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