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U. Stahl, L. Jones, M. Willcox, F. Stapleton; Tear Osmolality Measurements - Effect of Instrumentation and of Freezing. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2611.
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To compare tear osmolality measurements made with a freezing point depression osmometer with those using a vapour pressure osmometer and to investigate the effect of freezing on tear osmolality.
Three µl of non-stimulated tears, collected from 15 subjects were measured with a Wescor Vapour Pressure osmometer and an Advanced Instruments Model 3100 Tear Osmometer. For effects of freezing, 4µl of non-stimulated tears were collected and measured with a vapour pressure or a freezing point osmometer. In a randomized order, the remainder of the tears were either immediately frozen or frozen after centrifugation. After storage for 1, 3, 10 or 20 days at -80C, tears were measured with the same instrument again. For each freezing period, 20 samples were used.
Tear osmolality was equivalent for a given person between eyes (p=0.4). Although readings with the freezing point osmometer were 1.5 mmol/kg higher compared to those with the vapour pressure osmometer, there was no significant difference in tear osmolality between the instruments (p=0.13). Also, for up to 10 days of freezing, there was no significant effect of freezing on tear osmolality (p> 0.05). There was also no effect of centrifugation on tear osmolality (p>0.05). However, osmolalities of tears frozen for 20 days were significantly affected by the freezing period (p=0.004). This was largely driven by the effect of centrifugation on tears at this time period (p<0.001), which significantly increased osmolality. Both osmometers gave identical results for freezing and centrifugation (p>0.14).
Tear osmolality can be measured with either a freezing point depression or vapour pressure osmometer. Storage of tears for 10 days does not cause a significant change in osmolality. However, if tears are centrifuged prior to storage there is a significant increase in apparent tear osmolality if tears are stored for 20 days, or presumably longer. Therefore, we recommend storing tears for up to 10 days but no longer without centrifuging if osmolality measurements are to be made.
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