[Tear Film Osmolarity in Normal Individuals: Comparison of Two Osmometers]

The hyperosmolarity of the tear film is an important clinical sign of dry eye disease. It is the most important diagnostic parameter. In addition to the TearLab osmometer (TearLab Corp, San Diego, CA), which has been available for several years, the I-Pen osmometer (I-MED Pharma Inc, Dollard-des-Ormeaux, Quebec, Canada) has been recently marketed. The intention of our study was to compare tear film osmolarities in normal individuals as measured by TearLab and I-Pen.

51 healthy subjects (mean age 40.6 years, range 17 - 63 years, 66.7% females) without dry eye disease (ocular surface disease index < 13, tear film break-up time > 7 s, normal Schirmer test > 10 mm/5 min) were included in this study. Tear film osmolarity was measured in the tear meniscus (TearLab) and the palpebral conjunctiva (I-Pen) with a 30 min interval. The osmolarity testing techniques were alternated. Statistical analysis was performed using Wilcoxon tests and Spearman correlations.

Mean TearLab osmolarities were 295.1 mosmol/l (right eye), 294.0 mosmol/l (left eye), and 294.6 mosmol/l (mean of both eyes) with a range of 268 to 394 mosmol/l. I-Pen measured mean osmolarites of 301.6 mosmol/l (right eye), 302.5 mosmol/l (left eye), and 302.1 mosmol/l (mean of both eyes) with a range of 276 to 394 mosmol/l. There was a significant difference in tear film osmolarities for the two techniques for both single eyes (p < 0.02) and the mean value of both eyes (p < 0.001). TearLab and I-Pen measurements did not correlate significantly in any eye (r = 0.27 and r = 0.08 for right and left eye, respectively). With a cut-off value of 308 mosmol/l (as suggested for TearLab), 98.0% of TearLab values and 68.6% of I-Pen values were in the normal range.

I-Pen measured significantly higher tear film osmolarities than TearLab. The location of the measurement site in the palpebral conjunctiva could be responsible for this difference. Our results indicate that cut-off values should be changed when using I-Pen for evaluation of tear film osmolarity. Further examinations and studies with greater cohorts of healthy subjects and patients suffering from dry eye disease are necessary.