Human tears are a complex solution consisting mainly of water and electrolytes. Tear fluid has been reported to be isotonic with plasma. Accordingly, measurement of tear osmolarity has been used as a tool to evaluate hydration status in clinical studies. Its reliability however was relatively low due mainly to a) large sample variability, b) evaporation of tear fluids in the time elapsed between tear collection and analysis (3-5min).
Recently, a portable, noninvasive tear collection and analysis device has been produced. Its advantages are a) small tear volume required (50 nanoliter), b) very short time for tear collection (<5sec) and analysis (10 sec). The validity of the new device to track hydration level in athletes was tested in a study published in Medicine and Science in Sports and Exercise August 2011 by Fortes et al.
Figure. The new tear collection and analysis device (TearLab).
The aim of this study was to compare changes in Tear osmolarity (Tosm) and another widely used noninvasive marker, urine specific gravity (USG), with changes in Plasma osmolarity (Posm) during hypertonic-hypovolemia. 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).
Posm and USG increased with progressive dehydration on FR. Tosm 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. The mean correlation between Tosm and Posm was r = 0.93 and that between USG and Posm was r = 0.72.
The conclusions of this study were that
· Tear osmolarity increased with dehydration and tracked alterations in plasma osmolality
· It seems that this new tear collection and analysis device offers a practical and rapid hydration assessment technique.