Tear film stabilizing eye drops that are instilled prior to keratometry measurements may significantly influence K-readings, especially in patients with dry eye, a team of researchers from Austria report.

The investigation included 123 eyes of 80 pre-cataract surgery patients who were divided into two groups: normal and dry eyes. 

Two baseline keratometry measurements were followed by instillation of either high- or low-viscosity eye drops. The investigators repeated keratometry 30 seconds, two minutes and five minutes after instillation.

The study found high repeatability between baseline measurements. In both groups, the researchers observed a statistically significant increase in measurement variability after instillation of both low- and high-viscosity eye drops. They also found measurement variability was most pronounced between baseline measurement and 30 seconds, which diminished over time, and variability of K-readings appeared higher in dry eyes compared with normal eyes.

Additionally, astigmatism changed by more than 0.50D in 13.2% of normal eyes and 34.4% of dry eyes that used high-viscosity eye drops.

Instillation of any eye drops before keratometry during biometry should be performed with great care, the researchers suggest. If eye drops are necessary, the team suggests clinicians delay keratometry for more than five minutes.

“This rule of thumb applies even more for dry eyes, because their measurements presented higher scatter and lower reproducibility and were, therefore, less reliable,” the researchers wrote in their paper. “Furthermore, the higher the viscosity of eye drops, the longer the influence on the ocular surface, and therefore the longer the interval required before measuring the ocular surface.”