relevance, but some research indicates that it might increase risk of thromboembolism. Normal PTT requires the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI and XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged aPTT may indicate: use of heparin (or contamination of the sample) antiphospholipid antibody (especially lupus anticoagulant, which paradoxically increases propensity to thrombosis) coagulation factor deficiency (e.g., hemophilia) sepsis — coagulation factor consumption presence of antibodies against coagulation factors (factor inhibitors) To distinguish the above causes, mixing tests are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does disappear a factor deficiency is more likely. Deficiencies of factors VIII, IX, XI and XII and rarely von Willebrand factor (if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies. The aPTT is usually normal in pregnancy but tends to slightly decrease in late pregnancy. aPTT-based APC resistance test The aPTT-based activated protein C (APC) resistance test is used in the diagnosis of APC resistance (APCR). It involves a modified aPTT test performed in the presence and absence of APC. The ratio of these aPTT values is calculated and is called the APC sensitivity ratio (APCsr) or simply APC ratio (APCr). This ratio is inversely related to the degree of APC resistance. The aPTT-based APC resistance test was developed in 1993. History The PTT was first described in 1953 by researchers at the University of North Carolina at Chapel Hill. This is the reason why the color of the Vacutainer tube top is Carolina blue. See also Thromboplastin References Blood tests Medical signs