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• W2033536448 abstract "Thrombocytopenia and microangiopathic hemolytic anemia together with a severely deficient ADAMTS-13 activity (<5%) confirm the diagnosis of acute thrombotic thrombocytopenic purpura (TTP) [1], even though not all patients with a clinical diagnosis of TTP have a severe ADAMTS-13 deficiency [2]. Since the initial reports by Furlan et al. [3] and Tsai and Lian [4], numerous assays for the determination of ADAMTS-13 activity have been developed. The majority of these assays comprise two steps [5]. First, a von Willebrand factor (VWF) substrate is subjected to proteolysis by plasma ADAMTS-13. This is followed by a second step of either quantifying VWF digestion products [3, 4, 6] or determination of residual VWF function [7, 8]. These assays are time-consuming, often difficult to perform and hence restricted to specialized laboratories. Recently, Kokame et al. [9] reported a new fluorescence resonance energy transfer assay using a truncated, synthetic 73-amino-acid VWF peptide as a substrate for the determination of ADAMTS-13 activity (FRETS-VWF73 assay). While the performances of several of the older assays have been evaluated and compared in two multicenter studies [10, 11], this new method has not yet been formally evaluated in this way. Therefore, we compared the results obtained with the FRETS-VWF73 assay with those obtained with other ADAMTS-13 activity assays in a series of 30 plasma samples. The FRETS-VWF73 assay was performed essentially as described [9] with the following modifications. Pefabloc SC (Boehringer, Mannheim, Germany) was added to the assay buffer (5 mmol L−1 Bis-Tris, 25 mmol L−1 CaCl2, 0.005% Tween-20, pH 6.0) at a final concentration of 1 mmol L−1. Assay calibration was obtained by using a normal human plasma pool (NHP; Swiss Red Cross Blood Services, Bern, Switzerland) diluted 1:25 (100%) in assay buffer. Further calibration samples were obtained by serial predilutions of NHP of 3:4 (75%), 1:2 (50%), 1:4 (25%), 1:10 (10%), 1:20 (5%) in heat-inactivated fresh frozen plasma (FFP, incubated for 30 min at 56 °C followed by 15 min of centrifugation at 15 000 × g) to correct for a plasma matrix effect in the lower activity range of the standard curve. All of these standard samples as well as heat-inactivated FFP (0% ADAMTS-13 activity) and all test samples were subsequently diluted 1:25 in assay buffer. Next, 100 μL of each diluted standard or patient sample was incubated at 37 °C in a 96-well white plate (NUNC, Roskilde, Denmark). After 10 min, 100 μmol L−1 of 4 μmol L−1 FRETS-VWF73 peptide substrate dissolved in assay buffer was added to each well and evolution of fluorescence recorded at 37 °C in a TECAN Spectrafluor microplate reader (Tecan, Zürich, Switzerland) equipped with a 340 nm excitation filter (band width 35 nm) and a 450 nm emission filter (band width 25 nm). Fluorescence was measured every 5 min and the reaction rate was calculated by linear regression analysis of fluorescence over time from 5 to 60 min. ADAMTS-13 activity was determined in the same 30 plasma samples of patients with acquired or hereditary TTP and other conditions that had been previously analyzed for ADAMTS-13 activity in the context of the first multicenter study by quantitative immunoblotting assay, residual collagen binding activity assay using two different VWF substrates, residual ristocetin cofactor activity assay, and immune radiometric assay [10]. For the FRETS-VWF73 assay, aliquots of the same samples, which had been stored at −70 °C, were analyzed three times with different FRETS-VWF73 peptide substrate lots (550 116, 550 703 and 550 827). Patient samples with an ADAMTS-13 activity <20% were screened for the presence of inhibitory autoantibodies for which the patients’ plasma was heat-inactivated as described above, mixed 1:1 (v:v) with NHP and incubated for 2 h at 37 °C. This mixture was then diluted 1:25 in assay buffer. The key to the plasma samples and former results remained confidential until completion of all analyses. FRETS-VWF73 inter-assay reproducibility expressed by the mean coefficient of variation, calculated as the ratio between the standard deviation and the mean of the triplicate measurements of all samples multiplied by 100, was 6% and thus excellent. Predilution of calibration curve samples in heat-inactivated FFP was superior to dilution of NHP in assay buffer alone as judged from the standard curve correlation coefficient as well as reproducibility, especially in the lower ADAMTS-13 activity range (0–25%). This lower range is not formally covered by the original method described by Kokame et al., as their standard curve point next to 0% corresponds to an ADAMTS-13 activity of 25% [9]. A good agreement was found between the ADAMTS-13 activity measured by this slightly modified FRETS-VWF73 assay and the other methods for all 30 samples and all ADAMTS-13 activity categories (Table 1). The Spearman rank order correlation coefficients between results obtained with the FRETS-VWF73 assay and each of the other methods were Rs = 0.90–0.97 (P < 0.001). A total of 11 plasma samples were found to have an ADAMTS-13 activity <5% by the FRETS-VWF73 assay, confirming previous findings by all five assays in nine instances. One sample (no. 22) had been found to be severely deficient by three methods, while two assays had found ADAMTS-13 values of 8% and 10%, respectively. Another sample (no. 30) was found to have <5% ADAMTS-13 activity by four of five methods. The results of inhibitor screening were also comparable. In agreement with the other ADAMTS-13 assays, strong inhibitors were detected in four plasma samples of patients with acute acquired TTP and severe ADAMTS-13 deficiency (no. 5, 9, 11 and 13), while in four samples of patients with hereditary TTP (no. 2, 15, 19 and 30) no inhibitor was found by FRETS-VWF73. In the remaining six samples tested for ADAMTS-13 inhibitors, a definite inhibitor was found in one sample (no. 23), while five samples (no. 1, 12, 22, 25 and 27) tested negative. These six samples had been discordantly classified as positive, uncertain, or negative for inhibitor by the other assays [10]. We conclude that the FRETS-VWF73 assay is an easy-to-perform, reliable ADAMTS-13 activity assay. The advantage over the other ADAMTS-13 activity assays is the availability of results within 1–2 h. From data shown by Dong et al. [12], the inter-individual variation of ADAMTS-13 activity in normal subjects seems to be most pronounced the shorter a patient's plasma is incubated with the VWF substrate. Therefore, it is not excluded that ADAMTS-13 activity determined by the initial rate assay, FRETS-VWF73, may differ from that determined by other assays using a prolonged incubation (16–20 h) of patient's ADAMTS-13 with the VWF substrate. Nevertheless, in the series of samples analyzed here, ADAMTS-13 activity determined by FRETS-VWF73 assay was in good accordance with that measured by the other assays. The definite value of this new assay remains to be determined in a large cohort of patients diagnosed with acquired or hereditary TTP. This work was supported by grants from the Swiss National Foundation for Scientific Research (3200B0-108261) and the Mach-Gaensslen Foundation, Switzerland." @default.
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• W2033536448 title "Measurement of ADAMTS‐13 activity in plasma by the FRETS‐VWF73 assay: comparison with other assay methods" @default.
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