Modifier 91 Repeat clinical diagnostic laboratory test is used to report the same lab test when performed on the same patient, on the same day, to obtain subsequent test results.

Modifier 91 causes a lot of confusion when differentiating its use from that of modifier 59 Distinct procedural service. When reporting lab procedures, modifier 59 is used when the same lab procedure is done, but different specimens are obtained, or the cultures are obtained from different sites.

The June 2002 CPT Assistant provides a great example of the correct use of modifier 91:

A 65-year-old male patient with diabetic ketoacidosis had multiple blood tests performed to check the potassium level following subsequent potassium replacement and low-dose insulin therapy. After the initial potassium value, three subsequent blood tests were ordered and performed on the same date following the administration of potassium to correct the patient’s hypokalemic state.

Coding for this scenario is:

84132 Potassium; serum, plasma or whole blood




Per CPT® guidelines, modifier 91 is not to be used to report lab tests that are repeated to confirm the initial results, due to malfunctions of either the testing equipment or the specimen, or when another appropriate one-time code is all that is needed to report the service. If the test is rerun to confirm the initial results or because of a malfunction of the equipment, the service cannot be coded and modifier 91 would not apply.

If multiple tests are run, but a single code describes the test, only one code should be reported and modifier 91 would not apply. For example, 82951 Glucose; tolerance test (GTT), 3 specimens (includes glucose) includes three specimens. Therefore, if three specimens were obtained during the encounter, only 82951 would be reported.