Accuracy of a Freezing Point Depression Technique Osmometer.

The purpose of this study was to examine the precision and accuracy of the Fiske 110 Osmolarity System under different protocols to determine the possible applications of this device in tear film research and clinical practice. Three separate studies were performed. In the first, Fiske 110 measurements were made on undiluted and diluted (1:1, 1:4, and 1:9 dilutions) standard samples of different osmolarity values: 50, 290, and 850 mOsm/kg and 297 and 338 mOsm/L. In the second study, measurements were made on different types of contact lens care solutions. Finally, in an agreement study, measurements were made in two sets of 60 subjects to compare TearLab versus Fiske 110 (using both 2- and 4-μL tear sample). Although osmolarity measurements for undiluted solutions differed statistically from reference standard values, all biases were in the tolerance range proposed by the manufacturer except for the 850-mOsm/kg solution. No significant differences from reference osmolarity values were observed for the 1:1 and 1:4 diluted 297- and 338-mOsm/L H2O solutions, respectively, although all diluted solutions showed a possible bias out of the range provided. Osmolarities for the soft contact lens solutions fell within the range 293 to 309 mOsm/kg with the exception of Opti-Free Express (225 mOsm/kg). In the agreement study, significant differences were observed between measurements obtained using the TearLab and both Fiske 110 procedures, although the Fiske 110 (4 μL) procedure was closer to the TearLab than the Fiske (2 μL) procedure. For undiluted solutions, the Fiske 110 shows good performance, making it a useful device for osmolarity measurements in lens care solutions or eye drops. A worse performance was observed for more diluted standard solution samples. When testing diluted samples, performance was acceptable for osmolarity values close to tear values.