The saline needle-wash specimen is analyzed with the Roche Diagnostics PTH reagent assay performed on the Roche cobas 6000. The Roche cobas assay for determining intact parathyroid hormone (PTH) employs a sandwich test principle in which a biotinylated monoclonal antibody reacts with the N-terminal fragment (1-37) and a monoclonal antibody labeled with a ruthenium complex(a) reacts with the C-terminal fragment (38-84). Application of a voltage to the electrode then induces chemiluminescent emission, which is measured by a photomultiplier. The antibodies used in this assay are reactive with epitopes in the amino acid regions 26-32 and 37-42.(Package insert: Roche PTH reagent, Roche Diagnostics Corp, Indianapolis, IN July 2010)
For all samples with PTH concentrations >40 pg/mL, a dilution series is performed. A linear dilution excludes hooking and most major interferences. Samples that contain PTH concentrations <40 pg/mL are spiked with exogenous PTH to identify possible interferences that may cause a false-low result.