The study examined a "percent correction" formula for evaluating mixing study results comparing a 1:1 mix with a new 4:1 mix of patient plasma with citrated normal plasma for a prolonged activated partial thromboplastin time (aPTT) and/or prothrombin time (PT). The study also examined 3 suggested definitions of correction for evaluating mixing study results for comparison. Applicability of percent correction for evaluating the aPTT 4:1 mix testing with and without incubation also was studied. Our results showed that percent correction of the aPTT or PT 4:1 mix had an overall good sensitivity and specificity for detecting anticoagulant and factor deficiency and was better than that of the aPTT or PT 1:1 mix. The 3 suggested definitions of correction all had a poor sensitivity for detecting anticoagulant. The percent correction of the aPTT 4:1 mix testing after incubation had better sensitivity and specificity that that of testing immediately. Nevertheless, these procedures were complementary to each other. The percent correction using the aPTT or PT 4:1 mix seemed to offer a simple, objective, and effective criterion for evaluating mixing study results.