Postnatal foot length measurement as a proxy to identify low birth weight for frontline health workers in rural Sindh Province, Pakistan: a diagnostic accuracy study

Objective: To assess the diagnostic accuracy of postnatal foot length (FL) measurements as a proxy to identify low birth weight (LBW) for frontline healthcare workers in rural Sindh Province, Pakistan.

Design: A community-based cross-sectional study.

Setting: This study was conducted in the catchment area of Global Network's Maternal and Newborn Health Registry, Thatta, Sindh Province, Pakistan, from January to June 2023.

Participants: Singleton live births irrespective of gestational age at birth.

Reference standard: Birth weight was measured using calibrated digital weighing scales in grams based on the average of three readings with minimal clothing.

Index test: FL was measured within 48 hours of birth using a rigid transparent plastic ruler in centimetres based on the average of three measurements.

Primary outcome: Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), receiver operating characteristics curve and area under the curve with 95% CI were calculated. Euclidean distance was used to identify the cutoff of FL to identify LBW. A simple linear equation was created to predict the birth weight.

Results: Out of 336 analysed newborns, 179 (53.3%) were male and 157 (46.7%) were female. The median birth weight was 2801 g (IQR: 2465-3057), of whom 88 (26.2%) were LBW. The median foot length was 7.9 cm (IQR: 7.6-8.1). For identifying LBW, the foot length cutoff was ≤7.6 cm with 90.3% sensitivity, 81.8% specificity, 63.8% PPV and 96.0% NPV. A FL of 7.6 cm predicted birth weight of 2459.4 g.

Conclusion: Postnatal FL cutoff of ≤7.6 cm has adequate predictive value served as a simple, low-cost and reliable method to identify LBW for frontline healthcare providers in the rural settings of Thatta without calibrated weighing scales to triage LBW newborns in need of higher-level care.

Trial registration number: NCT05515211.