Modifier 91
Repeat clinical diagnostic laboratory test on the same day for the same patient
✅ When to Use Modifier 91
When the same lab test is repeated on the same day for clinical reasons (not equipment malfunction).
⛔ When NOT to Use Modifier 91
Do NOT use for rerun due to testing problems.
📋 Documentation Requirements
Document clinical reason for repeat.
Coding Tips for Modifier 91
Real-world specialist guidance from the PayerReady Medical Coding Team, including audit triggers, denial patterns, and payer policy variations.
Repeat clinical diagnostic lab test on the same day. Used when a second result is medically necessary (serial troponins, glucose curves). Cannot be used to indicate a redo because the first run was technically flawed, that goes to QW or repeat without modifier per lab policy.
Modifier 91 FAQ
What does modifier 91 mean? +
Repeat clinical diagnostic laboratory test on the same day for the same patient
When should I use modifier 91? +
When the same lab test is repeated on the same day for clinical reasons (not equipment malfunction).
When should I NOT use modifier 91? +
Do NOT use for rerun due to testing problems.
What documentation is required for modifier 91? +
Document clinical reason for repeat.
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Reviewed by the PayerReady Medical Coding Team
Verified against the CMS 2026 code set on July 16, 2026.
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