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Measurement Of Sweat Sodium Concentration In Exercising Individuals: Inter-instrument Reliability Of The B-722 Laqua Twin

Optimal replacement of sodium losses during or after prolonged exercise is best achieved when sweat losses and sweat sodium concentration are known. Both can be estimated under field conditions during a controlled test where the changes in body mass from before to after exercise is taken as a representation of sweat losses and sweat sodium samples are collected using the absorbent patches technique. The B-722 Laqua Twin (LaT), a low cost, battery operated, hand-held and easy-to-use sodium analyzer has been shown to have excellent intraand inter-day reliability and to offer a reasonable degree of validity for the measurement of sweat sodium concentration under field conditions. The inter-instrument reliability of the LaT has never been determined. PURPOSE: To assess the inter-instrument reliability of the LaT sodium analyzer. METHODS: Seventy sweat samples collected in 14 athletes were analyzed in duplicate with 3 different LaTs. Sweat samples were collected during 40-min cycling or running exercises (~29-30oC and 20-40% relative humidity) from five anatomical sites using 5 x 7 cm absorbent pads covered with highly-adhesive impermeable transparent dressings. Sweat samples were extracted from the pads using centrifugation and then frozen at -20 oC until thawed and analyzed. RESULTS: All data were normally distributed and showed no sign of heteroscedasticity. Relative reliability was excellent between LaT 1 and 2, 1 and 3, and 2 and 3 with Pearson correlation coefficients of respectively 0.991, 0.995 and 0.996 and intraclass correlation coefficients of 0.986, 0.993, and 0.996. Mean biases between instruments were low (LaT 1 and 2: -2.6 mmol/L; 1 and 3: -1.8 mmol/L; and 2 and 3: 0.8 mmol/L) but statistically significant. The typical error of measurements as well as the coefficient of variations were also low between instruments, ranging between 1.8 and 2.6 mmol/L and 2 and 4%, respectively. For comparisons between LaT 1 and 2, 1 and 3, and 2 and 3 the 95% limits of agreement reached ± 7.3, ± 5.3, and ± 4.8 mmol/L, respectively. CONCLUSION: The inter-instrument reliability of the LaT is adequate enough such that sports clinicians can confidently interpret exercise sweat sodium concentration values obtained from, and develop sodium replacement strategies for athletes based on, sweat sodium testing realized by different LaTs.