Objective: In clinical laboratories, the tests of prothrombin time (PT), and partial thromboplastin time (aPTT) are generally analysed fastly after blood collection. In addition to this, sometimes the blood samples can be kept as frozen in order to be analysed at a later time. In our study, we aimed to examine the effect of storage time of samples, which are kept frozen at -20oC, on the stability of PT and aPTT.

Materials and methods: The first plasma sample which was obtained from the bloods of 256 healthy individual, analysed in 60 minutes. The second plasma sample was divided into 4 groups and kept frozen at -20oC up to time, which were 2 months for 1st group (n=49), 3 months for 2nd group (n=44), 4 months for 3rd group (n=93) and 5 months for 4th group (n=70). PT and aPTT were analysed by TriniCLOT reagent on Destiny Plus instrument. The changes, which exceed the total allowable medical error between before frozen and after thawing, were accepted clinically significant.

Results: After freeze-thawing, there were found statistically significant increases on the PT, INR and aPTT values in all groups (p<0.001). While the increase of PT and INR values was in the limits of the total allowable medical error, we found that the increase of aPTT values exceed the limits of the total allowable medical error.

Conclusion: While there was the effect of keeping process at -20ºC on PT, INR and aPTT values, it can said that the changes in the PT and INR tests are within the clinically more acceptable limits compared with aPTT. While deciding that, the clinical needs and choosen allowable total error rate are very important.