Modifier -91 is used to denote what in laboratory testing?

• A. Prolonged service.
• B. Repeat laboratory test to obtain subsequent results.
• C. Unrelated procedure.
• D. Emergency stand-alone.

Answer: (B)

Modifier 91 is used when a clinical diagnostic laboratory test is repeated on the same patient and on the same day to obtain subsequent results. This signals to payers that the second test is not a new or initial study, but a repeat to verify or update the result. For example, if a glucose test is drawn and then re-tested later the same day to confirm the value, the repeat test would be reported with modifier 91. It’s not about longer service, unrelated procedures, or emergency stand-alone scenarios, which are covered by other modifiers or coding rules.