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Modifier 91

Definition

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.

When to Use

  1. First glucose test (82962) shows abnormal result; physician orders immediate recheck same day (82962-91)
  2. Urinalysis specimen contaminated (81002); lab repeats with new specimen same day (81002-91)
  3. Blood culture flagged positive for contaminant; repeat culture ordered same day (87040-91)
  4. Potassium level critically high; stat recheck ordered within 2 hours (84132 first, 84132-91 second)

Documentation Requirements

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-Specific Rules

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.
  • 76 – Repeat procedure same physician. Use for non-lab repeats.
  • 77 – Repeat procedure different physician. Use for non-lab repeats by another provider.
  • 25 – Significant E/M. Use if separate E/M provided with lab order.

Common Denials

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.

FAQ

Can I use modifier 91 if the second lab test is from a different lab?

No. Modifier 91 is same lab. If different lab, do not use 91; submit separate claim.

What if the second test is ordered but not resulted yet?

Do not bill modifier 91 until test is actually performed and results are available.

How soon after the first test can the second test be ordered?

Modifier 91 implies same day or immediate clinical need. If repeat is days later, do not use 91.

Prevent These Denials

Reduce lab repeat denials. A co-pilot ensures your clinical documentation supports medical necessity.

Related Resources

This reference is current as of 2026-03-23. Payer policies change. Always verify against the payer's latest policy documentation.
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