Human milk sodium and potassium as markers of mastitis in mothers of preterm infants

Introduction: This prospective longitudinal study examined changes in milk sodium concentration (Na) and sodium:potassium ratio (Na:K), microbiological culture, milk production and breast health in relation to mastitis after preterm birth.
Method: We studied women who gave birth at 29-34 weeks gestation in a tertiary obstetric hospital in Perth, Western Australia. Milk samples, 24 hour milk production and breast health data were collected every second day to day 10 postpartum, then every third day until infant discharge from the neonatal unit. Milk Na and K were measured at point of care (POC) using handheld ion selective meters, and Na:K calculated. Cultures were performed on postnatal days 8, 13 and every 6 days thereafter. For episodes of mastitis milk was cultured at onset, and Na and Na:K measured daily until resolution. Women were followed up at 4 and 8 weeks postpartum.
Results: In a sample of 44 women, 4 mastitis cases were detected in 3 women during their infants’ neonatal stay; all had elevated milk Na and Na:K that resolved within 48 hours; 2/4 experienced reduced milk production and 1/4e had heavy growth of Staphylococcus epidermidis. A further 2 mastitis cases were reported in 39 women followed up to 8 weeks postpartum. Four women had elevated milk Na and Na:K without clinical signs of mastitis; 3 also had reduced milk production.
Conclusion: POC testing of milk Na and/or Na:K may offer a useful indicator of breast health. Mastitis may cause an acute reduction in milk production regardless of the presence of culture positive infection.