Impact of time between collection and collection method on human tear fluid osmolarity

To generate data on the variability of tear osmolarity in a control (normal, non-dry eye) and symptomatic dry eye population (Ocular Surface Disease Index: OSDI ≥20). A secondary outcome is the determination of the effect that tear collection technique has on the osmolarity of the sample.

This was a two-phase study that recruited 20 subjects (n = 10 normal, n = 10 dry eye) to evaluate the influence of time between measurements (Phase I) and 30 subjects (n = 15 normal, n = 15 dry eye) to evaluate the influence of collection technique (Phase II). As part of Phase I, serial tear osmolarity measurements were performed on each eye; four separated by 15 min followed by four separated by 1 min, at each of three visits. Phase II compared the consecutive measurement of four in vivo tear samples to four in vitro measurements on tears collected and dispensed from a glass capillary tube.

During Phase I, the dry eye group had a significantly higher maximum osmolarity (334.2 ± 25.6 mOsm/L) compared to the normal group (304.0 ± 8.4 mOsm/L, p = 0.002). No significant differences were observed whether collections were performed at 15 or 1 min intervals. During Phase II, the in vivo osmolarity was equivalent to in vitro measurements from glass capillary tube samples for both the dry eye group (323.0 ± 16.7 mOsm/L versus 317.7 ± 24.8, p = 0.496), and for the normal subjects (301.2 ± 7.2 mOsm/L versus 301.9 ± 16.0 mOsm/L, p = 0.884).

Symptomatic dry eye subjects exhibited a significantly higher tear osmolarity and variation over time than observed in normal subjects, reflecting the inherent tear film instability of dry eye disease. There was no change in the distribution of tear osmolarity measurements whether tears were collected in rapid succession or given time to equilibrate, and collection method had no impact on tear osmolarity.