Comparison of Ion Selective Electrodes for the Measurement of Sodium and Potassium in Human Milk

Abstract

Background:

Accessible and cost-effective measures of human milk biomarkers are needed for lactation screening and management. Sodium (Na), potassium (K), and the sodium-potassium (Na/K) ratio in human milk show promise as indicators of secretory activation and mammary gland health.

Research Aim:

This validation study compares the accuracy and precision of handheld and benchtop ion selective electrodes (ISE) as compared to inductively coupled plasma mass spectrometry (ICP-MS) for the measurement of Na and K in defatted and whole milk.

Methods:

Milk samples (N = 27) were assessed for Na and K in defatted and whole milk using ICP-MS, handheld, and benchtop ISE instruments. Bland-Altman analysis was performed to compare Na, K and Na/K measurements in whole versus defatted milk and between instruments.

Results:

All three assay methods provided high precision for Na and K concentrations in both whole and defatted milk. Na/K ratios were similar between whole and defatted milk for both ICP-MS and the handheld ISE. Measurements of Na and Na/K were systematically higher with both ISE instruments as compared to ICP-MS. However, the systematic difference was approximately three times greater in the benchtop ISE as compared to the handheld ISE. This resulted in identification of a larger number of cases of elevated mammary epithelium permeability (MEP) with the benchtop ISE as compared to handheld ISE or ICP-MS.

Conclusions:

Meaningful assessment of Na and Na/K in whole milk is possible using the handheld ISE. Its portability and ease of use suggest potential for assessing Na and K in human milk at the point-of-care for use in both research and clinical applications.