Tear Fluid Osmolarity as a Potential Marker of Hydration Status

FORTES, M. B., B. C. DIMENT, U. DI FELICE, A. E. GUNN, J. L. KENDALL, M. ESMAEELPOUR, and N. P. WALSH. Tear Fluid Osmolarity as a Potential Marker of Hydration Status. Med. Sci. Sports Exerc., Vol. 43, No. 8, pp. 1590-1597, 2011. It has been suggested that tear fluid is isotonic with plasma, and plasma osmolality (P-osm) is an accepted, albeit invasive, hydration marker. Our aim was to determine whether tear fluid osmolarity (T-osm) assessed using a new, portable, noninvasive, rapid collection and measurement device tracks hydration. Purpose: This study aimed to compare changes in T-osm and another widely used noninvasive marker, urine specific gravity (USG), with changes in P-osm during hypertonic-hypovolemia. Methods: In a randomized order, 14 healthy volunteers exercised in the heat on one occasion with fluid restriction (FR) until 1%, 2%, and 3% body mass loss (BML) and with overnight fluid restriction until 08:00 h the following day, and on another occasion with fluid intake (FI). Volunteers were rehydrated between 08: 00 and 11:00 h. T-osm was assessed using the TearLab (TM) osmolarity system. Results: P-osm and USG increased with progressive dehydration on FR (P < 0.001). T-osm increased significantly on FR from 293 +/- 9 to 305 +/- 13 mOsm.L-1 at 3% BML and remained elevated overnight (304 +/- 14 mOsm.L-1; P < 0.001). P-osm and T-osm decreased during exercise on FI and returned to preexercise values the following morning. Rehydration restored P-osm, USG, and T-osm to within preexercise values. The mean correlation between T-osm and P-osm was r = 0.93 and that between USG and P-osm was r = 0.72. Conclusions: T-osm increased with dehydration and tracked alterations in P-osm with comparable utility to USG. Measuring T-osm using the TearLab (TM) osmolarity system may offer sports medicine practitioners, clinicians, and research investigators a practical and rapid hydration assessment technique.