Tear Film Osmolality A Newer Gold Standard?
BY KELLY K. NICHOLS, OD, MPH, PHD
In 1994, Farris published a paper titled, "Tear film osmolarity � a new gold standard?" This question is as relevant as it was more than 10 years ago. The dry eye workshops of the 1990s saw the same controversy related to osmolality that we argue today. Is this test important in dry eye, or is it just an "ivory tower" idea that means nothing to clinicians?
Figure 1. Advanced Instruments Tear Osmometer
Osmolality vs. Osmolarity
"Osmolality" and "osmolarity" are often used interchangeably. "Purists" refer to osmolality as the concentration of particles in dry weight (measured in mOsm/Kg). Osmolarity is a measure of the osmotic pressure that a solution exerts across a semi-permeable membrane and depends on the number of particles in solution (mOsm/L).
Most traditional osmometers use freezing point depression to measure osmolality of a sample (mOsm/Kg). With such instruments, a small volume liquid sample is frozen. The time to full thaw after a freeze is directly related to the osmolality. Older instruments, such as the Clifton Osmometer, required subjective detection of melting.
The Osmolality Debate
The NEI/Industry report on clinical trials in dry eye (Lemp, 1995) listed osmolality as a characteristic of dry eye syndromes. In the current international workshop to discuss the definition of dry eye, the group of approximately 40 researchers, clinicians and industry representatives has yet to achieve consensus on whether to include osmolality (and/or inflammation) in the definition of dry eye.
The concept of osmolality as an "overarching" characteristic common to dry eye-related disorders is appealing. Recently, Tomlinson and Khanal (2005) published an overview of osmolality findings reported over the last 10 years. Their summary indicated a range of tear film osmolality values for normal and dry eye patients (average 302 to 343 mOsm/Kg), and that a cut-point value of 316 mOsm may provide the best sensitivity, specificity and predictive value.
Tear sampling for evaluating osmolality occurs at the inferior tear meniscus, generally along the lateral lid margin, where the osmolality may be lower (more normal) than the thin tear layer over the front of the cornea. Thinning because of evaporation may lead to increased corneal osmolality, which may give rise to patient-reported dry eye symptoms.
In theory, we could place a small sample of tears from the tear meniscus into an instrument to measure osmolality, which would then say "dry eye" or "no dry eye." Sounds great, right?
Advanced Instruments is currently field testing its 3100 Tear Osmometer, which is based on freezing point depression of a 500nL tear sample. OcuSense, a new company developing an osmometer based on electric potential, plans to release its instrument within a year. It requires a similarly small tear sample for evaluation. Both instruments have the potential to provide valuable information about tear osmolality in ocular surface disease. Perhaps soon we'll finally know whether osmolality is the "gold standard" dry eye diagnostic test.
To obtain references, visit www.clspectrum.com/references.asp and click on document #119.
Dr. Nichols is associate professor of clinical optometry at The Ohio State University College of Optometry in the area of dry eye research.