Prolonged partial thromboplastin time: To mix or not to mix – is that the question?

Routine mixing studies are frequently used to evaluate patients presenting with prolonged partial thromboplastin times (PTT) and/or prothrombin times (PT). Unfortunately, mixing studies have a number of inherent limitations including lack of standardization in terms of what defines normal pooled plasma (NPP), the processing of a patient's plasma for platelet removal (platelet poor plasma versus platelet-free plasma), performance of appropriate controls, conducting an incubation step to evaluate for a time and temperature dependent inhibitor, and finally interpretation of test results. Moreover, misinterpretation of study results can lead to a delayed or incorrect diagnosis or worse, inappropriate treatment. Within this manuscript, we present four cases illustrating the shortcomings associated with inappropriate utilization and interpretation of routine mixing studies; and present practical steps for managing abnormal PT or PTT results.