Abstract
Purpose: :
Tear osmolality and osmolarity have been reported to change with ocular surface disease such as dry eye. Unfortunately, it has been difficult to analyze these changes because earlier instruments were difficult to use and/or required large volumes of tear samples. In this study, we assessed the repeatability, accuracy and precision of a new tear osmometer which uses electrical impedance to measure the osmolarity of 50 nanoliter (nL)-sized samples and compared it to an older model using freezing point depression to measure the osmolarity of 500 nL samples.
Methods: :
Six (6) standardized solutions were evaluated using the TearLab Osmolarity System. Ten (10) 50 nL samples were obtained from each solution and tested using this system and (separately) the Advanced Instruments (AI) Model 3100 Tear Osmometer using 500 nL samples. The solutions included 4 standard salt solutions (290 and 323 mOsm/kg H2O and 292 and 338 mOsm/L H2O) as well as 2 artificial tear matrix samples (315 and 341 mOsm/kg H2O).
Results: :
The precision data (mean and standard deviation) for the standard salt solutions in the TearLab were: (323:) 327+4, (290:) 287+5, (292:) 297+6, (338:) 345+6 as compared to 326+6, 298+4, 300+4, 332+8 for the AI. Precision values for the tear matrices (315 and 341 mOsm/L) were: 311+3, 329+5 (TearLab) and 321+4, 330+7 AI. The percent recoveries for these solutions were: 101.1, 96.4, 99.2, 104.0, 96.9, 99.5 respectively. Confidence of TearLab [(323:) 8.0, (290:) 10.8, (292:) 13.2, (338:) 12.3, (315:) 6.60, (341:) 9.58] and AI [(323:) 11.1, (290:) 7.78, (292:) 9.55 (338:) 15.6, (315:) 7.93, (341:) 13.7] and Coefficient of Variance [TearLab/AI (323:) 1.17/1.71, (290:) 1.80/1.25, (292:) 2.13/1.44, (338:) 1.80/2.37, (315:) 1.12/1.25, (341:) 1.40/1.99] were also calculated.
Conclusions: :
All 6 samples tested on the two tear osmometers had osmolalities that correlated well with, but were not identical to, each other. The repeatability data was in agreement with data obtained from clinical osmometers requiring larger sample- (10uL) sizes. Both the TearLab and AI osmometers appear suitable to test the osmolarity of ultra small-sized tear samples such as those often encountered in Dry Eye Disease patients, potentially making them useful in the diagnosis and classification of such abnormalities.
Keywords: cornea: tears/tear film/dry eye • detection