Modifier 91 (Repeat Clinical Diagnostic Laboratory Test) is used when the same clinical diagnostic laboratory test is performed more than once on the same patient on the same date of service to obtain additional medically necessary test results during the course of treatment.

The purpose of Modifier 91 is to distinguish medically necessary repeat laboratory testing from duplicate billing. It is commonly used in emergency departments, intensive care units (ICUs), inpatient settings, and outpatient facilities where serial laboratory monitoring is essential for patient management.

Modifier 91 must not be used when a laboratory repeats a test merely to verify an initial result, correct an equipment problem, or resolve a specimen issue. It is intended only for obtaining new, reportable clinical results.

Modifier Number

Modifier Name

Repeat Clinical Diagnostic Laboratory Test

Note: The official CPT® descriptor is maintained by the American Medical Association (AMA). Always refer to the current licensed CPT® codebook for the official wording.

Plain English Explanation

Modifier 91 tells the payer:

“The same laboratory test was medically necessary and repeated on the same patient, on the same day, to obtain new clinical information for treatment.”

Purpose of Modifier 91

Modifier 91 is used to:

Report medically necessary repeat laboratory testing.
Distinguish repeat testing from duplicate billing.
Support reimbursement for additional reportable laboratory results.
Document serial monitoring during treatment.
Comply with Medicare and commercial payer requirements.

Understanding Modifier 91

Modifier 91 applies only when:

The same laboratory CPT® code is repeated.
The same patient is tested.
The testing occurs on the same calendar day.
Each repeat test produces a new, medically necessary result used for patient care.

Common situations include:

Serial blood gas analysis.
Repeated blood glucose testing.
Electrolyte monitoring.
Potassium monitoring.
Repeat coagulation studies.
Repeat cardiac laboratory testing.

Modifier 91 is not intended for quality-control reruns or duplicate testing without medical necessity.

When to Use Modifier 91

Modifier 91 is appropriate when:

A physician orders the same laboratory test multiple times on the same day.
Each repeat test provides clinically necessary information.
The laboratory reports separate test results.
Medical necessity is documented.
The CPT® code does not already describe a series of tests.

Common Examples

✔ Serial arterial blood gas (ABG) testing.

✔ Repeat blood glucose monitoring.

✔ Repeat serum potassium testing.

✔ Repeat sodium testing.

✔ Repeat troponin testing.

✔ Repeat creatinine monitoring.

When NOT to Use Modifier 91

Do not use Modifier 91 when:

A test is repeated to confirm the initial result.
The specimen is rerun because of laboratory error.
Equipment malfunction requires retesting.
Quality-control testing is performed.
Another CPT® code already represents a testing series (such as glucose tolerance testing or provocative/suppression testing).

CMS specifically prohibits Modifier 91 for reruns performed because of testing problems or when only one reportable result is required.

Medicare Rules

Medicare requires:

Modifier 91 only for laboratory tests paid under the Clinical Laboratory Fee Schedule (CLFS).
The repeat test must be medically necessary.
Each repeated test must generate a separate reportable result.
Modifier 91 should not be appended to the initial test—only to the subsequent repeat test(s).
Modifier 91 does not replace other modifiers such as RT, LT, or 50.

Commercial Insurance Rules

Commercial payer policies generally follow Medicare guidance.

Many insurers:

Recognize Modifier 91.
Require documentation supporting medical necessity.
Review serial laboratory orders.
May request medical records for repeated testing.
Deny claims if Modifier 91 is used for duplicate or confirmatory testing.

Always verify payer-specific laboratory billing policies.

Documentation Requirements

Documentation should include:

Physician’s order.
Date and time of each test.
Medical necessity.
Laboratory results for each test.
Clinical reason for repeating the test.
CPT® code.
Diagnosis supporting repeat testing.
Laboratory report.

Real Billing Examples

Example 1 – Blood Glucose Monitoring

A patient in the emergency department has blood glucose measured at 8:00 AM.

Because insulin therapy is started, the physician orders another glucose level at 10:00 AM.

Billing:

82947
82947-91

Modifier 91 is appropriate because the second test provides a new medically necessary result.

Example 2 – Arterial Blood Gas

An ICU patient undergoes arterial blood gas testing three times during ventilator management.

Billing:

82803
82803-91
82803-91

Each repeat test produces new reportable values used for treatment.

Example 3 – Potassium Monitoring

A patient receiving intravenous potassium replacement has potassium levels checked every four hours.

Each laboratory test represents a medically necessary repeat.

Modifier 91 is appropriate on the subsequent repeat tests.

Example 4 – Incorrect Use

A laboratory reruns a potassium specimen because of analyzer malfunction.

Modifier 91 should NOT be reported, because no new reportable clinical result is being obtained.

CMS-1500 Claim Example

Field	Example
CPT Code	82947
Modifier	91
Diagnosis Pointer	Appropriate ICD-10-CM diagnosis
Units	1

Billing Example

Line	CPT® Code
First Test	82947
Second Test	82947-91
Third Test	82947-91

Common Denial Reasons

Modifier 91 appended to the initial laboratory test.
Repeat testing performed only to verify initial results.
Duplicate claim submission.
Missing medical necessity documentation.
CPT® code already describes a testing series.
Test not payable under the Clinical Laboratory Fee Schedule.

How to Correct the Denial

Review the EOB or ERA.
Confirm the repeated test was medically necessary.
Verify the repeat produced a new reportable result.
Ensure Modifier 91 appears only on repeat laboratory tests.
Submit supporting laboratory reports and physician documentation.
Appeal with complete clinical documentation if appropriate.

Coding Tips

Use Modifier 91 only for repeat laboratory tests.
Append Modifier 91 only to subsequent tests.
Never use Modifier 91 for confirmatory reruns.
Do not use Modifier 91 when another CPT® code already describes serial testing.
Verify payer-specific laboratory billing requirements.

Modifier 91 vs Modifier 90

Modifier	Purpose
90	Laboratory test performed by an outside reference laboratory.
91	Same laboratory test repeated on the same patient, same day, to obtain additional medically necessary results.

Modifier 91 vs Modifier 59

Modifier	Purpose
91	Repeat laboratory testing.
59	Distinct procedural service, not repeat laboratory testing.

CMS notes that Modifiers 59 and 91 are not interchangeable. Modifier 59 is used for distinct services, while Modifier 91 is specifically for medically necessary repeat laboratory tests.

Frequently Asked Questions (FAQs)

Q1. What is Modifier 91?

Answer: Modifier 91 identifies a medically necessary repeat clinical diagnostic laboratory test performed on the same patient on the same day to obtain additional reportable results.

Q2. Can Modifier 91 be used for rerunning a specimen after equipment failure?

Answer: No. Modifier 91 cannot be used for reruns caused by equipment malfunction, specimen problems, or quality-control purposes.

Q3. Should Modifier 91 be appended to the first laboratory test?

Answer: No. Modifier 91 is reported only on the subsequent repeat test(s), not the initial laboratory service.

Q4. Can Modifier 91 be used for glucose tolerance testing?

Answer: No. Modifier 91 should not be used when another CPT® code already represents a series of laboratory tests, such as glucose tolerance testing.

AR Caller Tips

When following up on a denied Modifier 91 claim:

Verify medical necessity for each repeat laboratory test.
Confirm each test generated a separate reportable result.
Ensure Modifier 91 is attached only to repeat laboratory services.
Review physician orders and laboratory reports.
Document the payer representative’s name, reference number, and appeal instructions.

Interview Questions

Question 1

What is Modifier 91?

Answer: Modifier 91 identifies medically necessary repeat clinical diagnostic laboratory tests performed on the same patient on the same day.

Question 2

When should Modifier 91 not be used?

Answer: It should not be used for confirmatory reruns, equipment failures, specimen problems, or quality-control testing.

Question 3

What is the difference between Modifier 90 and Modifier 91?

Answer: Modifier 90 identifies testing performed by an outside reference laboratory, while Modifier 91 identifies medically necessary repeat laboratory testing performed on the same day.

Practice Scenario

Scenario

A patient admitted to the ICU undergoes arterial blood gas testing at 8:00 AM, 12:00 PM, and 4:00 PM to monitor ventilator settings. Each test produces a new result that guides treatment decisions.

Question

Should Modifier 91 be reported?

Answer

Yes. The first arterial blood gas test is billed without Modifier 91. Each subsequent medically necessary repeat test is reported with Modifier 91, because each provides a separate reportable clinical result used in patient management.

Related Modifiers

Modifier 90 – Reference (Outside) Laboratory
Modifier 59 – Distinct Procedural Service
Modifier 99 – Multiple Modifiers

Common Billing Mistakes

Using Modifier 91 on the initial laboratory test.
Billing Modifier 91 for duplicate testing.
Using Modifier 91 for quality-control reruns.
Using Modifier 91 for equipment or specimen errors.
Applying Modifier 91 when another CPT® code already represents a testing series.

Key Takeaways

Modifier 91 identifies medically necessary repeat clinical diagnostic laboratory tests.
It applies only to repeat tests performed on the same patient on the same day.
Do not use Modifier 91 for confirmatory or quality-control reruns.
Append Modifier 91 only to subsequent repeat tests.
Proper documentation of medical necessity is essential to support reimbursement.

References

CMS Medicare Claims Processing Manual – Clinical Laboratory Services and Modifier 91 guidance.
CMS Billing and Coding Article A53482 – Repeat or Duplicate Services on the Same Day.
Noridian Medicare – Modifier 91 Billing Guidance.
AMA CPT® Professional Edition (licensed codebook).

Conclusion

Modifier 91 is an important laboratory modifier used to identify medically necessary repeat clinical diagnostic laboratory tests performed on the same patient during the same day. Proper application requires distinguishing clinically necessary repeat testing from duplicate or confirmatory testing, documenting medical necessity, and following Medicare and commercial payer policies. Accurate use of Modifier 91 helps reduce denials, supports compliant billing, and ensures appropriate reimbursement for serial laboratory monitoring.

Educational Disclaimer

This article was prepared with the assistance of artificial intelligence (AI) for educational and informational purposes. It is based on publicly available CMS guidance, Medicare billing resources, and general medical billing principles. It is not an official publication of the American Medical Association (AMA), CMS, or any insurance payer. CPT® is a registered trademark of the American Medical Association. Always consult the latest AMA CPT® codebook, CMS manuals, Medicare Administrative Contractor (MAC) guidance, CLIA regulations, National Correct Coding Initiative (NCCI) policies, and payer-specific billing policies before coding, billing, or submitting claims.