Evidence against enhanced platelet activity in sickle cell anaemia

Summary. Although numerous studies have provided indirect evidence for enhanced platelet activity in sickle cell anaemia, little attention has been directed to examination of platelet alpha and dense granule release in the sickling disorders. We simultaneously measured by radioimmunoassay plasma levels of the alpha granule constituents β‐thromboglobulin (β‐TG) and platelet factor 4 (PF₄) in 43 children with sickle cell anaemia in steady state and 24 patients during severe vaso‐occlusive crisis. β‐TG levels during steady state (50 ± 3.6 ng/ml, mean ± SEM) were greater (P<0.001) than in normal controls (36 ± 1.6), but there was no additional significant rise during crisis (55 ± 5.9). PF₄ levels were similar (P= 0.12) in both steady state (10 ± 1.2 ng/ml) and crisis (9.3 ± 2.3) to those of normal controls (6.0 ± 0.8). The similarity of β‐TG/PF₄ ratios in normal and sickle cell anaemia patients as well as the positive correlation (P<0.05) between platelet count and β‐TG and PF₄ suggested that an artefactual in vitro platelet activation was responsible for some of the observed increased β‐TG and PF₄ levels. Further evidence against enhanced platelet activity in these sickle cell patients included normal intraplatelet content of the dense granule constituent 5‐HT and a normal ATP/ADP ratio. From this data we conclude that platelet activation in children with sickle cell anaemia appears minimal.