Modifier 91: Repeat clinical diagnostic laboratory test
When it is appropriate
Append modifier 91 to a repeated clinical diagnostic laboratory test to show that the same test was medically necessary to run again on the same day on the same patient, such as monitoring a changing lab value, and is not a duplicate charge.
When it is not appropriate
Do not use 91 to rerun a test because of a specimen problem or equipment malfunction, or simply to confirm an initial result; those situations are not separately payable repeat tests.
CMS preference
There is no X modifier alternative for repeat lab testing; 91 is the modifier CMS expects for this specific circumstance.
Retrieved 2026-07-18.
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