What is required for a repeat clinical diagnostic test to be coded with Modifier 91?

Using Modifier 91 is appropriate when coding for repeat clinical diagnostic tests that are performed on the same patient. The crucial requirement for applying this modifier is that the tests must share the same CPT code. This modifier specifically indicates that the repeated test was necessary for clinical management, often due to the need for additional information or to confirm initial results.

When a test is done more than once for the same patient, it is important for clarity in billing and documentation to denote that these are indeed repeats of the same procedure. In contrast, differing patients, providers, or locations would not validate the use of Modifier 91, as the modifier is solely concerned with the repetition of the identical test on the same individual.