Higher parathyroid hormone (PTH) concentrations with the Architect PTH assay than with the Elecsys assay in hemodialysis patients, and a simple way to standardize these two methods

Background: The Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines recommend maintaining serum parathyroid hormone (PTH) concentration between 150 and 300 pg/mL in patients with chronic kidney disease (CKD) stage 5. However, a marked inter-method variability in PTH measurement has been reported recently. The aim of this study was to evaluate whether harmonization of the results measured with two commercial kits known to produce significantly different serum PTH concentrations could be reasonably achieved by a simple procedure. Methods: The study comprised a total of 216 hemodialyzed patients in whom blood was collected immediately before a dialysis session. The patients were from three dialysis centers, which defined three groups (119, 34, and 63 patients for groups 1, 2, and 3, respectively). PTH was measured by two automated assays, the Elecsys (Roche Diagnostics) and Architect (Abbott Diagnostics) assays, in three different laboratories and with different lots of reagents. We arbitrarily chose the Roche assay as the reference method, because several studies had previously shown that the concentrations measured with this assay were very close to the Allegro assay used in the studies that defined the K/DOQI thresholds. Data are median (interquartile range). Results: The median PTH concentrations were higher (p<0.001) in the Architect assay [238 (140-434) pg/mL] when compared to the Elecsys assay [182 (109-338) pg/mL]. Bland-Altman plots in the three groups showed a similar proportional bias between both kits. The Architect PTH/Elecsys PTH ratios were similar in the three groups [1.30 (1.25-1.35), 1.30 (1.19-1.39), and 1.31 (1.25-1.35)], and the ratio was 1.30 (1.25-1.35) in the cohort (pooling the three groups). In the whole population, 53 patients (24.5%) were classified differently by the two kits according to the K/DOQI cut-off values. We divided the Architect values by 1.3 to obtain "corrected'' values. These corrected Architect values were not different to the measured Elecsys values, and the Bland-Altman plot comparing the Elecsys and the corrected Artchitect values did not show any systematic proportional bias. Only six patients (2.8%) were still classified differently by the Elecsys and the corrected Architect concentrations. Conclusions: We propose to divide the PTH values measured with the Architect PTH assay by 1.3 so that the corrected values are almost identical to those measured with the Elecsys assay.