Introduction:
The recommended biological material for the parathyroid hormone (PTH) assay is venous blood plasma. According to diagnostic kit manufacturers, the PTH concentration can also be determined in blood serum. The aim of the study was to determine whether differences exist between PTH measured in serum and in plasma.

Material and methods:
The study was conducted on venous blood samples (plasma and serum) obtained from 92 patients at the Clinic of Endocrinology, University Hospital WAM – CSW. The PTH concentration was measured with an Immulite 1000 analyzer (Siemens), and iPTH kit.

Results:
The PTH values in plasma and serum were the same only in 3 cases. In 46 samples (50%), a higher hormone level was observed in the plasma, and in 43 cases (47%), a higher concentration was obtained in serum. The absolute differences between the PTH values in the tested biological materials ranged from 0 to 38 pg/mL, and increased together with the hormone concentration. The relative differences (percentage ratio of absolute differences to plasma PTH concentration) ranged from 0 to 34.3%. However, neither the absolute nor the relative differences were statistically significant (p > 0.05). The classification of PTH concentrations according to the reference values for each biological material (i.e. normal, reduced or elevated) was found to be concordant in 83 tested samples (90%).

Conclusions:
The identified PTH concentrations in the plasma and serum were not identical but comparable and in most cases the differences did not affect the clinical interpretation of the results.