Article Date: 3/1/2004

Street Smarts
Contact Lens Mathematics
Use these simple formulas to translate what your patients say into something resembling the truth. Then prescribe contact lenses best suited to their actual wearing habits.
BY DAN BECK, O.D., Leland, N.C.

People abuse contact lenses. This comes as no great revelation to anyone. However, the degree of abuse may surprise some new O.D.s, particularly those who don't wear contact lenses themselves.

Patients will embellish the truth, exaggerate the truth and flat-out lie right to your face about their contact lens-wearing habits. They're either eager to please or afraid of being scolded.

So how do you get to the truth? For that you must apply "Contact Lens Mathematics." Here are three equations that, when properly applied, will lead you to the truth.


Equation 1. Take the stated age of the contact lenses and multiply by 2.5. For example, when a patient says, "My lenses are about 2 weeks old," you can be fairly certain the lenses are actually 5 weeks old (2 weeks x 2.5 = 5).

Equation 2. Take the stated number of nights a patient sleeps in his contact lenses and multiply by 3. Thus, when a patient says, "I leave my lenses in for about 2 to 3 nights," he's actually admitting to sleeping in his lenses for 6 to 9 nights in succession (2 to 3 nights x 3 = 6 to 9).

Equation 3. Take the stated time of a patient's last contact lens eye examination and multiply by 2 to 3. So the patient who says, "It's been about a year since my last eye exam," probably hasn't seen an eye doctor for 2 to 3 years (1 year x 2 to 3 = 2 to 3).


Once you've calculated your patient's true contact lens habits, the next step is to prescribe the best lens for those habits. Scolding or using scare tactics rarely changes a patient's behavior. People are going to do what works best for them, not what works best for their doctors. And taking contact lenses away from an abuser for any extended length of time only results in a patient lost to a competitor.

On the other hand, if you become comfortable fitting a wide array of contact lenses, you'll have happier and safer contact lens patients. New silicone hydrogel contact lenses offer much higher oxygen transmission than that of traditional hydrogels. Patients who refuse to remove their lenses during sleep now have healthy options.


With so many contact lens options, the task of putting a patient in the best possible choice can seem daunting. Sticking with just a few tried-and-true lenses may make your job easier, but it puts limits on your fitting strategies and ultimately on patient satisfaction.

Adding new lenses and subtracting your concerns about trying something unfamiliar will almost always equal a better contact lens experience for your patients.

Do the math.

Dr. Beck, a 1993 graduate of PCO, massages the numbers at his group practice in Leland, N.C. You can contact him at



Optometric Management, Issue: March 2004