Randomized, masked, in vitro comparison of three commercially available tear film osmometers

The purpose of this study was to compare the precision and accuracy of commercially available tear film osmometers.

Contrived tear solution target values representing the physiological range of tear osmolarity (normal eyes 297 mOsm/L, moderately dry eyes 342 mOsm/L, and severe dry eyes 383 mOsm/L) were constructed using a mix of mono- and divalent electrolytes, metabolites, serum albumin, and pH balanced to 7.4. Solution values were randomized and masked from the investigators during testing. Osmometers (Wescor 5520 Vapro Pressure Osmometer: device A, TearLab Osmolarity System: device B, and i-Med Pharma i-Pen: device C) were calibrated according to manufacturer instructions. Each level was tested 64× on each osmometer across two sites. Accuracy was reported as a correlation coefficient against expected linear dilutions, precision was calculated as percent coefficient of variation.

Device A reported a correlation with known solutions of r²=0.98, with averages of 305.6±4.0, 352.2±5.5, and 389.8±4.0 mOsm/L, and coefficient of variations (CVs) of 1.3%, 1.6%, and 1.0%, respectively. Device B reported an r²=0.96, with averages of 300.6±3.7, 341.4±7.9, and 376.8±5.1 mOsm/L, and CVs of 1.2%, 2.3%, and 1.4%, respectively. Device C reported an r²=0.03, with averages of 336.4±21.5, 342.0±20.7, and 345.7±22.0 mOsm/L, and CVs of 6.4%, 6.1%, and 6.4%, respectively.

In this randomized, masked, in vitro study, device A and device B had significantly better accuracy and precision in measuring osmolarity of contrived tear solutions of known target values compared to device C. Device C showed insufficient performance to accurately and precisely delineate osmolarity levels in the physiological range. Furthermore, in vivo studies would be required to compare performance in human subjects.