Use your trial lenses and these conversion factors to measure corneal curvatures beyond the ranges of a conventional keratometer.
How It Works
Sometimes a patient has a corneal curvature so steep (keratoconus) or flat (post-LASIK or PKP) that they're beyond the range of a conventional keratometer. You can extend the keratometer range by taping a trial lens over the central aperture on the patient's side of instrument (be sure not occlude the mires). For steep corneas, use a +1.25 lens. For flat ones, use a -1.00 lens. Multiply the K readings by the following conversion factors to get the extended values.
It's a simple calculation, but this is something that comes up so rarely that most of us don't have access to the conversion factors when we need them.
Thanks to Jay Highland, O.D. who suggested this calculator and researched the conversion factors.