Effect of age on tear osmolality

Certain aspects of tear physiology change with age (reflex secretion and stability), whereas others remain unchanged (tear volume, evaporation rate, basal tear production). To establish the variation in tear osmolality with age, 0.2 to 0.4 microliter samples were collected from 50 normal males and 50 normal females (age range 17 to 75 years). Nanoliter aliquots of the samples were analyzed by freezing point depression nanoliter osmometry. Mean tear osmolality for the whole group was 303.6 +/- 13.0 mOsm/kg and was not significantly affected by age (r2 = 0.014, p = 0.378). For males the mean was 307.1 +/- 14.4 and for females 300.1 +/- 10.4 mOsm/kg, a significant gender difference (p = 0.006). Males under age 41 had tear osmolality which was not significantly different from that of males over 41 years of age (307.7 +/- 15.6 and 305.7 +/- 11.5 mOsm/kg, respectively, p = 0.671) but the mean tear osmolality for females under age 41 years (297.6 +/- 11.2 mOsm/kg) was significantly different from that of older females (304.8 +/- 6.7 mOsm/kg, p = 0.017). There was no significant correlation between age and osmolality for males (r2 = 0.003, p = 0.699), but there was a significant correlation for females (r2 = 0.159, p = 0.004). Males under age 41 years have a significantly higher mean tear osmolality than females under 41 years of age (p = 0.003). The age and gender differences found for tear osmolality can be accounted for by the low values in young females, which may be related to the increased tear flow observed in young females. A reduction in this high flow rate to a more "normal" level in older females may account for the rise in tear osmolality to a level which is not significantly different from that of older males. Generally in normal eyes, the osmolality of tears remains within normal limits and is not affected by age.