To mix or not to mix in lupus anticoagulant testing? That is the question

Mixing patient and normal plasma has been used for many years to assist with making decisions on which direction to proceed for further investigation of abnormally prolonged coagulation tests, namely, either individual coagulation factor measurement or the search for circulating anticoagulants. Mixing tests, however, gained wide acceptance only after the so-called lupus-like anticoagulant (LA) phenomenon was described and were, therefore, included in the guidelines for LA detection issued by the working group of the International Society on Thrombosis and Haemostasis. Important though they may be considered, the dispute between those who advocate the use of mixing tests and those who deny their superiority (or their need) for LA detection is difficult to resolve. This article aims to provide a balanced view on this dispute. Based on the limited information provided by the literature one may conclude that the disadvantages of performing mixing studies are the fact that (1) they are time-consuming; (2) they require a suitable source of normal pool plasma; and (3) weak LA may be lost at diagnosis due to the dilution of index plasma into normal pooled plasma. On the contrary, performing mixing tests makes the occurrence of false-negative LA in patients who present with the so-called "lupus cofactor phenomenon" relatively unlikely and, therefore, justifies their performance even though the frequency of the occurrence of this phenomenon in LA-positive patients is still unknown.