Evaluation of a New Tear Osmometer Instrument for Repeatability and Accuracy Using Nanoliter-Sized Samples

Abstract

Purpose:: Tear osmolality and osmolarity have been reported to change with ocular surface disease such as dry eye. However, it has been difficult to analyze these changes in dry eye patients because past instruments were difficult to use and/or required large volumes of tear samples. In this study, we evaluated a new tear osmometer instrument that measures the osmolality of nanoliter-sized samples by freezing point depression, a standard technique for determining osmolality.

Methods:: Four standardized solutions were tested using 0.5 uL samples for repeatability of measurements and comparability to standardized technique. Two known standard salt solutions, a normal artificial tear matrix sample and an abnormal artificial tear matrix sample were repeatedly tested (n=20 each) for osmolality using the new Advanced Instruments^TM Model 3100 Tear Osmometer (0.5 uL sample size) and the FDA approved Advanced Instruments^TM Model 3D2 clinical osmometer (250 uL sample size). Both machines provide endpoint measurements independent of operator biases. Data was analyzed using the EP Evaluator^TM Release 7 version 0.0.288 (David Rhoads Associates, Inc.) to assess the repeatability and percent recovery of each of the 4 samples.

Results:: The tear osmometer was easy to use and required minimal training to enable users to become proficient in the technique. The precision data including the mean and standard deviation for the 290 standard solution, the 304 reference solution, the normal value-assigned 306 sample, and the abnormal value-assigned 336 sample were 291.8 +/- 4.4; 305.6 +/- 2.4; 305.1 +/- 2.3; and 336.4 +/- 2.2, respectively. The percent recoveries for the 290 standard solution, the 304 reference solution, the normal value-assigned 306 sample, and the abnormal value-assigned 336 sample were 100.3, 100.2, 99.8, and 100.3, respectively.

Conclusions:: The repeatability data is in accordance with data obtained on clinical osmometers using larger sample sizes and all 4 samples tested on the tear osmometer have osmolality values that correlate well to the clinical reference method. The tear osmometer is a suitable instrument for testing the osmolality of nanoliter-sized samples, such as tears and therefore may be useful in diagnosing, monitoring, and classifying dry eye patients.

Keywords: cornea: tears/tear film/dry eye • clinical laboratory testing • cornea: clinical science