Measuring tear osmolarity has been identified as a potential method for objectively diagnosing dry eye. TearLab osmolarity system is an in situ osmometer which may be a promising candidate for clinical use as it is portable and only requires a 20nL sample. However, some studies have obtained conflicting results regarding the accuracy of this system. This study examined the precision and accuracy of the TearLab system using 10 different salt solutions of varying osmolarity compared to a freezing point depression osmometer. Their results showed that the TearLab system can precisely and accurately measure osmolarity of salt solutions, but larger differences were found between the two methods at both extremes of osmolarity. Furthermore, this was an in vitro study and therefore was not influenced by any external factors that have led to the large repeatability coefficients in other studies including the time of measurement, differences in observer techniques and possible change in osmolarity from disrupting the tear film. The authors state that more studies need to be performed, ideally in vivo before TearLab can be considered as clinically useful. Firstly, there is a significant overlap in osmolarity between mild to moderate dry eye patients and normal controls and therefore any osmolarity value for diagnosis may lead to low sensitivity and a high false negative rate. Secondly, patients with dry eye have significant variability that is greater than normal controls and this may reduce the ability of TearLab to monitor disease progression. 

Precision and accuracy of TearLab osmometer in measuring osmolarity of salt solutions.
Yoon D, Gadaria-Rathod N, Oh C, Asbell P.
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Khadijah Basheer

Moorfields Eye Hospital, London, UK.

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