Monocytosis in preterm infants.

In sick preterm neonates receiving intensive care a spectacular rise in monocyte count has frequently been observed in sequential full blood examinations. The etiology of this has not previously been investigated and this study examines clinical factors that may contribute to this finding. Thirty (5.1%) of the 587 neonates who required intensive care during the study period had significant monocytosis (absolute count greater than 1700/mm3). Their mean gestation was 29 weeks (range 26-32 weeks). Monocytic response occurred at an age of 5.5 +/- 3 (mean +/- S.D.) days with 20% occurring at birth, 57% in the first week and 23% in the second week of life and lasted for 19.8 +/- 16 days (mean +/- S.D.). Most reached peak levels within two weeks and these ranged between 2,170 and 7176 per mm3. Analysis of the clinical variables against 37 controls revealed lower mean birth weight and gestational age, and higher incidence of leukocytosis, multiple transfusions, albumin infusions and theophylline therapy in the study group in comparison to the controls (P less than 0.001). No significant difference was found in maternal risk factors (pre-eclampsia, diabetes and amnionitis), birth asphyxia, respiratory disease, parenteral nutrition, proven infection and antibiotic therapy. An unexpected association with maternal steroid therapy was demonstrated. It is speculated that monocytosis represents a physiological though immature response of the marrow of small premature infants to a variety of exogenous stimuli including drugs and foreign protein infusions.