One of the many changes to life since the pandemic has been an increased familiarity with point of care tests; tests that can easily be carried out directly on, or even by, the patient and yield results without the need for sending off to a lab. Most readers, for example, will be completing a lateral flow test this week (figure 1).
Point of care tests need to be able to identify some form of bio-marker, whether that be a protein, a pathogen or an antibody remnant, that is present in sufficient quantities in an accessible body fluid to be taken up by a swab and detected by a chemical response able to trigger a visible confirmation. Eye care practitioners have easy access to a body fluid that contains a wealth of potential biomarkers; the tears. Currently, almost 1,800 proteins are known to constitute the human tear proteome. Of these, a significant proportion are found to selectively decrease or increase in ocular surface diseases and thus are potential biomarkers.1
Some years ago, I reported on a point of care test for adenovirus (the AdenoPlus, see Optician 05.07.2013) and mentioned that the simplicity of carrying out the test would make it ideal for testing for other biomarkers (figure 2). Since then, the growth in our understanding of dry eye disease and its evaluation and management has presented one area where such testing is proving to be increasingly important.
Figure 2: Testing for adenovirus
TFOS DEWS II
The TFOS DEWS II report offered a revised definition of dry eye that included the terms ‘homeostasis’ and ‘inflammation’.2 Disruption to the homeostasis, as might result from dry eye disease, should be reflected in an alteration of profile of constituents within the tears. By understanding the changes to the tears that occur with dry eye disease, a useful test might be developed to test for this change. A specific chemical tested for in such an instance is described as a biomarker. Several biomarkers have been suggested for the assessment and monitoring of dry eye disease, and these include lysozyme, lactoferrin and MMP-9.
In the TFOS DEWS II Tear Film Report, Willcox et al noted: ‘Biomarkers should help identify and screen out individuals at risk of developing DED prior to refractive eye surgery, that itself can promote extended dryness. Prophylactic treatment for at risk individuals might also be possible, for example treatments that promote natural basal tearing, ocular surface homeostasis and corneal sensory innervation. Similarly, diagnosis of the type and severity of dry eye, or other ocular surface pathology, should become much more precise.’ 1
A biomarker that shows promise here is MMP-9.
MMP-9
Matrix metalloproteinases (MMPs) are proteolytic enzymes that are produced by stressed epithelial cells on the ocular surface. Metalloproteinases were first identified back in 1969 as substances produced in ocular surface diseases that were important contributors to corneal ulceration, destruction and perforation. There are more than 20 MMPs able to dissolve collagen and other tissues, but it is MMP-9 (also called gelatinase-B) that is of interest here. Matrix metalloproteinase-9 (MMP-9) is an inflammatory marker that has consistently been shown to be elevated in the tears of patients with dry eyes.3
Clinical studies suggest specificity and sensitivity of 80% in patients with pseudoexfoliation syndrome,4 positive agreement of 81% and a negative agreement of 98% in dry eye patients,3 and 57% detection of MMP-9 in post-LASIK dry eye patients.5
InflammaDry
InflammaDry (distributed in the UK by Positive Impact) is a point-of-care test that measures levels of matrix metalloproteinase-9 (MMP-9) on the surface of the eye. The test allows a sample to be taken from the tears, which may indicate the presence of raised MMP-9 levels within 10 minutes. Each test comprises a sample collector, a buffering solution and a test cassette (figure 3).
Figure 3: The InflammaDry test kit
I recently decided to try the kit out on one of the many patients we are all seeing at present who is suffering dry eye symptoms related to mask use (figure 4), so-called mask-associated dry eye or MADE. Tear sampling is easily undertaken (figure 5), and the sample collector is then inserted into the test cassette and the buffer solution added to the test strip (figure 6). As the solution permeates the strip, the presence of MMP-9 is indicated within a small window. If the procedure has been carried out correctly, there should always be a dark line at the end of the window labelled the ‘control zone’ (figure 7). A positive result is revealed by the appearance of a second line at the other end of the window labelled the ‘result zone’ (figure 8).
Figure 4: Symptomatic patient with MADE
Figure 5: Sampling the tears
Figure 6: Testing cassette in use
Figure 7: Normal result
Figure 8: Normal result with positive result (inset)
InflammaDry detects ~40ng/ml or more MMP-9, with indicated sensitivity and specificity values respectively of 85 and 94%, and negative and positive predictive values respectively of 73% and 97%.3 However, in a population with no previous diagnosis of dry eye disease, and who were classified as having mild dry eye in one study, the InflammaDry assay was positive for only 11% of subjects with symptoms of dry eye,6 and in a study of symptomatic (DEQ-5 score 6) subjects, only 39% tested positive with the InflammaDry test.7 These data suggest that the InflammaDry test is better at detecting moderate to severe dry eye subjects.1
Practice
A significant study of InflammaDry by Kaufmann et al concluded: ‘elevated levels of MMP-9 may be a more sensitive diagnostic marker than clinical signs.’ 8 The researchers also suggested its usefulness in detecting hidden cases of dry eye disease that may not be easily identified through the clinical examination, ‘particularly because inflammation is often present long before clinical signs appear.’
The main point from all this; a simple and sensitive test is available that can predict future problems, indicate the nature of dry eye symptoms and so influence the level of intervention, and will be a useful indicator of ongoing management.
For further information go to www.positiveimpact.co.uk.
References
- M.D.P. Willcox et al. The Ocular Surface. TFOS DEWS II Tear Film Report. 15 (2017) 366e403
- Download from; http://dx.doi.org/10.1016/j.jtos.2017.05.001
- Sambursky R et al. Prospective, multicenter, clinical evaluation of point-of-care matrix metalloproteinase-9 test for confirming dry eye disease. Cornea 2014;0:1–7
- Zimmermann N, Erb C. Immunoassay for matrix metalloproteinase-9 in the tear film of patients with pseudoexfoliation syndrome - a pilot study. Klinical Monbl Augenheilkd 2013;230:804e7
- Chan TC et al. Evaluation of point-of-care test for elevated tear matrix metalloproteinase 9 in post-LASIK dry eyes. British Journal of Ophthalmology, 2016;100:1188e91
- Schargus M et al. Correlation of tear film osmolarity and 2 different mmp-9 tests with common dry eye tests in a cohort of non-dry eye patients. Cornea 2015;34:739e44
- Lanza NL et al. Dry eye profiles in patients with a positive elevated surface matrix metalloproteinase 9 point-of-care test versus negative patients. Ocular Surface, 2016;14:216e23
- Kaufman H. The practical detection of mmp-9 diagnoses ocular surface disease and may help prevent its complications. Cornea 2013;32:211–216
