A recent publication in this journal identified numerous potential sources of variables that may alter the results of coagulation testing. 1 Storage of blood samples is one identified area where potential sources of test bias may occur, especially when storage is compromised or not optimal. For freshly collected samples that cannot be tested within the recommended time frame for measurand stability, plasma is separated by centrifugation (ideally with double centrifugation to ensure depletion of platelets2), stored in capped polypropylene storage vials, and maintained frozen, preferably at À70C or colder.

Historically, it has been suggested in recent publications2, 3 and reagent manufacturer package inserts that plasma samples for coagulation should avoid multiple freeze–thaw processes, as sample integrity may be compromised. Despite the long historical laboratory lore about avoiding multiple freeze–thaw cycles for coagulation testing, there is actually very little published evidence to support this restriction. There have been a couple of recent freeze–thaw publications on a limited number of coagulation tests. For example, in a study related to antiphospholipid antibody testing, Maelegheer and Devreese demonstrated that five freeze–thaw cycles did not cause any significant difference or impact on final result interpretation of anticardiolipin or anti-Beta2 glycoprotein I antibody testing. 4 In another study, Zhao and colleagues indicated that factor (F) XII can undergo only a single freeze–thaw cycle before test accuracy is compromised (albeit FXII is not considered clinically significant); FII, FV, and FIX can undergo only two freeze–thaw cycles; and FXI can undergo three freeze–thaw cycles. 5 The authors used a 10% change threshold, but their study design introduced a significant bias in that the testing was not performed concurrently, and thus day-to-day imprecision potentially affected study findings. What is encouraging is the increasing number of publications that are describing freeze–thaw effects (eg, of direct oral anticoagulant levels),