Birth weight and length of stay as determinants of nosocomial coagulase-negative staphylococcal bacteremia in neonatal intensive care unit populations: potential for confounding.

Coagulase-negative staphylococci are the most common cause of bacteremia in the neonatal intensive care units of the Brigham and Women's Hospital and the Children's Hospital, Boston, Massachusetts. In 1982, nosocomial bacteremia with coagulase-negative staphylococci occurred in 45 of 882 (5.1%) infants admitted to these units who survived and remained in intensive care for more than 48 hours and were therefore at risk. The overall cumulative incidence (attack rate) of nosocomial bacteremia rose dramatically with decreasing birth weight. The smallest infants, those with birth weights of less than 750 g, experienced an overall risk of nosocomial bacteremia 44.5 times that of infants with birth weights of greater than 2,000 g. A large part of the excess risk for small babies was attributable to their longer hospitalizations. However, after adjustment for length of stay, the smallest infants still suffered a daily rate of bacteremia (incidence density) 5.3 times that of the largest, indicating a considerable residual effect of birth weight on the daily risk of nosocomial coagulase-negative staphylococcal bacteremia. The results were similar in the two nurseries. Thus, there is still a substantially increased intrinsic risk of nosocomial coagulase-negative staphylococcal bacteremia among infants with very low birth weights, even after adjustment for duration of hospitalization, and differences in birth weight can confound comparative studies.