Tear Film Quantitative Tests
As our understanding of the tear film has improved (see Chapter 1), commercial assays to measure its various components have been developed. The objectivity and reproducibility of many of these tests have yet to be confirmed, and the results need to be evaluated in conjunction with symptoms and other clinical findings.
Microliter samples of tears can be used to measure tear film osmolarity. Values higher than 306–308 mOsm/L are considered highly indicative of ATD. Other studies, however, have shown a lack of correlation between tear osmolarity and symptoms and objective signs of dry eye.
Lactoferrin, an iron-binding protein secreted directly by the acinar cells of the lacrimal gland, plays an important antibacterial role in the tear film. Although it does not play a role in tear production, its levels are directly correlated to aqueous production. Microassays are now available to measure levels of lactoferrin, providing an indirect measure of lacrimal gland function.
Immunoglobulin E (IgE) levels in the tear film can also be measured with microassay technology, and increased levels can help distinguish between ATD and ocular allergies. Lactoferrin and IgE biomarkers can be measured in a single commercially available test requiring only 0.5 μL of tears.
Matrix metalloproteinase 9 (MMP-9) is an inflammatory cytokine released by distressed epithelial cells. Dry eye is multifactorial and can be associated with inflammation of the ocular surface and eyelids (eg, blepharitis), which in turn can cause the release of the inflammatory marker MMP-9. Elevated values (>40 ng/mL) might indicate evaporative dry eye (see Chapter 3). However, this test is not specific for dry eyes, as high levels of MMP-9 are reported in conditions other than blepharitis, such as allergy, infection, and peripheral ulcerative keratitis.
Sjögren syndrome is commonly associated with dry eye. Early biomarkers for this autoimmune disease may support the diagnosis in patients with systemic symptoms such as dry mouth and joint pains. A proprietary biomarker test is available that may be more sensitive than other blood tests, such as SS-A and SS-B antibody and rheumatoid factor tests.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.