Clinical application of a homogeneous colorimetric assay for tear lysozyme

Purpose: Tear lysozyme and tear lactoferrin are enzymes synthesized by the lacrimal gland. Their concentration in human tears reflects tear gland function. Tear gland dysfunction can lead to ocular surface disease. We developed a colorimetric lysozyme assay. The objective of this study was to determine the diagnostic power and the clinical application of this assay that allows rapid and precise quantification of tear lysozyme. Methods: Tear specimens of 120 eyes (30 Sjogren's patients and 30 controls) were collected using standardized filter paper discs. Tear lysozyme concentration was determined using p-nitrophenyl penta-N-acetyl-beta-chitopentaoside as substrate in the colorimetric assay. The results were compared to clinical findings and to two commonly used tests, the Micrococcus agar diffusion assay for tear lysozyme and the tear lactoferrin immunodiffusion assay. Results: The colorimetric assay showed a good dose-response relationship. The use of the assay as a method of diagnosing aqueous tear deficiency, using the clinical findings and the medical history as gold standard, demonstrated 85% sensitivity and 92% specificity. The results of the colorimetric assay when compared with the Micrococcus agar diffusion assay showed a linear relationship of r=0.77; when compared with tl-le lactoferrin immunoassay r=0.73. Conclusions: The colorimetric assay is simple to perform and does not require sophisticated laboratory equipment and personnel. Results can be precisely quantified within one hour after tear collection. The diagnostic power of the test is comparable to previously reported assays for lysozyme and lactoferrin and will be useful in the diagnosis of ocular surface disease.