Objective To examine whether the number of very low birthweight (VLBW) infants treated annually in neonatal intensive care units (NICUs) (hospital volume) has an effect on their in-hospital mortality under the regulatory conditions in Germany.

Setting The study included VLBW infants with <33 weeks of gestational age and birth weight below 1500 g admitted to NICUs in the state of Baden-Wüerttemberg, Germany, from 2003 to 2008. Cases were extracted from the compulsory German neonatal quality assurance programme with variables essential for calculation of the Clinical Risk Index for Babies (CRIB) and PREM birth model (PREM(bm)) scores. The cohort was divided into four subgroups corresponding to their disease severity (low, intermediate, high and very high) according to each score. Low-volume NICUs (LV-NICUs) were defined as treating up to 50 cases per year, while high-volume NICUs >50 cases.

Results After exclusion of infants with lethal malformations, 5340 cases from 32 units were analysed. While raw mortality was comparable, infants in LV-NICUs had an increased mortality after risk adjustment with the CRIB and PREM(bm) scores (OR 1.48 (95% CI 1.16 to 1.90), p=0.002 with CRIB; and OR 1.39 (95% CI 1.11 to 1.76), p=0.005 with PREM(bm)). In a subgroup analysis mortality was significantly higher for LV-NICUs in the intermediate disease severity group (OR 1.49 (95% CI 1.02 to 2.17), p=0.037 with CRIB) and in the high-risk group (OR 1.70 (95% CI 1.16 to 1.90), p=0.002 with CRIB; and OR 1.39 (95% CI 1.11 to 1.76), p=0.005 with PREM(bm)), but not in the low-risk and very high-risk subgroups.

Conclusion Depending on the severity of the disease, the risk-adjusted mortality in German NICUs with 50 or less annual cases of VLBW infants may be significantly increased.