Mothers’ milk Na without Na:K ratio detects secretory activation in mothers of preterm infants

BACKGROUND: Secretory activation is a critical lactation milestone that can be measured objectively using point-of-care (POC) techniques, improving research rigor and informing interventions for insufficient mothers' own milk (MOM) volume in mothers of preterm infants. MOM sodium (Na) and sodium:potassium (Na:K) ratio are commonly used biomarkers, but whether Na:K adds value beyond Na alone remains unclear. Because POC K measurement requires additional time, cost, MOM volume and calculations, we evaluated whether Na alone can serve as a stand-alone secretory activation measure during the first 14 days postpartum. METHODS: We conducted a secondary pooled analysis of five prospective studies from two sites (n = 176; 2458 MOM samples). Paired Na and Na:K measures were compared using prespecified thresholds for secretory activation achievement (Na ≤ 16 mM; Na:K ≤ 0.8) across domains: daily agreement, first day of achievement, variation across key clinical factors, and continuous-value agreement. RESULTS: Na and Na:K showed strong agreement across all analytic domains during days 1-14 postpartum, with no evidence that Na:K added diagnostic value beyond Na alone. CONCLUSION: In breast pump-dependent mothers of preterm infants, Na alone is a robust biomarker of secretory activation and can be used without Na:K ratio to detect delayed and/or impaired secretory activation. IMPACT: This study provides evidence that point-of-care (POC) mothers' own milk (MOM) sodium (Na) concentration alone is a sufficient and reliable biomarker of secretory activation in breast pump-dependent mothers of preterm infants. While the sodium-to-potassium (Na:K) ratio has also been used, POC K involves increased time, cost, MOM volume and hand-calculation. Our analysis of 2458 paired measures of Na and K shows the Na:K ratio offers no additional diagnostic value beyond Na alone for detection of secretory activation in this population, informing biomarker-based lactation measures in research and neonatal intensive care unit settings.