What do the numbers on my prescription mean?

Posted on: August 1, 2016

Decoding the language and jargon of medicine can be difficult. But everyone gets curious once and a while. If you wear prescribed glasses, you may have long wondered what the numbers and coding actually mean.

Basic questions such as which prescription is for the right eye and which for the left. Why does your friend’s or sibling’s prescription differ from yours? Here are some basic explanations to unravel the mystery a bit.

You may, for example, have seen “OD” and “OS” on your prescription. These are from the Latin Words Oculus Dextrus, which means right eye, and Oculus Sinister, which means left eye. The use of Latin is a common practice in medicine. Words from Latin and Greek are routinely used for descriptions, conditions, and prescriptions. The field of optometry is no different. You may also see the letters “OU.” This means Oculus Utro and refers to a condition involving both your eyes.

In general, the numbers on a prescription are best interpreted in the following manner: the further the number is from zero the worse your eyesight. Specific conditions vary. And the number you see before you can mean anything from your needing a higher quality of eyeglasses to a prescription for Toric contact lenses to combat astigmatism.

The first number represents the diopter, which is the unit measuring the correction needed for the eye to properly focus. A plus sign means you’re farsighted, or hyperopic. A minus sign means you are nearsighted, or myopic. If there are three numbers in your prescription you have an astigmatism, or an irregularly shaped cornea requiring further vision correction. The second and third numbers in the line refer to the “cylinder”, or astigmatism itself, and the “axis”, the orientation of the astigmatism on the cornea.

Your physician will of course know what to prescribe for your given condition. However, you should maintain a working knowledge of how it all works. The more you know about the ways and methods of your optometrist the better you will be able to monitor yourself and have the kind of conversation that can lead to a correct diagnosis of a worsening condition and the right solution for it.

A bit of study and practice will make all these matters clearer to you. With all of the information available on the web, no one should be completely in the dark about their health and the things being done to look after it. Although you may have a competent, well-qualified, well-credentialed eye doctor, you should always keep yourself as educated as you can about this branch of medicine. You should certainly have a working knowledge and understanding of what you are being prescribed. This is not only a good means of holding your physician to account; it will also ensure that the vendor who sells you the eyeglasses fills your prescription correctly. The more knowledge you have about what the numbers and codes mean the better you will be able to deal with your eye condition and the less dependent you will be on others.