Tests of Platelet Function Hematology Review 344
TEST METHOD CLINICAL SIGNIFICANCE
Platelet aggregation
Platelet function assay (PFA)
VWF:Ag
Aggregating agent (e.g., ADP, collagen, risto-
cetin, epinephrine) added to plt suspension.
As plts aggregate, ↑in light transmittance. Plt
aggregation curves generated (time vs. %
transmittance).
Citrated whole blood drawn through capillary
tubes coated with ADP/collagen or epineph-
rine/collagen. Plts adhere & aggregate when
exposed to collagen. Closure time = length of
time for plts to form platelet plug & close
aperture of capillary tube.
Immunologic tests (e.g., EIA) using mono-
clonal antibodies to VWF.
Abnormal curves with plt dysfunctions such as
von Willebrand disease, Bernard-Soulier syn-
drome, plt storage pool defects, idiopathic
thrombocytopenia purpura, drugs.
Screening test for qualitative plt defects. Re-
places bleeding time. Von Willebrand disease:
prolonged with collagen/ADP & collagen/
epinephrine. Defects related to drugs (e.g.,
aspirin): normal with collagen/ADP, prolonged
with collagen/epinephrine.
VWF connects plts to collagen. ↓in von
Willebrand disease, so plts don’t function
normally.
