Dilute activated partial thromboplastin time (dAPTT)

Lupus anticoagulants (LA) are classified as antiphospholipid antibodies (APA), although they are in fact directed against phospholipid-binding proteins, in particular, β2 glycoprotein I and prothrombin. The presence of persistent LA has a greater association with thrombosis, pregnancy morbidity and recurrence than the criteria antibodies detected in solid-phase assays (aCL & aβ2GPI). LA are a heterogeneous group of autoantibodies that are detected by inference based on their behaviour in phospholipid-dependent coagulation assays after other possible causes of elevated clotting times have been excluded. LA detection involves use of screening, confirmatory and mixing tests.

Screening tests commonly employ dilute phospholipid to accentuate the in vitro anticoagulant effect of LA, which if present, will prolong the clotting time. Screening tests can be prolonged for reasons other than LA, (i.e. factor deficiencies, anticoagulant therapy), so all elevated screening tests receive follow-up analyses to help define the nature of any abnormality. The confirm test generally involves performing the screening test in an identical fashion except that the phospholipid concentration is markedly increased. This has the effect of partially or completely overwhelming the LA and thus leads to a shorter clotting time than the screening test, thereby evidencing phospholipid dependence. Clotting times are converted to ratios to mitigate for issues of analytical variability. Correction of the screen ratio by the confirm ratio by ≥10% is considered consistent with the presence of a LA, providing that other causes of elevated clotting times are excluded. Diagnostic specificity is improved by performing the screen and confirmatory tests on 1:1 mixtures of test and normal plasma to evidence inhibition and reduce interferences, although the inevitable dilution effect can compromise this aspect of analysis.

Antibody heterogeneity and reagent variability necessitate use of at least two assays, of different analytical principle, to achieve acceptable detection rates. First-line assays are dilute Russell's viper venom time (dRVVT) and LA-responsive APTT, a pairing that will detect most clinically significant antibodies. Our laboratories employ a dilute APTT (dAPTT) in this respect, which employs a silica activator and a low concentration of phospholipid comprised of a composition of phospholipid types that is LA-responsive. The confirm test involves addition of concentrated platelet-derived phospholipid. All elevated DAPTT screen ratios are reflexed to receive the confirm test, and the screen and confirm mixing tests, and are reported with interpretive comment. Patients with LA may be postitive in one or both of the dAPTT & dRVVT test medleys.