Immulite 2000 parathyroid hormone assay: stability of parathyroid hormone in EDTA blood kept at room temperature for 48 h

Parathyroid hormone (PTH) concentrations were compared in serum and EDTA pl asma from 36 patients attending a renal stone clinic. Serum PTH concentrati ons ranged from 0.9 to 10.9 pmol/L, with a mean of 4.6 pmol/L. When serum a nd EDTA plasma results were compared, in samples frozen within 30 min of co llection, EDTA plasma results were found to be significantly higher than th ose in serum (P<0.0001; Wilcoxon test), with an average increase of 19.5% o ver the serum result. Results from EDTA-preserved blood left to stand at ro om temperature for 48 h were on average 14.8% lower than results from the c orresponding EDTA plasma samples frozen within 30 min, with a highly signif icant difference (P<0.0001). Freshly frozen serum and 48 h EDTA plasma PTH results were not significantly different. Parathyroid hormone in EDTA-preserved blood is not completely stable, and t his could lead to misclassification of results for samples which are not fr ozen quickly.