Activated Partial Thromboplastin Time (APTT) is a screening method used to detect defects in the intrinsic pathway. Different preanalytical variables such as centrifugation and thawing can affect the given test results from APTT analysis. The purpose of the study was to evaluate how the results for APTT were affected when frozen citrate plasma was thawed in a heating cabinet or a water bath, and how the results for APTT were affected if the samples were single or double centrifuged before freezing. A total of 40 fresh samples were collected for each study purpose and analyzed. To study how thawing conditions affected frozen samples, aliquoted plasma was frozen at –20°C for a week and then thawed in either a water bath or in a heating cabinet. To study how single and double centrifugation affected the test results, one of the two aliquots per sample was centrifuged again while the other remained single centrifuged. The samples were then stored frozen for one week and afterwards thawed. The results showed statistically significant differences between fresh samples and frozen samples thawed in a heath incubator or water bath. Regarding centrifugation conditions, both single and double centrifugation yielded statistically significant differences in comparison to fresh plasma. All the evaluated sample treatments showed statistically significant differences in comparison to fresh samples. However, in all cases the mean percentage change was ≤10%, which indicates no clinical significance. In conclusion APTT-samples seem to be sensitive to preanalytical variables and further studies are needed to evaluate their effects.