Transposing an Rx refers only to prescriptions where there is a cylindrical component. Spherical prescriptions (no cylinder and no axis) cannot be transposed. There are three steps to transposing an Rx and each eye must be transposed individually.
Example:
| Original Rx |
+2.50 -1.50 x 20 |
The example Rx above is in 'minus cylinder form' and can be converted to 'plus cylinder form' by following the 3 steps below.
NOTE: It does not matter if you are converting the cylinder from minus to plus or from plus to minus, the exact same steps are followed.
1) Add the sphere and cylinder components together algebraically
| New Sphere |
= original sphere + original cylinder |
| |
= +2.50 + (-1.50) |
| |
= +2.50 - 1.50 |
| Answer |
= +1.00 |
2) Change the sign of the cylinder:
If the cylinder is + change it to -, if the cylinder is - change it to +
| New Cylinder |
= the same absolute value of the original cylinder but the opposite sign |
| |
= - (original cylinder) |
| |
= - (-1.50) |
| Answer |
= +1.50 |
3) Change the axis by 90 degrees:
Either add 90 degrees to the axis or subtract 90 degrees from the axis. Use the result that is in the range between 1 and 180 degrees.
- If the original axis is less than or equal to 90 degrees then add 90
- if the original axis is greater than 90 degrees then subtract 90.
| New Axis |
= the original axis + or - 90 |
| |
= 20 + 90 = 110 YES |
| |
= 20 - 90 = -70 NO |
| Answer |
= 110 |
The new Transposed Rx, which is identical to the Original Rx:
| Original Rx |
+2.50 -1.50 x 20 |
| Transposed Rx |
+1.00 +1.50 x 110 |