CPT Modifier 91
Repeat Clinical Diagnostic Laboratory Test
Source: AMA CPT / CMS HCPCS Level II definitions. Maintained by the D3rx Clinical Billing Team.
What modifier 91 means
Modifier 91 indicates a clinical diagnostic laboratory test was repeated on the same day to obtain subsequent (serial) results when medically necessary. It distinguishes legitimate serial testing from a duplicate, a confirmatory re-run, or a re-test due to laboratory error.
When to use it
Medically necessary serial same-day testing to monitor a changing value — for example, serial troponins or repeated potassium levels after treatment.
Documentation checklist
The record should support every item below before you append modifier 91.
- The time and result of each draw
- The medical necessity for serial testing
- That the repeat is not a re-run for QC, equipment, or specimen error
Do NOT use modifier 91 when
- The test was repeated due to a lab error or for confirmation/quality control
- Only a single test was performed
- A code already specifies the number of tests (e.g., a defined panel)
Common denial reasons
- Used for confirmatory re-runs rather than true serial testing
- Documentation lacks separate draw times and medical necessity
- Denied as a duplicate test
How to appeal a modifier 91 denial
Submit the time-stamped serial results and a note establishing medical necessity for repeating the test (e.g., monitoring a trending value during treatment). Make clear the repeats were not re-runs for lab error or confirmation, which 91 does not cover.
Payer notes
Modifier 91 is not for re-running a test because of laboratory error or to confirm a result; it is for medically necessary serial testing. Payers may limit the number of repeats and request documentation.
Related & commonly confused modifiers
Where modifier 91 is used
- Clinical chemistry serial analytes (e.g., potassium, glucose)
- Serial cardiac markers (e.g., troponin)
- Hematology (e.g., 85025) repeated for monitoring
Look up these codes & their 2026 Medicare rates
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Medical billing disclaimer
Modifier definitions follow standard AMA CPT and CMS HCPCS Level II guidance and are for educational reference. Payer policies, billing formats, and coverage rules vary and change. Always verify the current rule with the specific payer before submitting. D3rx is not responsible for claim outcomes.