Stability of activated partial thromboplastin time (APTT) test under different storage conditions

We designed this study to assess the effect of storage time and temperature on the activated partial thromboplastin time (APTT) test and plasma activity of factor VIII (FVIII). A total of 71 subjects, comprising 34 healthy controls and 37 patients receiving unfractionated heparin were enrolled. After centrifugation of collected specimens aliquots of plasma were stored at room temperature (20-22 degrees C), refrigerated at 2-6 degrees C and frozen at -40 degrees C. Determination of APTT and plasma activity of FVIII were performed immediately after sampling (zero time) and after 6, 12 and 24 h. We found no significant difference in APTT after 6 h at room temperature and 4 degrees C compared to zero time values (P > 0.05) in control group, while APTT was significantly changed at other storage conditions. With regard APTT test in patients on heparin therapy and samples for FVIII activity in healthy subjects; there was a statistically significant change in their results after 6, 12 and 24 h at room temperature, 4 and -40 degrees C compared to zero time value (P < 0.05). Our data demonstrate that the APTT test can be done within 6 h when stored at room temperature and 4 degrees C without change in the result in healthy subjects. APTT test in patients on heparin therapy and samples for FVIII test in healthy subjects must be done immediately and without delay to avoid reduction in their activities.