Aim: In this study, it was aimed to examine the changes of activated partial thromboplastin time (APTT), prothrombin time (PT), and plasma fibrinogen during the day, which are the most used coagulation tests in daily routine.

Material and Methods: Blood was drawn from the volunteers included in the study at 9.00, 12.00, 15.00, 18.00, and 24.00 hours. For the analysis of PT, APTT, and fibrinogen parameters, an optical method was used with original kits on ACL Top 500 device (Instrumentation Laboratory, Bedford, MA, USA). Samples taken at 09:00 were considered basal. Diurnal variation was investigated by comparing the APTT, PT, and fibrinogen results obtained at different times of the day with the basal results obtained at 09:00, statistically and clinically.

Results: A total of 15 (10 men, 5 women) healthy volunteers aged between 18-50 years were included in our study. In our study, no clinically or statistically significant difference was found in PT values compared to the basal level (p<0.0125). When compared with the baseline value, both APTT and fibrinogen were the highest at 09.00, and all the results during the day were clinically significantly lower than the baseline level (APTT at 12.00 maximum -13.36%, Fibrinogen at 24.00 maximum - 9.81%). When compared with the basal value, APTT, and fibrinogen levels were found to be statistically significantly lower at some hours compared to baseline (p<0.0125).

Conclusion: PT did not change during the day. Differences in APTT and fibrinogen values were detected during the day, and it is recommended to consider sampling times in clinical evaluation.