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Kerstin Wunderlich, Esther Kuechler, Daniela Nosch, Stephan Gutzwiller; Accuracy Of The Tearlab Osmometer. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3797.
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Hyperosmolarity of the tear film is recognized as a key feature of dry eye syndrome (DES) and considered to be its single most accurate objective diagnostic test. Until recently, tear film analysis could not be carried out in clinical practice, due to the elaborate tear sample collection and laboratory procedures it involved. With application of the Tearlab, nanoliter volumes of tear fluid can now be measured with the same analytical performance as laboratory osmometers. Because it is assumed that osmolarity values differ between the tear meniscus and the bulbar tear film, the aim of this study was to determine if the angle of the Tearlab chip towards the tear meniscus during sample collection may influence the readings. Furthermore, the inter- and intra-examiner repeatability of measurements obtained with the Tearlab was investigated.
To determine the dependence of the chip position in a liquid meniscus during sample collection, three different angle positions (70°, 90° and 120°) of the Tearlab chip towards to the horizontal glass slide were analyzed. For each angle, 10 measurements were carried out. The liquid meniscus was created by applying a Ringer’s solution at the junction of two clean glass plates that had been placed on top of each other and displaced sideways. To evaluate repeatability, tear osmolarity measurements were carried out independently in 13 healthy subjects by two different examiners.
Osmolarity values varied significantly by 8.9 mosm/l with different angle positions of the Tearlab chip (p<0.001, ANOVA - nested model). At 90°, osmolarity was 314.4 ± 7.0 mosm/l compared to 323.3 ± 5.4 mosm/l at 120°. Inter-and intra-examiner variability were found to be considerable at ± 16.21 mosm/l (mean 303.77 mosm/l) and ± 11.82 mosm/l (mean 302.69 mosm/l), respectively (linear mixed effect model data).
These results suggest that tear film osmolarity values vary significantly, depending on where precisely within the tear meniscus the tear film sample is collected. This may - in part - account for the considerable inter- and intra-examiner variability found in this study. For accurate and reliable osmolarity measurement, this technical challenge during the tear sample collection should be addressed.
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