The laboratory criteria of the antiphospholipid syndrome include one coagulation assay (lupus anticoagulant [LA]) and two solid phase assays (anticardiolipin [aCL] and anti-ß(2)glycoprotein I antibodies [aß(2)GPI]). External quality control (EQC) surveys show that negative and clearly positive LA samples are classified correctly by about 95% of laboratories. For 'weak' LA there is a wide variability in samples' classification. Furthermore, when a weak LA sample is used in two different EQC surveys more than 50% of laboratories classify it differently. In some surveys weak LA samples were found to be positive for aCL and for aß(2)GPI by a majority of laboratories; the main reason for laboratories which classified these samples as LA negative was a negative result in the mixing test. It is likely that, depending on the sensitivity of the assay, a weak LA cannot be detected anymore after 1:1 dilution of the sample with normal plasma. Therefore, we recommend the use of integrated assays, such as screen/confirm ratios, for the detection of weak LA samples.