Antiphospholipid syndrome is diagnosed based on recurrent venous thromboses (hypercoagulability), frequent miscarriages, thrombocytopenia, and a false-positive rapid plasma reagin test. It is associated with clinical criteria including a history of thrombosis (arterial or venous) or spontaneous abortion along with laboratory findings of lupus anticoagulant, anticardiolipin, anti-β2 glycoprotein I antibodies.
It is also associated with an increased PTT. PTT is prolonged because the autoantibodies bind to the phospholipid reagent used in the PTT test, inactivating it and falsely prolonging the PTT. This permanent binding and PTT prolongation are not corrected by mixing 1:1 with fresh-frozen plasma. Correction of an elevated PTT would be seen in clotting factor deficiencies. APS is more directly diagnosed by detecting specific serum autoantibodies (eg, anticardiolipin or anti–beta-2-glycoprotein).
The false-positive rapid plasma reagin test result occurs because the reagent used in the test contains cardiolipin, which reacts with the patient’s autoantibodies.
The deep vein thrombosis and findings of SLE suggest a concurrent acquired antiphospholipid syndrome (APS), which causes hypercoagulability. This may lead to arterial and venous thrombi, thrombocytopenia, livedo reticularis, stroke, and pulmonary embolism. The syndrome is also associated with recurrent miscarriage and false-positive syphilis tests. APS can be primary or secondary to lupus or other autoimmune disorders. The low platelets can be due to the APS or due to SLE.