Modifier 91 is used to indicate that a clinical laboratory test is repeated on the same patient by the same lab on the same day. The repeat test is performed because the initial specimen was inadequate, results were inconsistent, or medically necessary re-evaluation occurred. Modifier 91 tells the payer the second test is clinically justified, not a billing error.
Document reason for repeat in physician's chart and lab requisition. Note if first result was out of range, specimen was contaminated, or clinical change required re-evaluation. Lab should note both results with timestamps. Clinical decision-making for repeat should be clear (e.g., 'Recheck potassium stat due to critical level concern').
| Payer | Acceptance | Common Denials | Notes |
|---|---|---|---|
| Medicare | Accepted with clinical justification | CO-12: Service not payable when performed in facility | Allow 91 for medically necessary repeats. Deny if no clinical reason evident. |
| Aetna | Accepted; routine acceptance | CO-16: Lab test not medically necessary | Will pay. May require documentation of clinical need on appeal. |
| United Healthcare | Accepted; fewer denials than other repeats | CO-151: Insufficient documentation | Lab repeats have high approval. Deny mainly for frequency outliers. |
| Cigna | Accepted with documentation | CO-8: Service denied based on plan | Review medical necessity. Pay if result variance large enough. |
| Humana | Accepted routinely | CO-3: Lab service not covered | High approval rate. Deny mainly on plan coverage, not 91 logic. |
| CARC Code | Reason | Primary Cause |
|---|---|---|
| CO-12 | Service performed in facility | Lab test billed from hospital facility; payer deems globally included. |
| CO-16 | Test not medically necessary | No clinical justification evident; repeat appears planned rather than medically driven. |
| CO-151 | Documentation insufficient | Claim lacks reason for repeat test. |
No. Modifier 91 is same lab. If different lab, do not use 91; submit separate claim.
Do not bill modifier 91 until test is actually performed and results are available.
Modifier 91 implies same day or immediate clinical need. If repeat is days later, do not use 91.
Reduce lab repeat denials. A co-pilot ensures your clinical documentation supports medical necessity.