As discussed in previous articles, there are several modifiers that routinely give trouble to even experienced coders. Chief among these are modifier 59 and modifier 25, but these two are only the beginning. Next in line is modifier 91, used to report “repeat laboratory tests or studies performed on the same day on the same patient.”

To start, this modifier is only applicable when the repeat test is ordered by the same provider, meaning that follow-up tests by a specialist would not qualify for modifier 91. Furthermore, the tests must be performed on the same day but at different times, meaning that multiple tests performed concurrently would also not qualify.

Additionally, modifier 91 should not be used if the test is repeated due to outside circumstances or issues. This means that if the machines in the lab malfunction and produce inaccurate results, redoing the test would not use this modifier. The same rules apply to problems stemming from the sample itself.

Finally, as with all modifiers of this type, 91 should not be used if the main code already contains all the relevant information. In other words, if the base CPT code already accounts for multiple tests being performed, then there is no need to add modifier 91.

So when is the proper time to use 91? CareCloud lists the following example: a patient with high blood pressure is maintaining a low-salt diet and needs a plasma renin activity (PRA) test. Since PRA levels vary depending on time of day and position, the provider may order a morning test with the patient lying down and an afternoon test with the patient standing up in order to provide a more complete assessment. In this case, affix modifier 91 to the second usage of the base CPT code in order to account for the repeated test.