Reproducibility in measuring tear samples using a freezing point depression osmometer

Abstract

Background: Hyperosmolarity of tear fluid has been recognised as a common feature of all types of dry eye disease. This study was designed to assess the inter-session reproducibility of a freezing point depression osmometer (Fiske 110) as the most common and precise way of measuring osmolality, by using two different volumes of tear samples on healthy subjects, and to determine the possible applications of this device in tear film research and clinical practice. Methods: Measurements were made by using the Fiske 110 osmometer under two different tear sample volumes (4 μl and 2 μl). In both cases, samples were diluted in purified water to obtain the 20 μl required by the device to perform the measurement (1:4 and 1:9 dilutions, respectively). Inter-session reproducibility was determined in two groups of 40 healthy subjects, in two sessions, one week apart. In each group, one of the two different tear sample volumes was used to determine the reproducibility of each technique. Results: No significant differences were detected between the measurements obtained in the two sessions using the 4 μl (paired t-test, p = 0.772; mean difference AE SD = −0.85 AE 18.77 mOsm/L; 95 per cent limits of agreement [LoAs] = −37.64/+35.94) or the 2 μl volume sample (paired t-test, p = 0.054; mean difference AE SD = 9.27 AE 29.44 mOsm/L; 95 per cent LoAs = −48.43/+66.97). Conclusions: Whereas both techniques show an acceptable inter-session reproducibility, the bias range with the present protocol was higher using the 2 μl tear sample volume than the 4 μl one. Therefore, it seems that the diluted 4 μl sample was the only dilution that could be acceptable for use in routine clinical practice for tear film analysis.

Diurnal variations of tear film osmolarity on the ocular surface

“…It is important to note that the low values reported in this study were similar to those obtained with a similar procedure (tear sample collected by a capillary tube and measured by freezing point depression osmometer non-tear specific) in three other recent studies where non-diurnal variation was analysed. 68,69,71…”

“…Even though it also has shown high repeatability in non-tear specific osmometers available based on freezing point depression techniques, some of them have shown lower values than those commonly reported in the literature. 49,68,69,71 There is some controversy in the relationship between the values obtained by both techniques, electrical impedance and freezing point depression based osmometers: some studies reported a perfect relationship between them, 22,62 while others found significant differences in their results. 68,69 Hence, it is difficult to perform a direct comparison between the results obtained in studies using these two different techniques.…”

“…Not all the devices employed in the different research studies used the same amount of tear sample; those differences could imply some errors in the final value because of the collection, dilution, or evaporation processes during a measurement. 58,68,71 This may generate some differences in the final results of the studies and a limitation for a real comparison between exact values depending on the device employed. 22,62,68…”