Poor agreement between methods used in population studies to estimate the volume of human milk consumed: a Bland-Altman analysis

Measuring the volume of human milk consumed by infants is essential for understanding infant feeding patterns, and their impact on long-term nutrition and health. The study aimed to evaluate the agreement and quantify biases among three methods used to quantify the volume of human milk among children on partial breastfeeding up to 33 months of age. A cross-sectional, observational study was conducted between November and December 2022, with socially vulnerable children in the Federal District, Brazil. Dietary intake was assessed using two non-consecutive 24-hour recalls. Human milk volume was estimated using three methods: age/dose-to-mother (DTM), age/partial feeding, and age/feeding. Pearson’s correlation coefficient was used to assess associations between methods. Bland-Altman analysis was applied to evaluate agreement between estimates by each method. Statistical significance was set at 5%. Eighty children were evaluated (mean age of 19 months; SD = 6.12), including 15 aged 7–12 months, and 65 aged 13–33 months. Among children aged 7–12 months, moderate and statistically significant correlations were observed between age/DTM × age/partial feeding (r = 0.544, p = 0.02) and between age/partial feeding × age/feeding (r = 0.649, p = < 0.001). The correlation between age/DTM and age/feeding was weak and not statistically significant (r = 0.309, p = 0.097). Most individual estimates fell within the limits of agreement. However, the age/DTM method yielded higher estimated of human milk volume - mean of 321 g/day (95% CI 273, 368) and 333 g/day (95% CI 273, 393) more than the age/feeding and age/partial feeding, respectively. The agreement between methods decreased as estimated milk volume increased. Although moderate positive correlations were observed among the methods, the age/partial feeding and age/feeding approaches demonstrated the highest level of agreement. No consistent agreement was found between the age/DTM method and the other two approaches.