Parathyroid hormone is more stable in EDTA plasma than in serum.

Measurements of circulating parathyroid hormone (PTH) are important in the evaluation of individuals with hyperor hypocalcemia and for the investigation of PTH function in bone and mineral disorders (1, 2). Serum may be a good alternative to EDTA plasma in the assessment of PTH status and is an acceptable alternative to EDTA plasma in the reformulated IMMULITE 2000 intact PTH (iPTH) assay. However, reports on the stability of PTH in serum and EDTA plasma are conflicting (3–5). Blood samples from 36 patients were collected in parallel into 10-mL tripotassium EDTA tubes, ensuring complete sample filling of the collection vessel (6 ), and into Becton Dickinson SST II gel separator tubes. PTH was assayed with the reformulated IMMULITE 2000 iPTH assay (beginning with lot L2KPP 106) within 3 h of collection and reassayed on both samples after storage for 3 days at room temperature. The mean iPTH was 14.8 pmol/L (range, 0.9–27 pmol/L) for EDTA-plasma samples at baseline (target value). Serum iPTH values were 35% lower. After 3 days of storage at room temperature, the serum iPTH values were 60% lower than the baseline EDTA-plasma target values (Fig. 1A), whereas the iPTH in EDTA was marginally higher (by 8%). Statistical analysis (repeated-measures ANOVA and Dunnett’s post hoc test) of all results indicated that serum values both at baseline and after 3 days were statistically lower than the target value (P 0.0001). In contrast, there was no significant difference between EDTA-plasma values at baseline and after 3 days at room temperature. The magnitude of difference between serum and EDTA-plasma iPTH was similar for high iPTH values and iPTH values within the reference interval (Fig. 1B). The Royal Australasian College of Pathologists Quality Assurance Program determines the allowable limits of performance for iPTH assays by two criteria: (a) 25% difference between the sample and the target value when iPTH is 10 pmol/L; and (b) 2.5 pmol/L absolute difference between the sample and target value when the target value is 10 pmol/L. When the Royal Australasian College of Pathologists Quality Assurance Program criteria were applied, 19 of 36 (52%) serum samples at baseline and 27 of 36 (75%) serum samples stored at room temperature for 3 days failed assurance criteria, in contrast to one EDTA sample stored at room temperature for 3 days. We defined our reference interval for iPTH (0.8–8.0 pmol/L) according to the values in healthy, vitamin Dsufficient blood donors (7 ). When we applied this reference interval for iPTH as the diagnostic classification criterion, 6 of 36 (17%) serum samples at baseline and 13 of 26 (50%) serum samples stored at room temperature for 3 days were misclassified, in contrast to two EDTA samples stored at room temperature for 3 days. Despite attempts to analyze serum samples expeditiously, serum values for iPTH were significantly lower than in EDTA-plasma samples. Thus, the IMMULITE 2000 iPTH assay does not give comparable results for serum and EDTA plasma. The further decline in iPTH values in serum at 3 days is consistent with the susceptibility of PTH to degrade in serum samples. As long as adequate sample volume during collection is ensured, EDTA samples are the most appropriate for iPTH meaFig. 1. Effect of sample type (EDTA or serum) and storage for 72 h at room temperature on iPTH values with the IMMULITE 2000 iPTH assay.