In the clinical laboratory practice, endocrine biomolecules are mainly measured by immunoassay. Storage of the samples can not be avoided in many cases. Measurement in the low concentration ranges require exact knowledge on how storage would influence the functional sensitivity of the measurement.
Aim: To evaluate the effect of storage of serum samples on their stability and the functional sensitivity of the applied method.
Methods: The biomolecules parathormone intact (PTHi), thyroglobulin (Tg) and thyroglobulin antibody (TgAb) were studied.The measurements were performed by an electrochemiluminescence immunoassay (Elecsys 2010, Roche). The stability of Tg and TgAb were studied in serum (N=71) and that of PTHi in plasma (N=31) as well. The parameters were measured in the fresh samples as well as after 4 and 8 hour of storage at room temperature and after 48 hour of storage at 410 °C. A longer-term storability test was also performed by keeping the samples for 14 weeks in deep freezer. The functional sensitivity of the methods was calculated from the results of deep frozen samples.
Results: In the first 8 hours the immunoreactivity of Tg, TgAb and PTHi changed only marginally (28%). During 48 hours storage, the Tg immunoreactivity increased by 23%, the PTHi molecule by 512% and the TgAb immunoreactivity decreased by 813%. During the long-term deep freezing, the immunoreactivity of all biomarkers decreased by 1239%. A stronger degradation of molecules was observed in the lower range. PTHi appeared to be more stable in plasma than in serum samples. The functional sensitivity of the PTHi (2.6 pg/ml) and Tg (0.66 ng/ml) methods were excellent, but the TgAb (85 IU/ml) sensitivity makes questionable the application as a tumor marker.
Conclusions: The immunoreactivity of Tg, TgAb and PTHi is not influenced by a short storage at room temperature, but freezing even for longer-term significantly alters the analytic results.