False low parathyroid hormone values secondary to sample contamination with the tissue plasminogen activator

Fluctuating parathyroid hormone values (PTH) are common in patients undergoing haemodialysis. Widely varying PTH results in an 82-year-old haemodialysis (HD) patient could not be explained. When PTH in the same blood sample was no longer detectable 24 h after blood draw, it was hypothesized that contamination with the catheter lock solution containing tissue plasminogen activator (tPA, alteplase) caused degradation of PTH in vitro.

Leftover samples from 21 patients on maintenance HD as well as control samples from healthy volunteers (n = 3) were incubated at 4 degrees C with small amounts of tPA (25 and 50 microL). In addition, pooled samples from HD patients with various PTH levels were incubated with 6.5, 12.5 and 25 microL of tPA and analysed with two different PTH assays with incubation times up to 48 h.

A rapid decline of PTH values to 2.5-33.5% of the original baseline was observed after 24 h with a further decrease to <1-15% after 48 h. The two different assays gave very similar results when the samples were incubated with tPA.

Minimal contamination of a blood sample with tPA results in degradation of PTH in a time-dependent manner. The tPA is therefore unique as a contaminant since its enzymatic activity means that even tiny amounts of contamination will lead to major errors in PTH results by digestion of the protein. This phenomenon was independent of the assay used. Strict attention to the technique when drawing a blood sample from a catheter is mandatory to prevent contamination and avoid spurious test results.