Preanalytical Influences on DPC IMMULITE 2000 Intact PTH Assays of Plasma and Serum from Dialysis Patients

Measurement of intact parathyroid hormone (PTH) is essential to the diagnosis and management of metabolic bone disease (1), hypercalcemia, hypocalcemia (2), and renal osteodystrophy (3). The effects of specimen type, collection temperature, and storage temperature on the in vitro stability of PTH differ by method and platform (4)(5)(6)(7)(8)(9)(10). Characterization of preanalytical effects unique to each method, platform, and patient population is important to prevent potential clinical misclassification.The Diagnostics Product Corporation (DPC) IMMULITE 2000 intact PTH assay is a solid-phase two-site chemiluminescent immunoassay with a monoclonal mouse capture antibody and a polyclonal goat signal antibody conjugated to alkaline phosphatase. The effects of preanalytical factors on this assay have been partially investigated. Underfilling of EDTA-plasma tubes decreased measured PTH in samples from both healthy persons and those with chronic kidney disease (CKD) (11). Storage of SST serum samples for 3 days at room temperature decreased measured PTH in SST serum samples, an effect not seen with EDTA plasma (6). Results from EDTA plasma paradoxically increased after storage at 4 °C (12). Despite evidence that PTH is more stable in EDTA-anticoagulated specimens (5)(6)(7)(9)(10), the manufacturer describes an instability associated with EDTA-plasma samples and therefore recommends cold (2–8 °C) collection, centrifugation, and storage until analysis (13).At our institution, PTH is most frequently measured for management of CKD and helps direct therapies, including calcium, vitamin D, bisphosphonates, and parathyroidectomy. Preanalytical influences on PTH assays may be atypical in CKD because samples contain increased concentrations of N-terminal truncated PTH fragments that cross-react with intact PTH assays (14). Accordingly, we have investigated preanalytical variables in a dialysis-dependent CKD population. The study was approved by the St. Paul’s Hospital ethics committee.Five predialysis samples were …