Stability of Parathyroid Hormone-Related Protein and Parathyroid Hormone at Room Temperature

The stability of plasma parathyroid hormone-related protein (PTHrP) as measured by the Nichols Institute assay at room temperature was assessed over a period of 72 h in blood samples collected in protease inhibitor tubes and EDTA tubes at 0, 6, 24, 48 and 72 h from 10 patients with hypercalcaemia of malignancy. Mean plasma PTHrP concentrations in blood samples collected in protease inhibitor tubes remained stable for up to 48 h but had decreased by 10% at 72 h. The mean EDTA plasma PTHrP at zero time was 67% of the protease inhibitor tube value and this had fallen to 39% at 72 h. The stability of parathyroid hormone (PTH) in separated blood samples was also assessed by collection into heparin and plain tubes as well as EDTA and protease inhibitor tubes. Serum PTH concentrations progressively declined throughout the 72 h study period although the zero time values were significantly higher than corresponding plasma PTH concentrations. Plasma PTH concentrations appeared to be stable when blood was collected in heparin, EDTA and protease inhibitor tubes during the 72 h period, except in one subject with markedly elevated plasma amylase activity.