BACKGROUND: An estimated 7 million episodes of severe newborn infections occur annually worldwide, with half a million newborn deaths, most occurring in low- and middle-income countries. Whilst injectable antibiotics are necessary to treat the infection, supportive care is also crucial in ending preventable mortality and morbidity. This study uses multi-country data to assess gaps in coverage, quality, and documentation of supportive care, considering implications for measurement. METHODS: The EN-BIRTH study was conducted in five hospitals in Bangladesh, Nepal, and Tanzania (July 2017-July 2018). Newborns with an admission diagnosis of clinically-defined infection (sepsis, meningitis, and/or pneumonia) were included. Researchers extracted data from inpatient case notes and interviews with women (usually the mothers) as the primary family caretakers after discharge. The interviews were conducted using a structured survey questionnaire. We used descriptive statistics to report coverage of newborn supportive care components such as oxygen use, phototherapy, and appropriate feeding, and we assessed the validity of measurement through survey-reports using a random-effects model to generate pooled estimates. In this study, key supportive care components were assessment and correction of hypoxaemia, hyperbilirubinemia, and hypoglycaemia. RESULTS: Among 1015 neonates who met the inclusion criteria, 89% had an admission clinical diagnosis of sepsis. Major gaps in documentation and care practices related to supportive care varied substantially across the participating hospitals. The pooled sensitivity was low for the survey-reported oxygen use (47%; 95% confidence interval (CI) = 30%-64%) and moderate for phototherapy (60%; 95% CI = 44%-75%). The pooled specificity was high for both the survey-reported oxygen use (85%; 95% CI = 80%-89%) and phototherapy (91%; 95% CI = 82%-97%). CONCLUSIONS: The women's reports during the exit survey consistently underestimated the coverage of supportive care components for managing infection. We have observed high variability in the inpatient documents across facilities. A standardised ward register for inpatient small and sick newborn care may capture selected supportive care data. However, tracking the detailed care will require standardised individual-level data sets linked to newborn case notes. We recommend investments in assessing the implementation aspects of a standardised inpatient register in resource-poor settings.