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William J. Benjamin, Tammy P. Than; Impedance Osmometry: Standard Solutions. Invest. Ophthalmol. Vis. Sci. 2012;53(14):562.
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We evaluated an impedance osmometer (TearLab Corp., San Diego CA) by measuring buffered saline solutions having osmolarity ranging nominally from 275-400 mOsm/L in increments of 25. We also measured references having stated osmolarity of 290 mOsm/L (Wescor, Inc., Logan, UT) and standards having stated values of 297 and 338 obtained from TearLab.
A calibrated Wescor vapor-pressure osmometer was used to verify osmolarity of the 6 test and 3 reference solutions. Solutions were also measured with 2 pens of the impedance osmometer. The 2nd pen replaced an original pen that was erratic.
The mean (n=12) vapor-pressure tonicities of test solutions were 277.5, 301.0, 320.7, 352.7, 371.4, and 401.8 mOsm/L. The 95% confidence intervals ranged from ±1.5-3.1%. Impedance outputs for the solution having greatest tonicity were "Above Range." The mean (n=12) impedance tonicities for the other test solutions were 292.8, 319.0, 344.4, 373.9, and 394.8 mOsm/L, with 95% confidence intervals from ±1.2-3.7%. The references averaged 290.0, 286.3, and 322.9 mOsm/L (n=12), respectively, using the vapor-pressure osmometer, and 95% confidence intervals were ±1.5-3.3%. Impedance means (n=12) for the references were 305.2, 286.6, and 327.8 mOsm/L with 95% confidence intervals from ±3.7-8.4%. There was no significant difference between outputs of Pens 1 and 2. Earlier, using test cards having a different lot number, the vapor-pressure and impedance means of the reference solutions were similar to those above except for Pen 1, which gave 338.8 mOsm/L for the higher standard. This mean was then significantly above that of Pen 2 for the 338 standard solution (p = 0.005).
Impedance osmolarities of test solutions were greater by an average of 6.24% (5.51-7.39%), the difference increasing with osmolarity from 15.3 to 23.4 mOsm/L until above the instrument’s upper limit. The mean impedance osmolarity was greater by 5.24% or 15.2 mOsm/L for the 290 reference. Osmolarities were less than stated for standard solutions used to verify calibration of the impedance instrument, except for an earlier occasion using Pen 1 on the higher standard. We feel that calibration of impedance osmometry is more complicated than realized. Reproducibility between instruments, pens, lots of test cards and reference solutions warrants further study, as does consistency over time.
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