Platelet
glycocalicin (GC) is the extramembranous portion of GPIb alpha that can be rapidly cleaved by
enzymes such as
calpain,
plasmin,
trypsin,
elastase, etc. Quantitative cleavage will ultimately result in an acquired Bernard-Soulier-like
bleeding disorder, and circulating GC may act as a potential inhibitor of platelet adhesion. We have developed and standardized a new
enzyme-linked
immunosorbent assay (ELISA), which uses two
monoclonal antibodies (mAbs), both of which bind to the amino-terminal 45-kD fragment of GC and inhibit platelet-von Willebrand interactions and the
streptavidin-
biotin system. First, the methodology was evaluated and standardized with special emphasis on the
anticoagulant and the inhibitors (
EDTA,
prostaglandin E1 [
PGE1],
aprotinin, N-ethyl-
maleimide), the mode of high-speed centrifugation (to avoid platelet microparticles), and the standards used (purified GPIb and GC). This assay was then used to analyze the GC levels of healthy subjects (2.04 +/- 0.46 micrograms/mL) and of patients with selected diseases. The results of patients with
aplastic anemia and
thrombocytosis confirmed that GC levels are clearly dependent on the platelet count, which was the basis for the introduction of the GC index, the standardization of GC for a platelet count of 250 x 10(9)/L. The GC index discriminates reliably patients with active
immune thrombocytopenic purpura from those in remission. GC levels are elevated in patients on
hemodialysis (3.62 +/- 0.75 micrograms/mL, P < .001). The high GC index (6.93 +/- 4.21, P < .001) in
cirrhosis patients suggests an increased platelet turnover and/or abnormal proteolysis. In contrast to other groups, we have not found that recombinant
tissue plasminogen activator (rtPA) treatment of patients with
myocardial infarction increases GC levels. However, concentrations are elevated in
leukemia and the highest levels found are approximately 40 micrograms/mL. These studies suggest that GC is a useful platelet marker in certain diseases, which directly reflects platelet damage and possibly platelet dysfunction.