Keratometry: Board Exam Essentials
Master keratometry for NBEO and OKAP: what it measures, how to read K values and the exam traps that matter.
The standard keratometric refractive index used to estimate corneal power is approximately:
Keratometry in 30 Seconds
Keratometry measures the curvature of the central anterior cornea and expresses it as corneal power in diopters. It is fast, clinically useful and heavily tested because it informs astigmatism assessment, contact lens fitting and cataract surgery planning. Remember its central limitation: keratometry is not corneal topography.
What the Keratometer Measures
A keratometer measures the radius of curvature of the central anterior corneal surface by analyzing the size of a reflected mire image, then converts that radius to corneal power in diopters using a standardized index of refraction. It measures only the central 3 to 4 mm of the cornea and assumes the cornea behaves as a spherical convex mirror. It cannot characterize peripheral corneal shape or irregular astigmatism.
Reading and Using the Result
- Determine the magnitude of corneal astigmatism from the dioptric difference between the principal meridians and the axis from their orientations.
- Suspect irregular astigmatism when mires appear distorted, broken or irregular instead of clear, regular ovals.
- Use keratometry as an essential input for intraocular lens power calculation before cataract surgery.
- Use corneal curvature measurements to fit rigid gas permeable lenses and some soft contact lenses.
4 Facts Exams Always Ask
- Keratometers use a standardized keratometric index of 1.3375. It is not the cornea's true refractive index because it compensates for the posterior corneal surface using a simplified model.
- Normal keratometric power is approximately 40 to 46 D. Steeper corneas have higher dioptric K readings, while flatter corneas have lower readings.
- Keratometry alone cannot diagnose keratoconus. Distorted or irregular mires should raise suspicion for corneal irregularity. See retinoscopy for how irregular astigmatism also shows up as a scissors reflex.
- Keratometry becomes unreliable after LASIK or PRK because anterior corneal curvature no longer has the usual relationship to total corneal power.
The Classic Trap
Do not assume keratometry measures the whole cornea or that a normal reading excludes corneal pathology. It samples only a small central zone and does not describe peripheral curvature or subtle irregular astigmatism. Interpret K readings together with mire quality, and obtain corneal topography or tomography when irregularity is suspected.
Clinical Pearl
A sharp, repeatable K reading does not guarantee a regular cornea. Always inspect the mires before accepting the numbers. The quality of the reflection can reveal irregularity that the displayed diopters conceal.
Checkpoint 1
Standard manual keratometry samples approximately which part of the cornea?
Checkpoint 2
After myopic LASIK, standard keratometry often:
A 24-year-old patient reports progressively blurrier vision despite updated spectacles. Keratometry shows 43.00 D at 180 degrees and 45.00 D at 90 degrees, but the reflected mires are distorted and asymmetric. The technician records the values as routine corneal astigmatism.
What is the most appropriate next step?
Sources & Reviewer Info
Diagram created with AI image generation, clinically reviewed for accuracy.
References
- StatPearls (NCBI Bookshelf). "Keratometer."
- Opterio. "Keratometer Principles and Use: Mire Alignment, K Readings, and Calibration."
- ScienceDirect Topics. "Keratometer" (overview).
- SPIE Optipedia. "Keratometry."
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