Elevation of serum thrombopoietin precedes thrombocytosis in acute infections

To clarify the mechanisms underlying thrombocytosis secondary to infections, we longitudinally studied serum levels of thrombopoietin (TPO) and interleukin (IL)-6 in 15 infants and young children with prominent thrombocytosis (platelets >700 × 10⁹/l) following acute infections and 116 age-matched controls using an enzyme-linked immunosorbent assay. The subjects included nine patients with bacterial infections, three with viral infections and three with non-determined pathogens. TPO values in the controls were 2.24 ± 0.87 fmol/ml (mean ± SD) with a 95% reference interval of 0.85-4.47 fmol/ml. In the first week of infection, platelet counts were normal, but TPO values increased (∼10.73 fmol/ml). TPO levels peaked on day 4 ± 2 at 6.44 ± 2.37 fmol/ml and then fell gradually. When platelet counts peaked in the second and third weeks, TPO levels were similar to the controls. IL-6 levels in the first week rose and dropped more rapidly than TPO. Serum TPO values were significantly correlated with C-reactive protein levels (r = 0.688, P < 0.001) and IL-6 levels (r = 0.481, P = 0.027). These results suggest that TPO contributes to thrombocytosis following infections in conjunction with IL-6, arguing for additional regulatory mechanisms of blood TPO levels.