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Keratometry is the measurement of the anterior corneal curvature and is one of the most clinically important procedures performed by ophthalmic assistants. K readings, as they are commonly called, provide critical data for contact lens fitting, intraocular lens (IOL) power calculations before cataract surgery, and screening for corneal conditions such as keratoconus. The COA exam expects candidates to demonstrate proficiency in manual keratometry technique, understand normal values, and interpret results in clinical context.
The keratometer (also called an ophthalmometer) measures the radius of curvature of the central corneal surface — approximately a 3mm optical zone — in two perpendicular principal meridians. This reveals both the degree and axis of corneal astigmatism, which is the difference in curvature between the flattest and steepest meridians. Readings are typically expressed in millimeters of radius and converted to diopters of refractive power.
This guide covers the complete keratometry skill set for the COA exam: instrument setup, mire alignment technique, normal reference values, the mm-to-diopters conversion formula, interpretation of steep and flat corneas, and the key clinical applications you need to understand for exam success.
The manual keratometer (e.g., Bausch & Lomb, Javal-Schiotz) projects a pattern of illuminated mires (rings or crosses) onto the corneal surface and measures the size of their reflection. Because the cornea acts as a convex mirror, the size of the reflected image is directly related to the radius of curvature — a steeper cornea produces a smaller image, while a flatter cornea produces a larger image.
One-Position Keratometer (B&L type)
The most common type in the US. Uses a fixed target; the observer adjusts the instrument's position until the mires are aligned. Measures in one position by rotating the eyepiece. Faster but requires perfect alignment.
Two-Position Keratometer (Javal-Schiotz type)
Used in Europe and academic settings. Requires the instrument to be positioned at 0° (horizontal) and then 90° (vertical). More complex but allows separate measurement of each meridian.
Automated Keratometer
Computer-driven instruments (e.g., Topcon KR series, Canon RK) that measure K readings automatically using infrared light and photodetectors. Faster and reduces operator error; still requires clean corneal surface and steady fixation.
Corneal Topography (Extended K)
Provides full anterior corneal surface maps rather than just central readings. Devices like Placido disc topographers give thousands of data points. Standard keratometry only captures the central 3mm zone.
Proper technique is essential for accurate, reproducible K readings. The COA exam may test both the procedural steps and the troubleshooting of distorted mires.
Patient Preparation
Remove contact lenses at least 30 minutes before measurement (longer for RGP lenses — minimum 2–3 weeks for accurate readings). Have the patient blink several times to ensure a fresh, even tear film.
Instrument Calibration
Check calibration using the steel ball provided with the instrument. The keratometer should read the ball's known radius within ±0.05mm. Clean the mire targets and ocular lens before each use.
Patient Positioning
Align the patient's eye at the correct height. The chin rest should position the lateral canthus at the level of the mark on the instrument upright. The patient fixates on the central target light.
Horizontal Meridian First
Rotate the instrument to align the eyepiece crosshair with the horizontal meridian (0°–180°). Adjust the drum (power wheel) until the plus and minus mires are aligned both vertically and horizontally (the "+" and "–" symbols should overlap correctly).
Read Horizontal K
Record the keratometry reading for the horizontal meridian: the power in diopters and the radius in mm at the 180° axis.
Rotate to Vertical Meridian
Rotate the instrument 90° to measure the vertical meridian (90° axis). Realign the mires — note that the image may have rotated; re-center and re-align before reading.
Read Vertical K
Record the keratometry reading for the vertical meridian: power and radius at the 90° axis. The difference between meridians represents corneal astigmatism.
Record and Report
Document as: Flat K @ axis / Steep K @ axis. Example: 43.00 @ 180° / 44.25 @ 90°. Corneal astigmatism = 44.25 – 43.00 = 1.25D WTR.
Mire Alignment Key
The B&L keratometer uses a "+" and "–" mire system. The mires are correctly aligned when: (1) the "+" mire touches the top-right corner of the "–" mire in the horizontal position, and (2) the "+" mire touches the bottom of the upper "–" mire in the vertical position. Irregular mires (distorted or non-circular) indicate irregular corneal astigmatism, surface disruption (dry eye, pterygium, scar), or keratoconus.
| Category | Radius (mm) | Power (Diopters) | Clinical Significance |
|---|---|---|---|
| Very Flat | > 8.44 mm | < 40.00 D | Post-refractive surgery (LASIK), high hyperopia |
| Flat | 7.94–8.44 mm | 40.00–42.50 D | Mildly flat, often normal variant |
| Normal | 7.50–7.94 mm | 42.50–45.00 D | Typical adult cornea |
| Steep | 7.18–7.50 mm | 45.00–47.00 D | Mildly steep; myopic cornea |
| Very Steep | < 7.18 mm | > 47.00 D | Keratoconus, pellucid marginal degeneration |
Conversion Formula: mm ↔ Diopters
D = 337.5 / r (mm)
or equivalently: r = 337.5 / D
Example 1
r = 7.50 mm → D = 337.5 / 7.50 = 45.00D
Example 2
r = 8.00 mm → D = 337.5 / 8.00 = 42.19D
Example 3
D = 44.00D → r = 337.5 / 44.00 = 7.67 mm
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Contact Lens Fitting
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IOL Power Calculation
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Keratoconus Screening
Sources of Keratometry Error
Patient-Related
Instrument/Technique
Keratometry results must be documented precisely and consistently for clinical use and medico-legal purposes.
Standard Documentation Format:
OD: 43.00 @ 180° / 44.25 @ 90° (K avg: 43.63D, Corneal Cyl: 1.25D WTR)
OS: 43.50 @ 175° / 44.50 @ 85° (K avg: 44.00D, Corneal Cyl: 1.00D WTR)
| Parameter | Definition | Clinical Use |
|---|---|---|
| Flat K (K1) | Lower diopter reading (flatter meridian) | Base curve reference for contact lenses; IOL formula input |
| Steep K (K2) | Higher diopter reading (steeper meridian) | Toric IOL/lens axis; keratoconus monitoring |
| Average K | (K1 + K2) / 2 | IOL power calculation input (most formulas use average K) |
| Corneal Astigmatism | K2 – K1 (in diopters) | Toric correction planning, corneal contribution to total Rx |
| Axis | Meridian of flat K (in degrees) | Toric contact lens/IOL orientation; WTR vs ATR classification |
Practice COA Exam Questions on Keratometry
Test your knowledge on K readings, corneal curvature, contact lens fitting, and all COA instrumentation topics.
Axial length measurement, IOL calculation formulas, and cataract surgery prep.
Contact lens fitting applications of keratometry from the NCLE perspective.
Corneal layers, curvature zones, and their relevance to contact lens fitting.
Eligibility, exam format, content domains, and preparation strategies.
What do K readings measure in keratometry?
K readings (keratometry readings) measure the curvature of the anterior corneal surface. They are expressed in both millimeters (radius of curvature) and diopters (refractive power). A typical keratometer measures the central 3mm zone of the cornea in two principal meridians — the flattest and steepest — which correspond to the axis of corneal astigmatism.
What are normal K reading values?
Normal corneal curvature values range from approximately 40.00D to 47.00D, with the most common range being 42.00D to 45.00D. In millimeters, this corresponds to roughly 7.1mm to 8.5mm radius of curvature. Values below 40D (radius >8.44mm) represent a flat cornea; values above 47D (radius <7.18mm) suggest a steep cornea and may indicate keratoconus.
How do you convert keratometry readings from mm to diopters?
The standard conversion formula is: D = 337.5 / r, where r is the radius of curvature in millimeters and D is the corneal power in diopters. For example, a radius of 7.5mm = 337.5 / 7.5 = 45.00D. Some instruments use the index of refraction constant 336 or 337.5 depending on their calibration; 337.5 is most commonly used in clinical practice.
What is the significance of "with-the-rule" vs "against-the-rule" astigmatism in keratometry?
With-the-rule (WTR) astigmatism means the vertical meridian is steeper (higher K value, 90° axis), causing the cornea to be shaped like a football oriented vertically. Against-the-rule (ATR) astigmatism means the horizontal meridian is steeper (180° axis). The axis of steepest K corresponds to the minus cylinder axis of the refraction. WTR is more common in young patients; ATR increases with age.
Why are K readings important for contact lens fitting and IOL calculations?
In contact lens fitting, K readings determine the base curve selection: the contact lens base curve is typically fitted flatter than the flattest K by 0.6–1.0mm for soft lenses, or matched to flat K for RGP lenses. For IOL (intraocular lens) calculations before cataract surgery, K readings combined with axial length measurements are entered into formulas (SRK/T, Haigis, Barrett) to calculate the correct IOL power needed to achieve the target refraction.
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