Since dry eye disease has multifactorial causes and pathology that lead to the tear film deficiency and the signs and symptoms of ocular surface disease and dry eye disease, diagnostic tests have to be done to help make the correct diagnosis.
However the diagnosis of dry eye disease is not just based on the tests, but is based on a combination of the patient symptoms and signs and the clinical tests. There is no single therapeutic strategy that fits all patients, instead treatment is individualised by targeting the specific mechanism that is found to be driving the dry eye disease for that individual patient.
When I am diagnosing dry eye disease in a patient, the clinical history is the first important thing. This is followed by the eye examination in order to confirm the diagnosis of dry eye disease using the binocular microscope or slit-lamp examination, with ancillary testing with dyes and tear osmolarity measurement.
The tear osmolarity shows us that the tear film is insufficient or unstable and hyperosmolar. The hyperosmolarity of the tears can vary during the day according to various environmental stresses. Osmolarity testing measures the number of charged particles in the tear sample to provide an estimate measured in mOsm/L.
Normal tear osmolarity is up to 300 or 302 mOsm/L, usually with very little difference between the two eyes. Mild dry eye is between 300 to 308 mOsm/L corresponding to early stages of dry eye disease. Measurements between 308 and 316 are dry eye disease and over 316 is advanced dry eye disease. The worse severity the dry eye, the more variable the tear osmolarity will be because it is an unstable tear film. So one of the factors important when looking at a patient with dry eyes is detecting a difference of 8 mOsm/L between the two eyes to be significant. However a normal value of tear osmolarity does not rule out dry eye disease. An elevated tear osmolarity is not a prerequisite for the diagnosis, but strongly suggest the presence of an inadequate tear film compatible with dry eye disease.
Tear Osmolarity is a dynamic test and there is minute to minute variability depending on the time of day, the patient’s food intake, physical activity, whether they have recently put in drops and whether the eye has already been touched by the patient or the doctor. Therefore an average tear film osmolarity over a specific period would be likely to be elevated and a single measurement is not always the best reflection of the overall status of the tear film. Tear osmolarity offers very valuable insight into the status of the tear film and provides a potentially good therapeutic guide of dry eye disease.