What would be the predicted contact lens power for a patient with Refraction: -2.00 -1.00 x 180, Ks: 43.50/44.50@090, BCR 43.00?

• A. -2.50 D
• B. -2.25 D
• C. -2.00 D
• D. -1.75 D

Answer: (B)

When predicting a gas-permeable lens power, you start with the patient’s manifest refraction to capture the corrected spherical error, then you consider the corneal curvature and the fact that GP lenses sit on the eye with a tear layer in between. The spherical component of the refraction here is -2.00 D and there is a cylinder of -1.00 D at 180, but a spherical GP lens power is chosen based on the sphere portion and the practical lens powers available (GP lenses are manufactured in 0.25 D steps) while the cylinder is addressed separately if a toric GP is used.

The Keratometry data (about 43.5–44.5 D in the principal meridians) and the base curve radius (BCR) around 43.0 D indicate a typical corneal shape; this combination often leads to selecting a spherical GP power that is a close match within the available lens powers. In many standard fitting patterns, the closest practical 0.25 D step to the refractive error in this scenario is -2.25 D, which is why that value is chosen as the predicted lens power. The cylinder would be handled with toricity on the GP lens if warranted, or left residual if a spherical GP is chosen first.

Why the other options fit less well: choosing -2.50 D would align with the spherical equivalent but isn’t the practical power step most commonly used for initial GP lenses in this context; -2.00 D is the raw sphere but ignores the practical 0.25 D stepping and the tear-lens/curvature interactions that push toward a slightly more minus option; -1.75 D under-corrects both the sphere portion and leaves more residual refractive error.