Depending on which causes are clinically suspected, additional tests may include measurement of homocysteine, testing for thrombotic disorders (antiphospholipid antibodies, protein S, protein C, antithrombin III, factor V Leiden), testing for rheumatic disorders (eg, antinuclear antibodies, rheumatoid factor, erythrocyte sedimentation rate), syphilis serologic testing, hemoglobin electrophoresis, and a urine drug screen for cocaine and amphetamines.