-expressed for use with standardized creatinine was published in 2007.27Levey A.S. Coresh J. Greene T. et al.Expressing the Modification of Diet in Renal disease study equation for estimating glomerular filtration rate with standardized serum creatinine values.Clin Chem. 2007; 53: 766-772Crossref PubMed Scopus (1450) Google Scholar As standardized creatinine methods were introduced, the 4-parameter equation for use with IDMS traceable creatinine results became dominant because it did not require additional laboratory tests and the 6-parameter equation was not updated for use with IDMS traceable creatinine. Figure 3 shows the percent of laboratories reporting eGFR for adults from 2003 to 2017 who were participants (5183 participants in 2017 and similar in other years) in the CAP general chemistry proficiency testing survey that reflects primarily North American practice although the survey has participants in approximately 70 other countries that represent 13% of all participants.28College of American Pathologists Comprehensive chemistry surveys, Supplemental Questions data used with permission.Google Scholar As expected, the percent of laboratories reporting eGFR increased during the initial period as issues were debated regarding which equation to use and whether to report eGFR before creatinine results were standardized. By 2013, 90% of US laboratories were reporting eGFR with creatinine results for adults and that percentage was about the same in 2017. Of those reporting eGFR, 86% reported eGFR with all creatinine results, 9% only when requested, and 2% only for outpatients because circulating creatinine concentration is less stable and GFR is more variable in acute illness conditions. National guidelines recommending routine reporting of the eGFR, as well as other standardized clinical and laboratory practices, have been developed in a number of countries including Australia and New Zealand,29Johnson D.W. Jones G.R.D. Mathew T.H. et al.Australasian Creatinine Consensus Working Group. Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: new developments and revised recommendations.Med J Aust. 2012; 197: 224-225Crossref Scopus (96) Google Scholar Malaysia,30Ministry of Health MalaysiaChemical Pathology Division, National Pathology Services. Laboratory Investigation Guidelines for Chronic Kidney Disease and utilisation of eGFR in adults.2012Google Scholar Croatia,26Biljak V.R. Honović L. Matica J. Krešić B. Vojak S.S. The role of laboratory testing in detection and classification of chronic kidney disease: national recommendations.Biochem Med. 2017; 27: 153-176Crossref Scopus (21) Google Scholar Turkey,31Abuşoğlu S. Aydın I. Bakar F. et al.A short guideline on chronic kidney disease for medical laboratory practice.Turk J Biochem. 2016; 41: 292-301Google Scholar and the United Kingdom.32NICE Clinical Guideline 182Chronic kidney disease. Early identification and management of chronic kidney disease in adults in primary and secondary care. Available at:.2014https://www.nice.org.uk/guidance/cg182Date accessed: April 27, 2017Google Scholar In 2011, 46% of UK laboratories reported eGFR for all adults, 36% only reported eGFR to general practitioners, and 21% did not report eGFR for in-patients.33Kilpatrick E.S. Verrill H. A national audit of estimated glomerular filtration rate and proteinuria reporting in the UK.Ann Clin Biochem. 2011; 48: 558-561Crossref PubMed Scopus (12) Google Scholar An assessment of kidney care status in 125 countries reported that eGFR from creatinine was typically available in strong economies and much less frequently available in low-income developing nations.34Bello A.K. Levin A. Tonelli M. et al.Assessment of Global Kidney Health Care Status.JAMA. 2017; 317: 1864-1881Crossref PubMed Scopus (216) Google Scholar A limitation of the MDRD Study equation is that it underestimates GFR at values above 60 mL/min/1.73 m2 and NKDEP recommends to not report numeric values above 60 mL/min/1.73 m2. Although there is little relationship of eGFR above 60 mL/min/1.73 m2 and risk for progression of CKD, clinical providers were interested to monitor kidney function at higher eGFR values. Consequently, a number of other eGFR equations have been introduced based on various population characteristics in an effort to optimize calculation of eGFR. The most fully characterized newer equation to calculate eGFR is from the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) research group. The CKD-EPI equation was published in 2009, uses the same 4 parameters as the MDRD Study equation, and was developed using creatinine values standardized to an IDMS RMP.35Levey A.S. Stevens L.A. Schmid C.H. et al.A new equation to estimate glomerular filtration rate.Ann Intern Med. 2009; 150: 604-612Crossref PubMed Scopus (16077) Google Scholar, 36Levey A.S. Eckfeldt J.H. Estimating glomerular filtration rate using serum creatinine.Clin Chem. 2017; 63: 1161-1162Crossref PubMed Scopus (8) Google Scholar Although the CKD-EPI equation is more complex, the calculation can be performed as easily as for the MDRD Study equation by laboratory computer systems. The principle advantage of the CKD-EPI equation is that its accuracy compared to measured GFR is suitable for reporting higher eGFR values.37Earley A. Miskulin D. Lamb E.J. et al.Estimating equations for glomerular filtration rate in the era of creatinine standardization. A systematic review.Ann Intern Med. 2012; 156: 785-795Crossref PubMed Scopus (367) Google Scholar The CKD-EPI eGFR reclassified some patients with reduced muscle mass who had MDRD Study eGFR values of 45–60 mL/min/1.73 m2 into a lower risk classification above 60 mL/min/1.73 m2.38Matsushita K. Mahmoodi B.K. Woodward M. et al.Comparison of risk prediction using the CKD-EPI equation and the MDRD study equation for estimated glomerular filtration rate.JAMA. 2012; 307: 1941-1951Crossref PubMed Scopus (735) Google Scholar The Kidney Disease Improving Global Outcomes (KDIGO) 2012 clinical practice guideline for the evaluation and management of CKD recommends the CKD-EPI equation to calculate eGFR unless there is high-quality evidence that another equation may perform better.1KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease.Kidney Int Suppl. 2013; 3: 1-150Abstract Full Text Full Text PDF Scopus (1594) Google Scholar Figure 4 shows the equations used to calculate eGFR in adults by participants in the CAP general chemistry proficiency testing survey in 2017. The CKD-EPI equation was used by 25% of laboratories even though 41% were reporting numeric values above 60 mL/min/1.73 m2. The IDMS traceable MDRD Study equation was used by 53% of participants some of whom were using that equation inappropriately to report values above 60 mL/min/1.73 m2. Also of concern were the 16% of laboratories using the original MDRD Study 4-parameter equation and 2% using the 6-parameter MDRD Study equation neither of which is suitable for use with IDMS traceable creatinine measurement procedures. Cystatin C is another biomarker that is used to estimate GFR. Cystatin C was identified and characterized by Grubb in Sweden and has been used there for kidney function assessment for many years.39Filler G. Bökenkamp A. Hofmann W. Le Bricon T. Martínez-Brú C. Grubb A. Cystatin C as a marker of GFR - history, indications, and future research.Clin Biochem. 2005; 38: 1-8Crossref PubMed Scopus (586) Google Scholar Because cystatin C is not influenced by muscle mass, it is particularly useful for patients with muscle mass that is different from that of an “average” individual. From the laboratory perspective, cystatin C has been of limited usefulness because initially different methods were not standardized. A working group of the International Federation of Clinical Chemistry and Laboratory Medicine collaborated with the Institute for Reference Methods and Materials of the European Commission to develop a commutable certified reference material, ERM DA471/IFCC, for cystatin C that became available in 2010.40Grubb A. Blirup-Jensen S. Lindstrom V. Schmidt C. Althaus H. Zegers I. First certified reference material for cystatin C in human serum ERM-DA471/IFCC.Clin Chem Lab Med. 2010; 48: 1619-1621Crossref PubMed Google Scholar Manufacturers are now introducing standardized commercially available measurement procedures for cystatin C measurement, but standardization is not yet universal among commercially available measurement procedures.41Eckfeldt J.H. Karger A.B. Miller W.G. Rynders G.P. Inker L.A. Performance in measurement of serum cystatin C by laboratories Participating in the College of American Pathologists 2014 CYS survey.Arch Pathol Lab Med. 2015; 139: 888-893Crossref PubMed Scopus (48) Google Scholar, 42Bargnoux A.S. Pieroni L. Cristol J.P. et al.Multicenter evaluation of cystatin C measurement after assay standardization.Clin Chem. 2017; 63: 833-841Crossref PubMed Scopus (41) Google Scholar Older equations for calculating eGFR from cystatin C are limited for use only with the cystatin C method used to develop the equation. As standardized cystatin C measurement procedures replace the older methods, equations developed for use with standardized cystatin C results must be used. The KDIGO 2012 guidelines recommend estimating eGFR from cystatin C when a patient's eGFR (creatinine) is 45–59 mL/min/1.73 m2 and urine albumin is <30 mg/g creatinine (<3 mg/mmol) using the CKD-EPI equation to estimate eGFR using standardized cystatin C results for adults. There are 2 versions of the CKD-EPI equation, one uses only cystatin C and the other uses cystatin C and creatinine to calculate eGFR.43Inker L.A. Schmid C.H. Tighiouart H. et al.A new equation to estimate glomerular filtration rate from standardized creatinine and cystatin C.N Engl J Med. 2012; 367: 20-29Crossref PubMed Scopus (2505) Google Scholar The combined equation had better performance than either the cystatin C–based or creatinine-based equations for estimating GFR in the CKD-EPI validation cohort. A comparison of eGFR using various equations also concluded that the combined equations had better agreement with measured GFR in adults.44Björk J. Grubb A. Larsson A. et al.Accuracy of GFR estimating equations combining standardized cystatin C and creatinine assays: a cross-sectional study in Sweden.Clin Chem Lab Med. 2015; 53: 403-414Crossref PubMed Scopus (68) Google Scholar However, a cystatin C–based eGFR is particularly useful in conditions when muscle mass limits the suitability of a creatinine-based eGFR. Equations to estimate GFR in children have also been developed. The original Schwartz equation was published in 1984 and estimated creatinine clearance from serum or plasma creatinine, factors for age and gender ranges, and height or length of the child to compensate for the large variation in muscle mass during development.45Schwartz G.J. Feld L.G. Langford D.J. A simple estimate of glomerular filtration rate in full-term infants during the first year of life.J Pediatr. 1984; 104: 849-854Abstract Full Text PDF PubMed Scopus (360) Google Scholar The original Schwartz equation should not be used with standardized creatinine methods because it will overestimate the creatinine clearance. A revised so-called “Bedside Schwartz” equation to estimate GFR in children was developed to be used with IDMS traceable creatinine results measured using enzymatic methods.46Schwartz G.J. Muñoz A. Schneider M.F. et al.New equations to estimate GRF in children with CKD.J Am Soc Nephrol. 2009; 20: 629-637Crossref PubMed Scopus (2404) Google Scholar This equation also uses height of the child but with a single factor for all ages and genders. The bedside Schwartz equation is better than the original equation for creatinine measured using standardized Jaffe alkaline picrate methods because it was developed for results with calibration traceable to IDMS. However, the techniques to “compensate” for noncreatinine chromogens in many Jaffe methods make them less suitable for use with pediatric patients (see previous section). Based on the 2017 CAP Survey data, only 9% of laboratories reported eGFR along with creatinine for children presumably because of the lack of data on height in laboratory information systems. The original Schwartz, bedside Schwartz, and an adult equation were used by 30%, 35%, and 24%, respectively, of laboratories. eGFR can also be calculated for children older than 1 year using cystatin C that has the advantage of not being influenced by muscle mass. As for adults, many older equations have not been updated for use with standardized cystatin C measurements. Equations for use with standardized cystatin C in pediatrics have been published, but there is no consensus on a preferred equation.47Filler G. Huang S.H. Yasin A. The usefulness of cystatin C and related formulae in paediatrics.Clin Chem Lab Med. 2012; 50: 2081-2091Crossref PubMed Scopus (33) Google Scholar A recently reported equation was developed using 7 standardized cystatin C measurement procedures and validated for use with both pediatric and adult populations.48Pottel H. Measuring and estimating glomerular filtration rate in children.Pediatr Nephrol. 2017; 32: 249-263Crossref PubMed Scopus (58) Google Scholar Estimation of kidney function is a vital part of good medical practice. Over the nearly 2 decades since the publication of the first MDRD Study equation and a series of international guidelines, kidney function assessment has become internationally standardized with appropriate tools in the form of standardized creatinine and cystatin C measurement procedures and well-validated equations to estimate GFR. The current recommendations from KDIGO are to use the CKD-EPI equations for estimating GFR. This work has involved collaboration among professional laboratory organizations, IVD manufacturers, metrology institutes, clinical nephrologists, and individual laboratories. Patients have an expectation that their medical status can be assessed accurately and reliably irrespective of their location and the health care setting. In the case of GFR estimation, standardized assessment is a reality in many countries, and progress is continuing in all locations." @default.
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