Lupus Anticoagulant Testing: Dilute Prothrombin Time (dPT)

Lupus anticoagulants (LA) rarely affect routine prothrombin time assays because the high phospholipid (PL) content in thromboplastin reagents tends to overwhelm the antibodies. Dilution of thromboplastin to create a dilute prothrombin time (dPT) screening test renders the assay sensitive to the presence of LA. Technical and diagnostic performances are enhanced if recombinant thromboplastins are employed in place of tissue-derived reagents. Presence of an LA cannot be deduced from an elevated screening test alone since other coagulation disturbances can prolong clotting times. Confirmatory testing with less dilute or undiluted thromboplastin reveals the PL-dependent nature of LA by reducing the clotting time relative to that of the screening test. Where appropriate, such as a known or suspected coagulation factor deficiency, mixing tests are valuable in correcting factor deficiencies and evidencing inhibitory properties of LA, to increase diagnostic specificity. Although LA testing is commonly restricted to dilute Russell's viper venom time and activated partial thromboplastin time, dPT is sensitive to LA unreactive in those assays, and inclusion in routine testing increases detection rates of clinically significant antibodies.