Antiphospholipid syndrome (APS), formerly detected by the association between clinical events (venous/arterial thrombosis or pregnancy morbidity) and positivity in at least one of the laboratory tests used to detect antiphospholipid antibodies (aPL), namely lupus anticoagulant (LA), IgG- or IgM-class anticardiolipin antibodies (aCL), and IgG or IgM-class anti-beta2GPI antibodies (abeta2GPI), is now diagnosed using revised APS classification criteria. However, there are a number of problems with the methods used to detect aPL. IgM-class aCL and abeta2GPI measurements are not covered by the health insurance system of Japan, while ELISA to measure abeta2GPI has not been standardized LA which is strongly associated with thrombosis in APS, can be determined using methods recommended by the Scientific and Standardization Committee of the International Society of Thrombosis and Haemostasis (ISTH-SSC). According to the guidelines for detection of LA recently published by ISTH-SSC, APTT reagents with silica as an activator and double centrifugation-treated samples (including control plasma) are recommended to detect LA, while 50% patient plasma mixed with control plasma is recommended for a cross-mixing test. However, some APTT reagents with ellagic acid are sensitive for LA activity. In the present study, we obtained favorable results for LA detection with cross-mixing tests by measuring APTT in mixtures of control plasma with 0%, 10%, 20%, 50%, and 100% concentrations of patient plasma treated with a 0.2 microm filter. In the future, laboratory examinations for APS will change with diagnostic criteria, as APS has not yet been established as a distinct disease concept.