Modifier 91
Repeat Clinical Diagnostic Laboratory Test
Repeat lab test
Important: Modifier usage depends on documentation and payer policy. Never add a modifier without supporting documentation. Always verify payer-specific rules, as modifier policies vary. The information on this page is for educational purposes only and does not guarantee payment. When in doubt, consult the official CPT guidelines, NCCI Policy Manual, and payer-specific policies.
Modifier Description
Modifier 91 indicates a repeat clinical diagnostic laboratory test. Used when a lab test is repeated on the same day for medical reasons.
Why This Modifier Exists
Modifier 91 was created to report a repeat clinical diagnostic laboratory test performed on the same day for medical reasons. Without it, the second test would be denied as a duplicate. Modifier 91 tells the payer the repeat was medically necessary — not a lab error.
Plain English Explanation
If a lab test was done and then had to be repeated the same day because results were abnormal or the doctor needed a follow-up, you add modifier 91.
When to Use
Use when a lab test is repeated on the same day for a medical reason, such as confirming abnormal results or monitoring a changing condition.
When NOT to Use
Do NOT use if the repeat is due to lab error — that should not be billed. Do NOT use for routine repeats. Do NOT use if the test was not actually repeated.
Documentation Requirements
The medical record must document the medical reason for repeating the test.
Common Denial Reasons
- CO-97 (bundled), CO-25 (duplicate), CO-16 (missing documentation), CO-50 (not medically necessary).
Common Rejection Reasons
- Rejected for using it on lab errors (which should not be billed).
- Rejected for not documenting the medical reason for the repeat.
- Rejected for routine repeats.
Common Mistakes
- Using it for lab errors.
- Not documenting the medical reason.
- Using it for routine repeats.
Example Scenarios
Critical lab value repeated to confirm — modifier 91.
Blood glucose repeated to monitor changing condition — modifier 91.
Payer Considerations
Medicare allows modifier 91 for repeat lab tests with medical necessity. Check payer policy. Documentation is essential.
Telehealth Considerations
Not applicable to telehealth.
Hospital Considerations
Hospital lab billing may use modifier 91. Check OPPS rules.
Professional Billing Considerations
For professional billing, document the medical reason for the repeat. Modifier 91 is for medical repeats, not lab errors.
Never Add Without Documentation
Never add modifier 91 to a claim without supporting documentation. Always verify payer-specific rules and consult the official CPT guidelines, NCCI Policy Manual, and payer policies. When in doubt, consult a certified coder or billing specialist.
Related Modifiers
Need Help with a Denial?
Use our AI-powered tools to investigate denials involving modifier 91.