The impact of time of sample collection on the measurement of thyroid stimulating hormone values in the serum.

Objective: The aim of our research is to determine whether the time of blood sampling and fasting of patients have an impact on TSH values.

Design And Methods: A total of 198 participants were enrolled in this study and classified into five groups: A--the first sample collection for TSH measurement was taken between 7:00 and 8:00 a.m. at fasting and the second after 140 min without food intake; B--between 7:00 and 8:00 a.m. at fasting and the second after 140 min with food intake; C--between 7:00 and 8:00 a.m. at fasting the previous day and the second one between 7:00 and 8:00 a.m. at fasting the following day; D--between 9:00 and 10:00 a.m. at fasting the previous day and the second one between 9:00 and 10:00 a.m. at fasting the following day, and E--between 9:00 and 10:00 a.m. at fasting the previous day and the second one between 7:00 and 8:00 a.m. on the following day. Serum TSH concentration was measured by electrochemiluminescence immunoassay (ECLIA, Roche Diagnostics, Mannheim, Germany).

Results: TSH values (mIU/L) were in group A: 2.50 (2.20-2.81) first samples, 1.74 (1.52-1.96) second samples, p<0.001; B: 2.11 (1.52-2.72) first samples, 1.56 (1.13-1.81) second samples, p<0.001; C: 2.60 (2.28-2.91) first samples, 2.23 (1.92-2.53) second samples, p<0.001; D: 1.80 (1.48-2.11) first samples, 1.77 (1.44-2.09) second samples, p<0.597; and E: 1.32 (1.11-2.16) first samples, 1.67 (1.48-2.93) second samples, p<0.001.

Conclusion: The time of sample collection must be standardised for the purpose of standardisation and harmonisation of TSH measurements.