Early elevation of serum thrombopoietin levels and subsequent thrombocytosis in healthy preterm infants

To verify pathophysiological mechanisms underlying thrombocytosis in low-birth-weight (LBW) preterm babies, we evaluated kinetic changes in platelet counts and thrombopoietic cytokines including thrombopoietin (TPO), interleukin 6 (IL-6) and IL-11 in 24 uncomplicated preterm infants. Platelet counts in cord blood (CB) (265 ± 64 × 10⁹/l) were similar to adult levels, increased by d 14 (473 ± 140 × 10⁹/l), and then remained fairly constant. Thrombocytosis (> 500 × 10⁹/l) was observed in 9/24 (38%) subjects. Mean TPO level in CB was 5.11 ± 1.51 fmol/ml, peaked at d 2 (7.64 ± 3.28 fmol/ml), decreased at d 5 (3.93 ± 1.67 fmol/ml), and thereafter kept fairly constant during the remaining neonatal period. Compared with term infants, mean TPO levels of preterm infants in CB and at d 2 were significantly higher (P < 0.01). There was an inverse correlation between platelet counts and TPO levels (r = 0.45, P < 0.001, n = 88). Preterm neonates with thrombocytosis had significantly higher TPO values in CB than those without thrombocytosis (P < 0.05). There was no significant relationship between platelet counts and IL-6. IL-11 was not detectable. These results suggest that an early elevation of serum TPO levels is related to the subsequent thrombocytosis in LBW preterm infants.