When to use Modifier 91? Modifier 91 is used for the reporting of repeat laboratory tests or of studies that are performed on the same patient on the same day. You will only add Modifier 91 when additional tests results are to be subsequently obtained to the initial administration or the performance of the tests done on the same day. When Not to Use Modifier 91 Modifier 91 is not used when laboratory studies or tests are rerun as a result of equipment or specimen malfunction or error. It is also not used when a test is repeated to confirm the results that the initial test reported. In addition, based …