IN-VITRO STABILITY OF ENDOGENOUS PARATHYROID HORMONE-RELATED PROTEIN IN BLOOD AND PLASMA

We describe a systematic comparison of the effects of anticoagulants, protease inhibitors and conditions of sample handling on the in vitro stability of endogenous parathyroid hormone-related protein (PTHrP) in blood from patients with hypercalcaemia of malignancy (HM). When bloo d was separated within 15 min of collection PTHrP1-86 levels measured by two-site immunoradiometric assay in serum and heparinized plasma we re significantly lower than in ethylenediaminetetraacetic acid (EDTA) plasma (P< O.02). PTHrP was unstable in blood kept at 20 degrees C for 4h and inclusion of protease inhibitors reduced, but failed to abolis h, this instability. In blood collected in the presence of EDTA, inclu sion of leupeptin either alone or in combination with pepstatin and ap rotinin increased the mean half-time of disappearance from 3.9 to 10.1 and 11.2 h, respectively (P< O.05). In contrast, when blood containin g EDTA was separated within 15 min, PTHrP was stable in plasma at 20 d egrees C for at least 4h. As a result of the instability of PTHrP1-86 immunoreactivity in whole blood at ambient temperatures we advise that for our immunoradiemetric assay (IRMA) blood collected in EDTA should be separated within 15 min, and the plasma frozen until assay.