Laboratory tests - lupus anticoagulant

What it is

Lupus anticoagulant - a functional (clotting-based) assay detecting antiphospholipid antibodies that paradoxically prolong in vitro clotting tests despite being associated with an in vivo prothrombotic state - a counterintuitive but well-established laboratory phenomenon
One of the three antiphospholipid antibodies in APS classification criteria, and generally considered to carry the strongest association with thrombotic risk of the three antibody types

Detected via a multi-step functional coagulation assay (not an antibody-specific immunoassay like anticardiolipin/anti-beta-2 glycoprotein I) - the antibody interferes with phospholipid-dependent clotting reactions in vitro (prolonging the clotting time), while in vivo, on cell surfaces with abundant phospholipid, the same antibody promotes a prothrombotic effect - the apparent paradox between the in vitro and in vivo effect is a classically tested concept

Testing requires a specific multi-step protocol (screening test, mixing study to exclude a simple factor deficiency, confirmatory phospholipid-dependent correction) - cannot be reliably tested while the patient is therapeutically anticoagulated (anticoagulants themselves interfere with the clotting-based assay), a key practical limitation requiring test timing to be planned around anticoagulation status
As with the other antiphospholipid antibodies, requires persistence on repeat testing (12 weeks apart) for diagnostic significance

Confirmed lupus anticoagulant positivity in the context of a qualifying clinical event supports the APS diagnosis and the same anticoagulation approach as for the other antiphospholipid antibodies
If testing is needed to guide a treatment decision in an already-anticoagulated patient, this practical limitation should be explicitly discussed with the laboratory, since results may be unreliable/uninterpretable while anticoagulated

Antiphospholipid syndrome, unexplained prolonged aPTT (lupus anticoagulant is a recognised cause of a prolonged aPTT that does NOT correlate with bleeding risk, unlike most other causes of prolonged aPTT), SLE

An isolated prolonged aPTT with no bleeding history should prompt consideration of lupus anticoagulant as the cause - this is one of the few causes of a prolonged clotting test that reflects increased thrombotic, not bleeding, risk, the opposite of the usual clinical association with a prolonged clotting time

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