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Interpreting tear film osmolarity: One test is not enough
Variability, fluctuation of osmolarity mean that numerous data points are needed to assess tear film.
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Welcome to another edition of CEDARS/ASPENS Debates. CEDARS/ASPENS is a joint society of cornea, cataract and refractive surgery specialists, here to discuss some of the latest hot topics in ophthalmology. Tear film osmolarity has become a commonly used test to diagnose and monitor dry eye patients. Despite the understanding of elevated osmolarity as an indicator of dry eye disease, the value of the test and the interpretation of the results have been widely debated. This month, I will discuss how I use and interpret tear film osmolarity testing for dry eye patients. I hope you enjoy the discussion.
Dry eye disease is among the most common conditions the eye care professional encounters. It has been estimated that nearly 30 million people in the U.S. suffer from dry eyes. Due to a growing understanding of the effect of dry eyes on vision and surgical outcomes, there has been an increased emphasis on diagnosing and treating dry eye disease. The FDA finally approved another medication for the treatment of dry eye disease, and several other medications are in the pipeline. In addition, numerous diagnostic tests have become available to help diagnose and grade dry eye disease. This has led to confusion among eye care professionals as to which tests to use and how to interpret these tests.
One test that has made a significant impact on the way I treat dry eye disease is tear film osmolarity testing (TearLab). The Dry Eye WorkShop defined dry eye disease as a multifactorial disease of the ocular surface and tear film accompanied by increased osmolarity of the tear film and inflammation of the ocular surface. Testing tear film osmolarity has become a critical part of my dry eye management. Unfortunately, many physicians struggle with when to use this test and how to interpret the results. The following is my system of how I use and interpret the tear film osmolarity test.
Testing protocol
When a patient comes to my office with complaints consistent with dry eye disease, my technicians immediately perform osmolarity testing. They base the decision to perform the test on a series of questions indicating dry eye disease. One may use a standardized test, such as the OSDI or SPEED test, or a self-made survey asking for similar symptoms to initiate the process. If the osmolarity score is 300 mOsm/L or greater in the higher scoring eye, I consider this abnormal. From 300 mOsm/L to 320 mOsm/L, I grade as mild; from 320 mOsm/L to 340 mOsm/L, I grade as moderate; and greater than 340 mOsm/L, I grade as severe. In addition, a difference of 8 mOsm/L or more between eyes is also considered abnormal. It is therefore essential to test both eyes every time to identify the intra-eye difference.
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There are obviously other tests that I perform in my dry eye evaluation, but those are beyond the scope of this article.
Going forward, once the initial osmolarity test has been performed, I can assess when the osmolarity needs to be checked again. My staff is instructed to perform the test on any new patient with the previously mentioned complaints or any established patient who presents with new complaints. I will also typically test patients on each dry eye evaluation. Once patients have been stabilized, they may only need to be tested on their annual exam. Early on, when they are still unstable, I may need to test several times to assess their progress with my treatments. I will typically order the test at the end of my exam for their next scheduled visit. If they are stable and present for an unrelated issue, such as a floater check or a pressure check, I do not test at that time. On the other hand, if they were not scheduled for osmolarity testing on a particular visit but are having new symptoms or worsening signs, then I will retest.
Interpretation of test
The difficulty with interpreting the test stems from the variability in the results. For a dry eye patient, the osmolarity may vary dramatically between eyes and from visit to visit. Often, physicians interpret this variation as a flaw in the test itself. In actuality, it is just an indicator of the severity of the disease. Variability and fluctuation of osmolarity are hallmarks of dry eye disease and need to be recognized as such.
Because of this variability, I do not believe that a single test is sufficient. Just as a one-time random blood sugar does not give us an indication of how well controlled a patient’s diabetes is, a one-time osmolarity test does not indicate how well a patient with dry eye is doing. For diabetic patients, we have the A1c test, which gives us an indication of relative blood sugar control over a period of months. We do not have such a test for osmolarity. We therefore need numerous data points. Without the A1c, we would need to resort back to frequent blood sugar tests (breakfast, lunch, dinner and bedtime) to plot a curve to figure out if the patient is controlled. With osmolarity, we face the same situation. By collecting numerous data points, we can plot the curve and truly assess the stability of the tear film. Over time, as dry eye improves, we may see a lowering of the overall osmolarity, less fluctuation between tests and a smaller intra-eye difference.
Just as high osmolarity will indicate dry eye disease, a consistently normal osmolarity may suggest another cause of the symptoms. This is where multiple tests are critical. A one-time normal osmolarity does not indicate the lack of dry eyes, just as a one-time normal blood sugar in a patient with diabetes does not indicate stable blood sugar control. On the other hand, if the osmolarity is consistently normal over several tests, then I would consider another source. While one normal test does not rule out dry eyes, it is significant enough to at least require the investigation for other sources.
In summary, tear film osmolarity testing has become a vital part of my dry eye evaluation. Osmolarity testing helps me diagnose dry eyes, grade severity and monitor response to treatment. Fluctuation of osmolarity is due to the volatility of the disease, rather than a flaw in the test itself. Numerous tests over time are often needed to truly assess the tear film, as variation between tests is another marker of an abnormal tear film. Finally, while tear film osmolarity is a wonderful dry eye test, it should not be evaluated in a vacuum, but rather in conjunction with other tests as well as the patient history to best interpret the results.
- References:
- Paulsen AJ, et al. Am J Ophthalmol. 2014;doi:10.1016/j.ajo.2013.12.023.
- Annual estimates of the resident population for selected age groups by sex for the United States, States, Counties, and Puerto Rico Commonwealth and Municipios: April 1, 2010 to July 1, 2014. https://factfinder.census.gov/faces/tableservices/jsf/pages/productview.xhtml?src=bkmk. Published June 2015.
- The definition and classification of dry eye disease: report of the Definition and Classification Subcommittee of the International Dry Eye WorkShop (2007). Ocul Surf. 2007;doi:10.1016/S1542-0124(12)70081-2.
- For more information:
- Kenneth A. Beckman, MD, FACS, can be reached at Comprehensive Eye Care of Central Ohio, 450 Alkyre Run Drive, No. 100, Westerville, OH 43209; email: kenbeckman22@aol.com.
Disclosure: Beckman reports he is a consultant for TearLab.