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Understanding biocompatibility: the importance of study design, methods, and proper interpretation.

1. The assumption that increased hyperfluorescence observed by slit lamp (increased corneal permeability [Pdc] by fluorophotometry) is solely attributable to fluorescein paracellular transport (compromised barrier function) is incorrect. Current research shows fluorescein enters healthy, mitotically active corneal epithelial cells within an intact epithelium. Adding polyhexamethylene biguanide (PHMB)-preserved multipurpose solution (MPS) (ie, renu fresh/ MultiPlus [RNF]) increases the number of hyperfluorescent cells without an increase in damaged/dead cells. Furthermore, studies show that (1) fluorescein enters healthy cells via normal active mechanisms and (2) PHMB binds fluorescein and cell membranes in a nondestructive, reversible manner, thereby increasing the number of fluorescein molecules at the cell surface that are able to enter by physiological transport processes. This is a more likely mechanism to explain their observation. 2. Based upon the definition of biocompatibility (of lenses and solutions), “the ability of material/lens care solution to interact with living cells/ tissues without toxicity, injury, or causing immunological reactions while performing appropriately,” no single test can measure biocompatibility, including fluorescein Pdc in the absence of other measures, especially symptoms. 3. The association or predictive value of corneal hyperfluorescence/staining for microbial keratitis (MK) has been disproven by 20+ years of epidemiology, clinical studies, and extensive laboratory work by Fleiszig’s group. Therefore, the inference that corneal hyperfluorescence increases MK risk is irresponsible. Several study methodologies are also of concern: