Human tears: osmotic characteristics.

Using a freezing point depression method, requiring volumes of only 200 nanoliters, the osmolalities of 324 tear samples collected at consecutive 10-minute intervals throughout 8.5-hour periods from each of six healthy young adults were measured. The population mean was found to be 318 mOsm/kg, with a population median of 315 mOsm/kg. Individual subject means ranged from 310 to 334 mOsm/kg. Short-term oscillations were observed for all subjects, with a net positive rate toward hypertonicity for the population of 1.43 mOsm/kg X hr-1 as the day progressed. Two subjects did demonstrate mild net rates of decrease, however. All eyes in the study were asymptomatic, suggesting a broader range of tear osmotic pressure among normals than previously suspected.