A 60-year-old post-cataract surgery patient has a posterior chamber IOL in the bag. The axial length is 25 mm. Using the SRK/T formula, the calculated A-constant is 118.4. If the keratometry reading is reduced by 1.0 D from the true value due to measurement error, approximately how much IOL power error is introduced?

• A 0.5 D error in IOL power
• B 0.7 D error in IOL power ✓
• C 1.5 D error in IOL power
• D 1.0 D error in IOL power

Explanation

In the SRK/T formula, a 1.0 B error in keratometry produces approximately 0.7 B error in the calculated IOL power for longer eyes (axial length > 24.4 mm). The relationship is not 1:1 because the formula's effective lens position and optical model distribute the error. Shorter eyes have a proportionally greater IOL error per B of K error, while longer eyes have a smaller proportional effect.

Reference: AAO BCSC Section 3: Clinical Optics, 2023-2024 ed.

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Written and medically reviewed by the StethoPrep medical team.