In October, we discussed the importance of tear film osmolarity in the diagnosis of dry eye. To continue that discussion, I asked my good friend from Canyon, Texas, Dr. Bill Townsend, to further enlighten me, and you.

In 1983, Gilbard and Farris reported the results of a study evaluating dry eye in individuals with Graves’ disease.¹ One important outcome was the association between tear film osmolarity and dry eye. During the ensuing years Gilbard and numerous other investigators^2-4 studied tear film osmolarity as it relates to ocular surface disease. Those studies were performed using freezing point depression osmometry. In this technique the temperature of a test solution (in this case, tears) is lowered beyond its freezing point and then slowly increased until the sample melts. It is widely used in laboratories throughout the world to evaluate serum osmolarity which is normally @ 290 mOsm/L, approximately the same osmolarity found in “normal” tears. To accurately determine osmolarity using this method requires extremely precise measurement of temperature. For instance, a shift of one mOsm/L produces a temperature change of only 0.0018° C. Tear osmolarity testing using freezing point depression osmometry requires a 10 µl sample, but the normal lacrimal lake contains a few microliters of tear, and sometimes as little as 500 nL in dry eye patients. As such, tear collection using glass capillaries can take a very long time, and may not be possible in some patients. Laboratory osmometers are potentially useful for evaluation dry eye patients, but are not practical for eye care providers practicing outside an academic setting. 

TearLab Osmolarity System
The TearLab Osmolarity System uses temperature-corrected impedance measurement to provide a measurement of tear film osmolarity. The system is composed of three parts, a test card, a pen, and a base unit. The single-use polycarbonate “test card” draws approximately 50 nanoliters of tears into a chamber for testing. The “pen” serves as a handle for test cards and facilitates safe, simple tear fluid collection. It also connects the test card to the reader unit. The counter top unit gathers data from the tear fluid sample, converts it into an osmolarity value, and displays it on an LCD display. The entire testing process is performed in under a minute for each patient. The 50 nL sample obtained by the TearLab is hundreds of times smaller than the volume sample required for freezing point depression or vapor pressure osmometers, and is much more readily obtained from the tear film individuals with reduced tear volume.

Artificial tears or other topical therapy can significantly alter tear film osmolarity, so every potential TearLab subject is questioned regarding use of any drops or other ophthalmic medication in the two hours prior to testing. Examination of the ocular surface should be deferred or carried out under minimal light to avoid reflex tearing secondary to photophobia. Vital dye installation, Schirmer testing, or any other testing, even if it is minimally invasive, should be performed after tear film osmolarity testing has been completed.

Testing begins with setting up and preparing the instrument. Because osmolarity is influenced by temperature, TearLab contains an internal thermometer to measure ambient temperature. The TearLab must be turned on at least 25 minutes prior to testing to allow it to equilibrate to ambient room temperature. In our practice we leave the instrument on all the time to avoid the 25-minute lag time between starting up the instrument until it is ready for use. For the same reason, placement of the base unit is critical. The instrument should not be positioned next to a computer or other equipment that gives off heat, nor should it situated immediately adjacent to an air conditioning or heating duct. The unit should not be placed with direct exposure to a window; radiant solar heat can warm the TearLab, and cold air from a window can reduce the ambient temperature. Most importantly, the test cards should be stored right next to the base unit to achieve best results.

When used properly, the TearLab is a very accurate and precise device. It has a precision of around ±1% or ±3–4 mOsm/L on test solutions. Because it is a precision device, quality control is an integral part of the system and is maintained with standardized test solutions and test cards. The test solutions are used to verify accuracy of each lot of test cards. Calibration of the entire system is periodically evaluated with reusable test cards that are provided with each system. Technicians perform daily quality control procedures and maintain logs of the results. These procedures take very little time, but are essential for ensuring consistent osmolarity readings, especially because test cards can be damaged by high heat during shipment or storage outside their labeling.

The use of tear film osmolarity as a means of diagnosing dry eye is well established. Another important utilization of this technology is to determine the impact of a treatment regiment. Nelson and Farris⁵ reported the value of tear film osmolarity in monitoring the efficacy of an unpreserved artificial tear substitute for the treatment for dry eye disease. 

Tear film osmolarity measurement is a lab test, not a procedure, and would normally fall under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) regulations. A newly introduced device ordinarily requires the testing facility to have a moderate complex CLIA certificate; creating a great deal of work by the practitioner and staff. The FDA has issued a CLIA waiver for the TearLab. This waiver reduces these responsibilities and allows patients to have better access to this technology.

Implementing TearLab into practice requires planning and staff education. There are excellent training modules found at www.tearlab.com that allow the practitioner and technicians to master the patient testing and quality control procedures. One important decision is determining which patients will undergo osmolarity testing. Ultimately the decision rests with the health care provider. One way to accomplish this is providing all patients with a dry eye questionnaire. Those whose scores are suggestive of dry eye disease are automatically tested for increased osmolarity. This can be done through the use of standing orders. Some practitioners may prefer to evaluate all patients and then, based on subjective and objective signs and symptoms, order testing on those who are determined to have elevated risk for dry eye.

The CPT code for the TearLab is 83861QW, and the test is reimbursable by Medicare and insurance companies. An appropriate diagnosis code must be used to obtain reimbursement.

In summary, tear film osmolarity testing is an objective, reliable means of evaluating individuals with dry eye. Prior to the introduction of the TearLab, this testing was only done in research and institutional settings, but now it is available for the practicing clinician and shows great promise in helping us identify and treat individuals with dry eye.

Note from the Editor
My thanks to Dr. Townsend for the TearLab summary. I will close here with a short note. This will be my last issue of Managing Dry Eye. For the last two and a half years, I have enjoyed writing these pieces and learned a ton about dry eye, but more than that I have enjoyed hearing from you, the readers. You can continue to contact me at drbowling@windstream.net. Take care and may God bless.



References
1.Gilbard JP, Farris R. Ocular surface drying and tear film osmolarity in thyroid eye disease. Acta Ophthalmol (Copenh). 1983 Feb;61(1):108-16.
2. Gilbard JP, Kenyon KR. Tear diluents in the treatment of keratoconjunctivitis sicca. Ophthalmology. 1985 May;92(5):646-50.
3. Holly FJ, Lamberts DW. Effect of nonisotonic solutions on tear film osmolality. Invest Ophthalmol Vis Sci. 1981 Feb;20(2):236-45.
4. Koumantakis G.,Wyndham LE. An evaluation of osmolality measurement by freezing point depression using micro-amounts of sample J Automat Chem. 1989; 11(2): 80–83.
5. Nelson JD, Farris RL. Sodium hyaluronate and polyvinyl alcohol artificial tear preparations. A comparison in patients with keratoconjunctivitis sicca. Arch Ophthalmol 1988; 106: 484-487.