A patient's Goldmann applanation IOP reads 24 mmHg. CCT is 495 µm. The Ehlers correction formula adjusts this to an estimated true IOP of approximately:

• A 27 mmHg
• B 28 mmHg ✓
• C 22 mmHg
• D 20 mmHg

Explanation

Goldmann applanation tonometry is calibrated for a CCT of 520 µm (Ehlers correction baseline). For each 10 µm thinner than 520 µm, measured IOP underestimates true IOP by approximately 0.7 mmHg per µm deviation (various correction factors range from 0.7–1.0 mmHg per 10 µm). With CCT 495 µm (25 µm thinner than 520 µm): correction ≈ 25 × 0.15 = ~3.75 mmHg (using ~0.15 mmHg/µm), giving approximately 27–28 mmHg. A commonly used rule: 500 µm CCT has true IOP ~2 mmHg higher than measured. The patient's true IOP ~28 mmHg is significantly elevated, altering glaucoma management decision-making toward earlier treatment.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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