Tests to Assess Risk of Thrombosis Hematology Review 360
TEST SIGNIFICANCE ASSAYS
Antithrombin (AT, formerly
antithrombin III)
Protein C
Protein S
Factor V Leiden
Lupus anticoagulants
Plasma inhibitor that neutralizes all serine proteases,
including thrombin. Deficiencies associated with
↑risk of thrombosis.
Coagulation inhibitor. Inactivates Va & VIIIa. Deficien-
cies associated with ↑risk of thrombosis.
Cofactor for protein C.
Most common cause of hereditary activated protein C
resistance (APC). Mutation that makes V resistant to
activity of activated protein C. ↑risk of thrombosis.
Risk factor for thrombosis & recurrent spontaneous
abortion. Acquired antiphospholipid antibodies that
interact with phospholipid in APTT reagent & prolong
time. In vitro phenomenon. Patient doesn’t have factor
deficiency or bleeding. Present in patients with lupus,
other autoimmune diseases, neoplasms, infections,
drugs. Also present in some normal individuals.
Chromogenic substrate assay, immunologic
assay, nephelometry.
Immunologic assay, chromogenic substrate
assay, clot based assay.
Clotting assay, immunologic assay.
APC resistance assay is most frequent screening
test. Patient plasma diluted in V-deficient
plasma. Activated protein C added. APTT or dilute
Russell viper venom time (dRVVT) performed.
Abnormals must be confirmed by molecular test-
ing (e.g., PCR, restriction fragment length
polymorphism).
Detected by unexplained prolongation of APTT
that isn’t corrected by addition of equal volume
of normal plasma. No definitive assay.
