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• W2727928642 abstract "The serum (plasma) anion gap is commonly used in assessment of acid–base disorders.1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar, 2Kraut J.A. Madias N.E. Serum anion gap: Its uses and limitation in clinical medicine.Clin J Am Soc Nephrol. 2007; 2: 162-174Crossref PubMed Scopus (240) Google Scholar In the assessment it is recommended to adjust the anion gap owing to variability in the serum concentration of albumin observed ([Alb]obs),1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar, 2Kraut J.A. Madias N.E. Serum anion gap: Its uses and limitation in clinical medicine.Clin J Am Soc Nephrol. 2007; 2: 162-174Crossref PubMed Scopus (240) Google Scholar, 3Figge J. Jabor A. Kazda A. Fencl V. Anion gap and hypoalbuminemia.Crit Care Med. 1998; 26: 1807-1810Crossref PubMed Scopus (299) Google Scholar and the assessment generally takes 6 steps for metabolic acid–base disorders, as follows. Step 1. Calculate anion gap (AG) by using observed serum concentrations (mmol/L) of sodium, chloride, and bicarbonate ([Na+]obs, [Cl−]obs, and [HCO3−]obs, respectively).AG=[Na+]obs−([Cl−]obs+[HCO3−]obs)(Eq. 1) Step 2. Adjust AG (AGadj) by using [Alb]obs (g/dL) and its reference value ([Alb]ref).AGadj=AG+2.5×([Alb]ref−[Alb]obs)(Eq. 2) Step 3. Compare AGadj with its reference value (AGref). Step 4. Calculate delta AG (ΔAG) by subtracting AGref from AGadj.ΔAG=AGadj−AGref(Eq. 3) Step 5. Calculate delta bicarbonate (Δ[HCO3−]) by subtracting [HCO3−]obs from its reference value ([HCO3−]ref).Δ[HCO3−]=[HCO3−]ref−[HCO3−]obs(Eq. 4) Step 6. Compare Δ[HCO3−] with ΔAG. AGadj > AGref and AGadj < AGref at step 3 indicate the presence of metabolic acidosis with unmeasured anions (eg, lactate and ketones) and abnormal circulating ions (eg, bromide, lithium, iodide, and paraproteins), respectively.1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar, 2Kraut J.A. Madias N.E. Serum anion gap: Its uses and limitation in clinical medicine.Clin J Am Soc Nephrol. 2007; 2: 162-174Crossref PubMed Scopus (240) Google Scholar Δ[HCO3−] > ΔAG and Δ[HCO3−] < ΔAG at step 6 indicate the presence of hyperchloremic metabolic acidosis and metabolic alkalosis, respectively.1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar Although anion gap is a useful tool to diagnose acid–base disorders, the assessment by anion gap is cumbersome, having 4 steps of calculation. Therefore, as an alternative for anion gap, I would like to propose calculated bicarbonate ([HCO3−]calc), which can be obtained using [Na+]obs and [Cl−]obs (mmol/L) and [Alb]obs (g/dL) as follows.[HCO3−]calc=[Na+]obs−[Cl−]obs−2.5×[Alb]obs(Eq. 5) The assessment by calculated bicarbonate can estimate the presence of metabolic acid–base disorders by only 3 steps with 1 calculation, as follows. Step 1. Calculate ([HCO3−]calc (Equation 5). Step 2. Compare [HCO3−]calc with [HCO3−]obs. Step 3. Compare [HCO3−]calc with [HCO3−]ref. [HCO3−]calc > [HCO3−]obs and [HCO3–]calc < [HCO3−]obs at step 2 indicate the presence of metabolic acidosis with unmeasured anions and abnormal circulating ions, respectively. [HCO3−]calc > [HCO3−]ref and [HCO3−]calc < [HCO3−]ref at step 3 indicate the presence of metabolic alkalosis and hyperchloremic metabolic acidosis, respectively. The calculated bicarbonate assessment provides the same diagnostic performance as the anion gap assessment, as demonstrated by the following formula conversions based on the assumption that the negative charges of albumin account for almost all the charges of anion gap.1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar, 2Kraut J.A. Madias N.E. Serum anion gap: Its uses and limitation in clinical medicine.Clin J Am Soc Nephrol. 2007; 2: 162-174Crossref PubMed Scopus (240) Google Scholar, 3Figge J. Jabor A. Kazda A. Fencl V. Anion gap and hypoalbuminemia.Crit Care Med. 1998; 26: 1807-1810Crossref PubMed Scopus (299) Google Scholar When [Alb]ref is given in g/dL, AGref can be substituted by 2.5× [Alb]ref.1Berend K. de Vries A.P. Gans R.O. Physiological approach to assessment of acid-base disturbances.N Engl J Med. 2014; 371: 1434-1445Crossref PubMed Scopus (135) Google Scholar, 2Kraut J.A. Madias N.E. Serum anion gap: Its uses and limitation in clinical medicine.Clin J Am Soc Nephrol. 2007; 2: 162-174Crossref PubMed Scopus (240) Google Scholar, 3Figge J. Jabor A. Kazda A. Fencl V. Anion gap and hypoalbuminemia.Crit Care Med. 1998; 26: 1807-1810Crossref PubMed Scopus (299) Google Scholar Therefore, by substituting Equation 2 for AGadj, Equation 3 can be converted to:ΔAG=AG+2.5×([Alb]ref−[Alb]obs)−2.5×[Alb]ref=AG−2.5×[Alb]obs Substituting Equation 1 for AG,ΔAG=[Na+]obs−([Cl−]obs+[HCO3−]obs)−2.5×[Alb]obs Rearranging the equality,[HCO3−]obs+ΔAG=[Na+]obs−[Cl−]obs−2.5×[Alb]obs From the definition of [HCO3−]calc (Equation 5),[HCO3−]calc=[HCO3−]obs+ΔAG(Eq. 6) Therefore, “[HCO3−]calc > [HCO3−]obs” indicates:[HCO3−]obs+ΔAG>[HCO3−]obs Rearranging the inequality,ΔAG>0 From the definition of ΔAG (Equation 3):AGadj−AGref>0AGadj>AGref In addition, “[HCO3−]calc > [HCO3−]ref” indicates:[HCO3−]obs+ΔAG>[HCO3−]ref Rearranging the inequality,ΔAG>[HCO3−]ref−[HCO3−]obs=Δ[HCO3−] The conversions indicate that “[HCO3−]calc > [HCO3−]obs” and “[HCO3−]calc > [HCO3−]ref” are equivalent to “AGadj > AGref” and “ΔAG > Δ[HCO3−],” respectively. Therefore, the comparisons of [HCO3−]calc with [HCO3−]obs (step 2) and with [HCO3−]ref (step 3) in the calculated bicarbonate assessment correspond to the comparisons of AGadj with AGref (step 3) and ΔAG with Δ[HCO3−] (step 6) in the anion gap assessment, respectively. Rearrangement of Equation 6 gives “[HCO3−]calc − [HCO3−]obs = ΔAG,” which indicates that the calculated bicarbonate assessment simplifies the process of the anion gap assessment; the calculated bicarbonate assessment makes the process possible without the reference value and adjustment of anion gap. Having only 3 steps with 1 calculation, the calculated bicarbonate assessment is simpler than the anion gap assessment, which has 6 steps with 4 calculations. Furthermore, it can estimate the presence of metabolic alkalosis and hyperchloremic metabolic acidosis without [HCO3−]obs. Therefore, calculated bicarbonate will be a useful tool in clinical practice. As an illustration, see the following case example. A 66-year-old man with pre-existing type 2 diabetes treated with insulin was taken to the emergency room because of drowsiness, fever, and frequent vomiting. He had not injected insulin since the previous day. Laboratory tests revealed [Na+]obs 138 mmol/L, [Cl−]obs 96 mmol/L, [Alb]obs 3.6 g/dL, [HCO3−]obs 29 mmol/L, and glucose 360 mg/dL (20 mmol/L). Ketone bodies in urine were positive. The Table shows assessment of metabolic acid–base disorders for the patient, who is suspected to have ketoacidosis with metabolic alkalosis, comparing the calculated bicarbonate assessment with the anion gap assessment. The case demonstrates that the calculated bicarbonate assessment is simpler and less mistakable than the anion gap assessment. In the anion gap assessment, calculation for Δ[HCO3−] in step 5, which resulted in a minus figure (−4), might be done in the opposite direction, and inaccurate calculation could lead to misdiagnosis. In comparison, the calculated bicarbonate assessment is free from this kind of error.TableCase of Mixed Acid–Base Disorder: Comparison of the Calculated Bicarbonate Assessment with the Anion Gap AssessmentCalculated Bicarbonate AssessmentAnion Gap AssessmentStep 1. [HCO3−]calc = 138 − 96 − 2.5× 3.6 = 33Step 2. 33 > 29 Diagnose metabolic acidosis with unmeasured anionsStep 3. 33 > 25 (venous [HCO3−]ref) Diagnose metabolic alkalosisStep 1. AG = 138 − (96 + 29) = 13Step 2. AGadj = 13 + 2.5× (4 − 3.6) = 14Step 3. 14 > 10 (AGref) Diagnose metabolic acidosis with unmeasured anionsStep 4. ΔAG = 14 − 10 = 4Step 5. Δ[HCO3−] = 25 − 29 = −4Step 6. 4 > −4 Diagnose metabolic alkalosisCase is a 66-year-old man with fever and vomiting who had not injected insulin prescribed for pre-existing diabetes since the previous day. Laboratory tests showed [Na+]obs 138 mmol/L, [Cl−]obs 96 mmol/L, [Alb]obs 3.6 g/dL, and [HCO3−]obs 29 mmol/L.AGref = reference value of AG; [Alb]obs = observed venous concentration of albumin in g/dL; [Cl−]obs = observed venous concentration of chloride; [HCO3−]obs = observed venous concentration of bicarbonate in mmol/L; [HCO3−]ref = reference value of [HCO3−]; [Na+]obs = observed venous concentration of sodium. Open table in a new tab Case is a 66-year-old man with fever and vomiting who had not injected insulin prescribed for pre-existing diabetes since the previous day. Laboratory tests showed [Na+]obs 138 mmol/L, [Cl−]obs 96 mmol/L, [Alb]obs 3.6 g/dL, and [HCO3−]obs 29 mmol/L. AGref = reference value of AG; [Alb]obs = observed venous concentration of albumin in g/dL; [Cl−]obs = observed venous concentration of chloride; [HCO3−]obs = observed venous concentration of bicarbonate in mmol/L; [HCO3−]ref = reference value of [HCO3−]; [Na+]obs = observed venous concentration of sodium." @default.
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• W2727928642 date "2017-10-01" @default.
• W2727928642 modified "2023-09-25" @default.
• W2727928642 title "Calculated Bicarbonate for Acid–Base Disorders" @default.
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