Certified Ophthalmic Medical Technologist (COMT) Practice Exam

Using Kestenbaum's Rule, what is the required lens power for a patient with a visual acuity of 20/400?

1. +10 D

2. +15 D

3. +20 D

4. +25 D

The correct answer is: +20 D

Kestenbaum's Rule is a useful method in ophthalmology for determining the appropriate lens power needed for patients based on their visual acuity. The rule provides a way to estimate how much magnification a patient requires in order to improve their visual function to a better level, often aiming for a visual acuity of 20/40 or better. For a patient with a visual acuity of 20/400, Kestenbaum's Rule can be applied to ascertain the necessary lens power for correction. The rule states that for every doubling of visual acuity, the power required increases by 5 diopters. Since 20/400 equates to a situation where the person can only see at 20 feet what a person with normal vision can see at 400 feet, this denotes a significant level of visual impairment. By juxtaposing 20/400 against a target of 20/40, the visual acuity must improve by a substantial factor. Going from 20/400 to 20/200 requires an approximate lens power of +5 D, then again to 20/100 adds another +5 D, and transitioning from 20/100 to 20/40 necessitates an additional +10 D. Thus, cumulatively,