Rx Interpretation, What it Really Means

As I outlined in my previous post, Rx Interpretation, Part 1, The most important part of interpreting the patient’s Rx is knowing what the Rx means. When we take the patient’s script in hand, and look at it, what’s on it makes sense to us. We instantly:

  1. understand the patient’s problem.
  2. envision the products that would be best for them.
  3. know how to recommend those products to the patient.

That is the true, PRACTICAL, essence of Rx interpretation, the thing we do every time we encounter a patient. This is the part we think is real easy, what we do every day.

But, of course, it’s not all that simple, is it? I mean, if it were something any clerk could do after being hired off the street, would there be a need for Certification? (See Why should You Become Certified?) Would there be a reason for taking National Boards for which you have only a 63% chance of passing with a score of 70, and for which you must pay a fee? Obviously what we do requires specific knowledge and expertise. It requires more than just knowing our product line and being able to write up an order. We have to know some of the science behind the products. We have to know how to work with those numbers.

Here’s where some of that MATH so many of you are obsessing over comes in!

Rx Interpretation: Manipulating the Numbers

Unlike the sort of prescriptions our pharmacist cousins have to decipher, an optical script is mostly numerical. We assess the “plus” or “minus” values of sphere and cylinder and note the degree of “axis”. We note if the numerical values are high or low. Many times we have to manipulate the numbers, and we have to know how to do it. Take the average presbyopic Rx. If the doctor writes:

+1.00 -0.75 x 90
+0.75 -1.00 x 180
Add +2.50

We know that this is a multifocal Rx that can be filled with conventional (lined) bifocals or with progressives, but we also know it can be filled with:

  1. Reading only (add the reading add power to the distance sphere)
  2. Intermediate/Computer only (add half the add power to the distance sphere)
  3. Variable Focus Readers (use the full reading Rx with a design of appropriate degression)
  4. Occupational Line Bifocal (intermediate in distance portion and full reading in seg)
  5. Occupational Line bifocals with intermediate power in the lower seg
  6. Double D occupational segs (with specific powers for each seg)

Many of us work with ordering software that makes it simple to fill NVO orders. We just check some box labeled “reading only” and all the math is instantly done for us! That doesn’t, however, let us off the hook. We should already have done all the calculations in our head before we actually went to the computer. If the patient’s distance sphere power is +2.00 and their add is +2.50 their total power is going to be a whopping +4.50. If the patient selects a size 60mm aviator rimless frame for their reading glasses, they might not be enchanted by the result!

Though most ordering software doesn’t automatically compute the intermediate power, opticians have the authority to deduce the intermediate from the full written Rx. You don’t always have to call the doctor. It might be good to do that in cases where the add won’t divide equally, but even then it is not absolutely necessary. You don’t have to delay the patient’s order (sometime for days!) just to get through to a busy specialist in order to ask them to divide a +1.75 add in half! That’s considered our job. Most of us go up +0.25 in the intermediate.

Rx Interpretation and Toric Transposition

Lenses are ordered and manufactured in minus cylinder form. That was done originally to make them fit better into frames. So what’s with all the doctors who persist (annoyingly) in writing plus cylinder prescriptions?

I’ve asked a lot of doctors over the years why so many keep writing in plus cyl and why it is largely the opthamologists who do it. The answers ranged from the schools the doctors were trained at, the equipment used, to “sheer, blamed stubbornness.” The best answer I ever got is that opthamologists measure the patient’s ocular problem while the optometrists measure how to correct the problem (as with lenses) which is not always exactly the same. Well, whatever. Since lenses come in MINUS CYLINDER, we need all prescriptions to be transposed into minus cyl if they’re not in it already.

Now for most of you, this is probably “old hat” as they say. You probably do this every day! You know that:

+1.00 +2.00 x 90

can be transposed to minus cylinder by adding the sphere and cylinder together algebraically, changing the sign of the cylinder, and changing the axis by a factor of 90º.

(+1.00 +2.00) -2.00 x (90º +90º) = +3.00 -2.00 x 180

Really simple, isn’t it? And if you see a question on your NOCE like this you will know exactly what to do. But what if you see something like… this?

+1.00 x 180 / +3.00 x 90 How do you handle this?

Well, the first thing you do is don’t panic! This is called crossed cylinder form and there is a formula to change it into conventional notation. In crossed cylinder form, each power is expressed in terms if its axis, not its meridian, so the axis combined with each power actually corresponds to the power 90º away from it. Following me? Let’s do some optical math and make each side of the crossed cylinders look like an equation.

+1.00 ___ x 90 = +3.00 ___ x 180

The value you put in the first blank must make the first sphere add up to the second sphere, and the value in the second blank must make the second sphere add up to the first. This way you get a completely transposed Rx showing both plus and minus cylinders looking like this:

+1.00 +2.00 x 90 = +3.00 -2.00 x 90

But what if you see something like this on your exam?

+1.00 @ 180 / +3.00 @ 90

That’s NOT crossed cylinder form. It looks like the K readings that get printed from our auto-refractors, doesn’t it? Well it’s sort of the same thing. In this form of antiquated notation each power is expressed in terms of its meridian, not its axis. The power in the 180º meridian is +1.00 in this case, and the power in the 90º meridian is +3.00. So we set up our equation differently:

+1.00 ___ x 180 = +3.00 ___ x 90
+1.00 +2.00 x 180 = +3.00 -2.00 x 90

Transposition on Your Exam

But are you really likely to see transposition questions on your ABO certification exam?


Transposing prescriptions is something we have to do each day. Most of the time it is basic and routine. The doc writes in plus cylinder and we have to order in minus. If there is significant stig in the script, we may need to do some geometrical figuring in our heads as to where the greatest thickness will be in the finished lens. We may need to recommend a frame with a short “B” measurement (higher frame difference) or a smaller “A” (lower eye size.)

The questions you see on your exam may not be straight forward transposition questions. They may have to do with understanding where the meridian of greatest power is and then have you choose lens or frame recommendations accordingly.

There may be questions regarding an Rx where some of the answers are in plus cylinder and others are in minus, leaving you to transpose them all to either minus or plus to figure the answer.

And you may see some antiquated notation. These tests are supposed to be DIFFICULT. And they are! Only 64% of applicants score a 70 or better on this exam. You don’t need an A or a B to pass, just a C, and even that is hard given that just over half ever achieve it…

That’s what this blog is all about, and that’s what Passyouropticalboards is all about! We are here at this site to help you prepare for your Boards.

If you see antiquated notation, just remember: If there is a @ involved, the axis IS the meridian of that power, the place where that power is felt. If an “X” is used instead of a @ symbol, think just the opposite! Use the other axis.

Happy studying! And make sure you practice your toric transposition and Rx interpretation!