An atypical circulating anticoagulant

Antiphospholipid antibodies: discovery, definitions, detection and disease

Antiphospholipid antibodies (aPL) are immunoglobulins of IgG, IgM and IgA isotypes that target phospholipid (PL) and/or PL-binding plasma proteins. Detection of aPL in the laboratory is done currently by both immunoassays and functional coagulation tests. Convention defines aPL specificity in immunoassays according to the particular PL substrate present, for example aPS represents antiphosphatidylserine antibodies. This may be technically incorrect inasmuch as a particular PL may be responsible for binding and highly concentrating a specific plasma protein, the latter then becomes the target for the aPL. The binding of beta(2)GP-I (apolipoprotein H) to the negatively charged PL, cardiolipin (CL) provides a good example of this circumstance. In contrast, aPL which specifically prolong coagulation times in in vitro are called lupus anticoagulants (LA). The precise PL target(s) of the aPL responsible for LA activities are unknown and often debated. The persistent finding of aPL in patients in association with abnormal blood clotting and a myriad of neurological, obstetrical and rheumatic disorders often compounded by autoimmune diseases has led to an established clinical diagnosis termed antiphospholipid syndrome (APS). The common denominator for these APS patients is the presence of circulating aPL on two or more occasions and the observation of events attributable to abnormal or accelerated blood clotting somewhere in vivo. The purpose of this review is to collect, collate, and consolidate information concerning aPL.

Should we include anti-prothrombin antibodies in the screening for the antiphospholipid syndrome?

Anti-prothrombin antibodies belong to the family of the antiphospholipid antibodies. Their prevalence ranges from 50 to 90% of antiphospholipid-positive patients, depending on the laboratory methodology employed for their detection. ELISA techniques are the most commonly used methods for their measurements, which allow a quick determination of their titer and isotype(s). Unfortunately, no well standardized assays are yet commercially available. The clinical relevance of anti-prothrombin antibodies as risk factors for thromboembolic events has been investigated by a number of retrospective and case-control studies. We reviewed 11 studies: data on 1,440 patients and 371 sex- and age-matched controls were available. Seven studies reported a significant association between anti-prothrombin antibodies and thrombosis by univariate analysis. Multivariate analysis was performed in five studies: only two of them confirmed the association. The sensitivity and specificity of anti-prothrombin antibodies for thrombosis were analysed according to the antibody isotype and to the arterial and/or venous site of thrombosis. The sensitivities are disappointingly low, particularly when the M isotype and arterial thrombosis are considered, whereas the specificities are somewhat better, even though they range widely. These data do not allow to recommend the measurement of anti-prothrombin antibodies in the routine laboratory workout of antiphospholipid-positive patients in order to define their thrombotic risk.

Prothrombin as cofactor for antiphospholipids

Prothrombin is a common antigenic target of antiphospholipid antibodies, since anti-prothrombin antibodies are detected in about 50-90% of the patients. To allow proper immune recognition, prothrombin must be adsorbed on suitable anionic surfaces. The epitope(s) have not yet been identified: the majority of anti-prothrombin antibodies appear to be of poly- or oligoclonal nature. Anti-prothrombin antibodies, either alone or in combination with anti-beta2-glycoprotein I antibodies, are responsible for the lupus anticoagulant activity of about 75% of the cases of phospholipid-dependent inhibitors of coagulation. The two antibodies may be discriminated by means of specific coagulation profiles generated by the comparison of the ratio of the Kaolin Clotting Time (KCT) and the dilute Russell's Viper Venom Time (dRVVT): the KCT profile, which mainly reflects the presence of anti-prothrombin antibodies and the dRVVT profile, which is mostly associated with anti-beta2-glycoprotein I antibodies. This distinction, although somewhat artificial, may be clinically useful, since the KCT profile identifies patients at low risk to develop thrombosis. Similarly, most of the studies that measured anti-prothrombin antibodies by ELISA failed to find a significant association with thrombosis. In conclusion, the clinical relevance of these antibodies has not yet been established.