Modifier 91 is used when a diagnosis needs to take multiple tests and results on a patient on the same day. But there are limitations to this as one cannot use Modifier 91 when medical practitioners found out that the equipment or specimen is malfunctioned. You cannot append Modifier 91 just to confirm the initial result on the test. If this is abused, violators will be penalized and even have their contracts terminated.
This should be used properly and be followed all the times as this cannot be used just for the sake of not accepting the initial result or malfunctions equipment on the same day. Problem with the patient might arise and correct results might not be obtained if this is not correctly.