Human basic tear fluid osmolality. I. Importance of sample collection strategy.

Osmolalities of 200 human tear prism fluid samples collected from two subjects were determined from their melting-point temperatures with the Clifton Nanoliter Osmometer by calibration with 200 standard solution samples (290 mOsm/kg). Comparisons were made between tear fluid osmolalities obtained using a single-sample simultaneous-recalibration method: 1) for tear samples collected using finely-drawn microcapillaries without biomicroscopy, vs secondly with biomicroscopic observation by illumination of only the sampling area on the inferior tear prism, the two collections separated by an interval of 10 min; and 2) for samples collected without biomicroscopy before, vs after a 10-min interval. Tear fluid collection using a biomicroscope resulted in values that were significantly lower than those collected without (overall mean = 299.5 and 306.6 mOsm/kg, respectively; p < 0.0001). The difference (7.1 mOsm/kg) may have resulted from mechanical, photic, and/or psychogenic reflex stimulation due to biomicroscopy, as the 10-min interval had no osmotic effect on samples collected without biomicroscopy. Based on these results from two subjects, we suggest that relatively rapid, repetitive collections of human tear prism fluid can be made without significantly disturbing the osmotic outcome. However, physiological representation of basic human tear fluid is more accurate by avoidance of reflex-inducing collection methods that were formerly considered unobtrusive. Basic human tear prism fluid is more hypertonic, by at least 7 mOsm/kg, than generally realized.