Abstract
Rationale & ObjectiveManagement of chronic kidney disease mineral and bone disorder requires parathyroid hormone (PTH) concentrations. “Biointact” PTH immunoassays detect “whole” PTH (wPTH), whereas “intact” immunoassays measure PTH plus PTH fragments (iPTH). We aimed to determine whether longitudinal changes in PTH concentrations can be evaluated using biointact and intact immunoassays alike.Study DesignOpen noninterventional longitudinal cohort study.Setting & ParticipantsPTH concentrations were measured quarterly up to 5 times in 102 hemodialysis patients.Predictors & Tests ComparedAge, sex, phosphate levels, and others as clinical predictors for PTH trend. Tests compared were iPTH immunoassays from Siemens and Roche and wPTH immunoassays from Roche and DiaSorin.OutcomesPTH concentration trend; regression equations; test bias.Analytical ApproachPredictive regression-to-the-mean model for PTH slope; Bland-Altman plots, Passing-Bablok regression, and reference change values for test comparisons.ResultswPTH concentrations were similar with both immunoassays (wPTH-Roche = 11.7 + 0.97 × wPTH-DiaSorin, r = 0.99; mean ± 1.96 SD bias, 8.2 ± 43.3 pg/mL [17.5% ± 40.9%], by Bland-Altman plots). iPTH-Siemens concentrations were higher than iPTH-Roche concentrations (iPTH-Siemens = −5.4 + 1.33 × iPTH-Roche, r = 0.99; mean ± 1.96 SD bias, 84.0 ± 180.2 pg/mL [21.1% ± 29.8%], by Bland-Altman plots). iPTH-Roche and iPTH-Siemens concentrations were 2- and 2.5-fold higher than wPTH concentrations, respectively. Full agreement among all 4 immunoassays in detecting both significant and insignificant changes in PTH concentrations, upward or downward from one quarter to the next, was reached in 87% of consecutive measurements. In a predictive model, baseline PTH concentrations > 199 pg/mL (wPTH-Roche), 204 pg/mL (wPTH-DiaSorin), 386 pg/mL (iPTH-Roche), and 417 pg/mL (iPTH-Siemens) correctly predicted declining PTH concentration trend in 62% to 68% of patients, but age, sex, hemodialysis vintage, and calcium and phosphate levels were no significant predictors.LimitationsLimited number of immunoassays, only 59 patients attended all quarterly samplings.ConclusionswPTH-Roche and wPTH-DiaSorin concentrations were similar, while iPTH was higher than wPTH concentrations. The iPTH-Siemens immunoassay is either higher calibrated or detects more fragments than iPTH-Roche. However, longitudinal PTH concentration changes largely coincided with all tested immunoassays.
Highlights
Detect only whole parathyroid hormone (wPTH) are entitled “biointact” tests.[9]
Median intact parathyroid hormone (iPTH) concentrations measured with immunoassays from Siemens and Roche were nearly 3 times higher than the upper limit of the reference range of the respective immunoassay
Median wPTH concentrations measured with immunoassays from Roche and DiaSorin were 1.9 times higher than the upper reference limit of the respective immunoassay
Summary
Detect only wPTH are entitled “biointact” tests.[9] The clinical performance of any one immunoassay is usually evaluated by a comparison of one immunoassay against another by means of correlation analyses at a single time point. Longitudinal immunoassay behavior to our knowledge has not been systematically assessed. In the current study in maintenance hemodialysis (HD) patients, we evaluated PTH, calcium, and phosphate concentrations over time, as recommended by KDIGO. Our specific aims were to: (1) provide methods for converting PTH concentrations obtained by some widely used immunoassays, (2) determine whether iPTH and wPTH immunoassays uniformly detect significant longitudinal intrapatient changes, and (3) assess using a prognostic model whether clinical factors and calcium and phosphate levels could forecast increasing versus decreasing PTH concentrations, by type of immunoassay
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