Decoding Your Contact Lens Prescription

Imagine this: you just had your annual eye examination and received a copy of your contact lens prescription. You glance down at the page, and it looks like this:

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Example of a contact lens prescription

With so many abbreviations and numbers (and not to mention the plus and/or minus signs), it can be confusing. While it may look like a secret code, your prescription is really quite simple to decipher. That’s why I put together the following guide to help you decode your contact lens prescription.

1. Brand Name

This is the specific brand of contact lenses your eye doctor has prescribed to you. It is not interchangeable with another brand, even if it is made by the same manufacturer. According to U.S. laws, contact lens retailers cannot make substitutions to the prescribed brand without the prescriber's authorization.

In the case of a private label contact lens, the prescription should include the name of the manufacturer, trade name of the private label brand, and if applicable, trade name of an equivalent brand name.

2. OD, OS, OU

This represents which eye the prescription is written for. "OD" is the abbreviation for "oculus dexter," the Latin term for "right eye." "OS" stands for "oculus sinister," or "left eye." Rarely, you'll see "OU" which means "oculus uterque" or "each eye," indicating both eyes are the same. Nowadays, these abbreviations are commonly being replaced by "RT" and "LT" because they're easier to decipher (right eye and left eye, respectively).

3. Base Curve (BC)

This is the back curvature of a contact lens, measured in millimeters. It is designed to match the curvature of your cornea (the clear front surface of your eye). Base curve numbers usually range from 8 to 10. The lower the number, the steeper the curve of your cornea.

4. Diameter (Dia)

This is the distance from one edge of your contact lens to the opposite edge, measured in millimeters. Diameter numbers usually range from 13 to 15. The lower the number, the smaller the lens.

5. Refractive Power (PWR or SPH)

This is the amount of power (measured in diopters) that you need to correct your vision. The further the number is from zero (known as "plano"), the more correction you need. A minus sign ("-") is for a nearsighted (myopic) correction, whereas a plus sign ("+") is for a farsighted (hyperopic) correction.

6. Toric Power (Cyl, Axis)

This represents the cylinder power (measured in diopters with a minus sign) and axis orientation (measured in angle degrees) needed to correct for astigmatism. It is only found in toric contact lenses. If the cylinder column is blank (or says "SPH") then you don't have any astigmatism or it is too slight to need correction. Axis numbers usually range from 1 to 180.

7. Add Power (Add)

This represents the increase in magnifying power (measured in diopters with a plus sign) needed to correct for presbyopia. It is only found in bifocal and multifocal contact lenses to see clearly up close. It is usually stated as "low," "medium," or "high," which refers to a range of dioptric powers that can differ from one manufacturer to another.

8. Color

This is included only if the contact lens brand changes or enhances your eye color. It is not the same as the visibility tint, which is intended to help you find your lens should you drop it. Color options and styles vary by brand.

9. Issue & Expiration Date

By law, contact lens prescriptions are valid for 12 months after the date of when the prescription was written. When your prescription expires, you won't be able to buy more lenses until your eye doctor writes an updated prescription. This will involve an eye exam to check your general eye health and a contact lens evaluation to be certain that the lenses aren't adversely affecting your eyes.

Additional Rx Information

In addition to the aforementioned abbreviations and numbers, contact lens prescriptions should include the name of the patient, name of the prescriber, and the office's contact information.

It is important to note that contact lens and eyeglass prescriptions are not the same. You need a separate contact lens prescription in order to purchase contact lenses.

Related Post: Decoding Your Eyeglass Prescription

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