Although contact lens (CL) wear is a well-known risk factor for microbial keratitis (MK), a new study puts a number to the risk, potentially providing clinicians more ammo when educating contact lens wearers about proper wear and care.

Researchers in France looked at 1,198 cases of MK and 1,069 controls between 2014 and 2017. Study participants completed a 52-item questionnaire that included questions regarding epidemiological data, CL modality and care regimen and patient lifestyle. The team first divided the groups based on their lens modality, whether daily disposable wearers (149 cases and 119 controls) or not (1,049 cases and 950 controls).

Results for all lens types show that men were nearly twice as likely to develop and infection, and patients with myopia were 2.6 times likely to have MK than those with hyperopia. In addition, participants who had been wearing CLs for less than 13 years were more than twice as likely to have MK compared with longer-term wearers. The researchers found purchasing lenses online put wearers at an increased odds of 16x of developing MK.

Not surprisingly, wearing lenses fewer than five days a week was a protective factor against infection. Other factors such as patient education on lens care, hand washing and using prescribed and multipurpose solutions were all protective, while the use of branded cleaning solution came with a 3.5 odds ratio (OR).

For daily disposable wearers, sleeping in their lenses led to an OR of 15.8, and diverging from the appropriate renewal schedule put patients at 9.2 increased risk.

To help clinicians translate these ORs into practice, the investigators also created a risk equation of CL-related MK, which assigns a numerical value to various CL-related questions, ultimately providing a percent risk for each patient. As an example, the researchers found a rigid gas permeable wearer of five years who is in their lenses for 12 hours a day with dry eye, allergy and a history of MK could have a risk as high as 13%.

“This tool thus allows for a possible identification and modification of risk behavior with the aim of achieving a reduction in the incidence of CL-related MK,” the researchers concluded in their study.