Measurements of plasma glycocalicin (GC) and reticulated platelets (RP) have been reported to be useful for classifying thrombocytopenic disorders. However, there have been no reports comparing the clinical usefulness of the two methods. We measured GC and RP levels simultaneously in 39 patients with idiopathic thrombocytopenic purpura (ITP), 15 patients with aplastic anemia (AA), and 17 patients with hypoplastic thrombocytopenia (HypoT) due to chemotherapy. The GC index (GC level normalized for the individual platelet count) and the percentage of RP (%RP), a parameter of platelet life span, were very high (7.5 +/- 11.4 and 20.8 +/- 13.0%, respectively) in patients with ITP as compared with those of healthy subjects (1.3 +/- 0.5 and 7.9 +/- 2.5%, respectively). However, 6 AA patients and 14 HypoT patients, in whom platelet life span is thought to be normal, also had an elevated GC index, suggestive of a false positive result. The RP, a parameter of platelet production, was low in all AA and HypoT patients except for one in each case. However, the GC level, an additional parameter of platelet production, was normal in 4 AA and 8 HypoT patients, indicating that it is not a sensitive indicator. We conclude that the RP and %RP are more feasible markers of thrombopoiesis and platelet life span, respectively, than the GC level and GC index.