Which laboratory criteria is required for a diagnosis of antiphospholipid syndrome (APS)?

A key component in diagnosing antiphospholipid syndrome (APS) is the presence of lupus anticoagulant, which must be detected on two separate occasions. This requirement is part of the classification criteria established by the Sydney criteria for APS. The lupus anticoagulant is an antibody that interferes with the normal clotting process, and its sustained presence indicates a persistent risk of thrombotic events, which is a hallmark of APS.

This laboratory finding assesses the patient's immune response related to phospholipids, which are essential components in blood coagulation. The requirement for detection on two occasions emphasizes the need for a consistent and reproducible finding, rather than a single positive test, to confirm a diagnosis. This helps to rule out transient or incidental findings that may not accurately reflect the patient's underlying condition.

The other choices, while relevant in the context of APS, do not represent the core laboratory criteria necessary for diagnosis. They are more supportive evidence rather than definitive criteria for the diagnosis itself. Thus, the presence of lupus anticoagulant on two occasions stands out as the critical laboratory requirement for diagnosing antiphospholipid syndrome.