I pose this question to see what others are doing in this scenario and what literature there is to support your practice. We commonly get mixing study requests in which the PT or PTT are just minimally out of our normal range, such as 0.1 seconds outside for PT. Is this 0.1 second enough to do the mixing study or is there a buffer such as one second for PT and slightly more for PTTs in which although it is outside the normal range, that a mixing study could be not done? I don’t recall ever seeing one of these minimally prolonged PT/PTT turn out to be an inhibitor on followup. I would appreciate your feedback. Thanks.Bruce King, M.D.
Hello, Dr. King, and thank you for the question. This was the last day of the very successful Thrombosis and Hemostasis Summit of North America in Chicago, and I took the opportunity to pose your question to colleagues Larry Brace, PhD of Edward Hospital, Naperville, IL, and Larry Smith, PhD, of Memorial Sloan-Kettering in New York. Both agreed with the policy in our local institution, that any result outside the normal range may be subjected to a mixing study if there is an indication. Both would reflex a prolonged PT or PTT to a mixing study in consultation with the physician managing the case, and would only proceed if the physician saw a diagnostic need. In most instances, the physician realizes that a second or two beyond the reference interval limit is within the routine variability of the assay and would not require follow-up, however it may be the mixing study provides an answer for a clinical question. Since the range for correction is narrow, I suggest that correction be defined on the basis of the pooled normal control used in mixing, and not the mean or limit of the reference interval. I hope this helps, and look forward to colleagues’ responses, which may include references.