Anti-phospholipid syndrome: Current opinion on mechanisms involved, laboratory characterization and diagnostic aspects.

Anti-phospholipid syndrome is a complex and severe clinical situation, associated with symptoms such as recurrent thrombosis, arterial or venous, at any site, pregnancy loss, and other related syndromes. These clinical burdens, are highly variable from patient to patient, and are associated with biological abnormalities, such as the presence of the Lupus Anticoagulant or phospholipid dependent antibodies, confirmed on two occasions at least 12 weeks apart. From the diagnosis standpoint, both, functional (clotting) or immunological assays, are difficult to standardize and to optimize, due to the absence of reference material, or a characteristic clinical group, and international reference preparations. Large cohort studies are necessary for defining the usefulness of each assay, in terms of specificity, sensitivity, accuracy and for following-up the disease evolution. Clotting assays are based on Activated Partial Thromboplastin Time (APTT) and diluted Russell Viper Venom Time (dRVVT), performed at low and high phospholipid concentration, or on 1:1 mixtures of tested sample and a normal plasma pool. They allow evaluation of the paradoxal effects of LAs, which are pro-thrombotic in vivo, and anticoagulant in vivo. Use of synthetic phospholipids improves assay specificities and sensitivities, especially in patients treated with anticoagulants. Immunoassays can also be used for testing phospholipid dependent antibodies, first identified and measured as anti-cardiolipin antibodies, but now characterized as targeted to phospholipid cofactor proteins: mainly β2GP1 (which exposes cryptic epitopes upon binding to phospholipids), and in some cases prothrombin, and more rarely Protein S, Factor XIII, Protein Z or Annexin V. Use of optimized assays designed with well-characterized anionic phospholipids, then complexed with highly purified phospholipid cofactor protein (mainly β2GP1), offers a better link between reactivity and clinical associations, than the former assays which were empirically designed with cardiolipin. Standardization also remains complicated due to the absence of international standards and harmonized quantitation units. Validation on large cohorts of negative and positive patients remains the key approach for defining assay performance and clinical usefulness. Laboratory practice for all these methods is now greatly facilitated thanks to the use of automated instruments and dedicated software. Along with clinical criteria, laboratory assays are of great usefulness for identification and confirmation of the anti-phospholipid syndrome and they allow disease follow-up when appropriate patient management is in place.