Ureaplasma urealyticum and chronic lung disease of prematurity: critical appraisal of the literature on causation.

A critical appraisal of four cohort studies examining the relationship between Ureaplasma urealyticum and chronic lung disease (CLD) of prematurity is presented. Three studies were concurrently conducted, but the fourth was conducted 4 years later when surfactant replacement was a widespread practice. Although infants were enrolled in all studies soon after birth before they had developed CLD, there were differences in patients population, the definition of colonization with U. urealyticum, neonatal management, and the definition of CLD of prematurity. Despite the differences, all four studies found an association between colonization and development of CLD of prematurity. A combined estimate of relative risk for the four studies was 1.91 (95% confidence interval, 1.54-2.37). When infants were categorized into groups by birth weight, the association was not observed in infants who weighed > 1,250 g. The association was also not observed in infants who weighed < 750 g, but the risk of CLD of prematurity in the uncolonized control group was already 82%. Because the cohort study design allows for the possibility that one or more additional factors associated with U. urealyticum may be the true cause(s) of CLD of prematurity, there is strong but not definitive evidence that U. urealyticum causes CLD of prematurity.