Stability of thyroid tests in serum obtained from plain tubes and tubes containing gel separator (Vacutainer®))

We investigated the stability of thyroid tests during a period of 72 hours from blood drawing in evacuated plain tubes and in tubes containing thixotropic gel. Many statistically significant differences between the two kinds of tubes were observed, but these differences were never clinically relevant. The measurement variations between tubes 24 and 72 hours after blood collection were always lower than method imprecision and biological variability. We decided to store serum for thyroid tests until 72 hours after blood collection and centrifugation at 4°C in both plain and thixotropic gel containing tubes.