7.2 Core Workflows and Decision Points

7.2 Core Workflows and Decision Points

Keratometry is a procedure with a fixed sequence. The COA must perform each step in order, because skipping calibration or tear-film prep introduces errors that look like real astigmatism.

Step 1: Focus the eyepiece (zero the reticle)

Before touching the patient, focus the eyepiece on the keratometer reticle (cross-hairs). Turn the eyepiece fully counterclockwise (toward plus) to fog, then slowly clockwise until the cross-hairs are sharp and black. This relaxes the examiner's accommodation so it does not shift the measured radius. If two technicians get different readings on the same eye, an unfocused eyepiece is the usual culprit.

Step 2: Position the patient and instrument

Seat the patient with chin in the chin rest and forehead against the bar, eye aligned with the canthus marker. Occlude the eye not being measured. Have the patient look at their own eye's reflection in the center of the barrel. Adjust the instrument up, down, and forward until the central mire is sharply focused.

Step 3: Optimize the tear film

The keratometer reads off the tear-film reflection, so a dry or broken tear film distorts the mires. Ask the patient to blink fully just before each reading. Distorted or fluctuating mires that clear after a blink point to dry eye, not astigmatism. In contact-lens wearers, remove lenses and wait so the cornea is not warped.

Step 4: Align and read both meridians

Most instruments superimpose a plus sign and minus sign (or two circles). The procedure:

If the plus and minus mires both align at 180/090, the cornea is spherical. If you must rotate the axis away from 180/090 to align them, the astigmatism is oblique. If the mires cannot be made to overlap cleanly at any rotation and look bent or doubled, suspect irregular astigmatism (e.g., keratoconus, scar, prior surgery).

Step 5: Classify the astigmatism

Record the steeper meridian and classify it:

• With-the-rule (WTR): steep meridian at or near 90 degrees (vertical). Common in younger patients from eyelid pressure. Minus-cylinder axis near 180.
• Against-the-rule (ATR): steep meridian at or near 180 degrees (horizontal). More common with age. Minus-cylinder axis near 090.
• Oblique: steep meridian between 30-60 or 120-150 degrees.

Decision points and handoffs

The highest-error handoff is recording: write both K values with their axes, not just the average. For IOL calculation the surgeon needs the steep and flat K and the axis; an averaged single number is incomplete. If readings are unstable across repeats, do not average noise; re-prep the tear film, refocus the eyepiece, and re-measure. Documentation that flags 'mires distorted, possible irregular cornea' is more useful than a falsely precise number, because it routes the patient to corneal topography.

Aligning the mires in detail

On the Bausch & Lomb single-position keratometer, the field shows three circles: a central focusing circle flanked by a plus sign on the right and a minus sign at the top. To measure the horizontal meridian, turn the horizontal measuring drum until the two plus signs exactly superimpose. To measure the vertical meridian, turn the vertical drum until the two minus signs superimpose. The axis wheel is rotated first so the plus and minus arms lie along the true principal meridians; you know they are correct when, in an astigmatic eye, the plus signs sit on a continuous line and the minus signs sit on a continuous line.

If the plus signs step up or down relative to each other (a vertical mismatch), the axis is off and the barrel must be rotated, not the power drum. Confusing an axis misalignment for a power difference is a classic procedural error.

Calibration and maintenance

Keratometers should be calibrated against steel test spheres of known radius, typically supplied with the instrument (for example a sphere reading near 43.00 D). If the device reads the test sphere incorrectly, it must be serviced before patient use; otherwise every K reading carries a fixed offset that quietly corrupts IOL calculations. The technician should also clean the optics and confirm the chin rest and forehead bar move smoothly so patient alignment is reproducible. Document calibration checks per clinic policy.

When to repeat versus when to refer

Repeat a reading when the cause is fixable in the chair: an unfocused eyepiece, a dry tear film, a blink, a tilted instrument, or a wandering fixation. Refer (or switch to topography) when the cornea itself is the problem: persistently irregular mires, readings off the instrument scale, recent refractive surgery, or a known ectasia. Choosing 'repeat' when the stem describes irregular mires is a trap; the right move is to recognize the corneal cause and escalate.