Once- Versus Twice-Daily Measures of Mothers’ Own Milk Biomarkers: Does It Make a Difference in Research and Practice?

<b><i>Background:</i></b> Point-of-care (POC) measures of mothers' own milk (MOM) sodium (Na) concentration are inexpensive, objective measure of secretory activation (SA) achievement with the potential to personalize lactation care, especially among breast pump-dependent mothers with risks for delayed/impaired SA. It is unknown whether POC Na should be measured once or twice daily for research and practice. <b><i>Objectives:</i></b> To determine the need and feasibility of twice- versus once-daily MOM Na measures and examine whether there are differences in MOM Na concentration between morning (AM) versus evening (PM) samples. <b><i>Methods:</i></b> Secondary analysis of a dataset with 312 matched pairs of AM and PM MOM Na measures collected over the first 14 days postpartum in 38 breast pump-dependent mothers of preterm (<33 weeks) infants. Analyses included general linear mixed and regression models, paired rank tests, and descriptive statistics. <b><i>Result:</i></b> Fewer than 50% of subjects had paired samples prior to postpartum day 5 due to MOM being prioritized for infant feedings. Differences between twice-daily measures were significant over postpartum days 1-5, with Na concentrations higher in AM versus PM samples (<i>p</i> < 0.0001), a trend that continued (nonsignificant) during postpartum days 6-14. Over postpartum days 1-14, our modeling revealed 80% of the variance in AM measures was predicted by PM measures (<i>p</i> < 0.0001). <b><i>Conclusions:</i></b> Feasibility of twice-daily sampling is limited prior to the first 5 postpartum days. Analyses suggest once-daily Na concentrations can be used in research and practice, especially after postpartum day 5. Research priorities include determining the effects of interpump interval and interbreast differences on MOM Na concentrations.