Modifier 91
Repeat Clinical Diagnostic Laboratory Test
Learn what Modifier 91 means, when it is appropriate, when it should not be used, how it differs from Modifiers 59, 76 and 90, common denial scenarios, AR investigation steps and practical laboratory billing examples.
Modifier 91 at a Glance
The essential points every medical biller, coder and AR caller should know.
Repeat Laboratory Test
Used when the same clinical diagnostic laboratory test is performed more than once on the same day for the same patient when medically necessary.
Same Day
Modifier 91 is specifically associated with repeat laboratory testing performed on the same date of service.
Medical Necessity
The repeat test must be necessary to obtain subsequent reportable results during the course of treatment.
Not for Reruns
Do not use Modifier 91 for laboratory errors, quality-control reruns or confirmation of initial results.
What Is Modifier 91?
Understanding repeat clinical diagnostic laboratory testing.
Simple Definition
Modifier 91 is used to identify a repeat clinical diagnostic laboratory test performed on the same patient on the same day when the repeat test is medically necessary to obtain subsequent reportable results.
CMS explains that the modifier is appropriate when the physician needs a repeat clinical diagnostic laboratory test that is distinct or separate from other laboratory services and is performed to obtain medically necessary subsequent test values.
Easy Way to Remember
Think:
“91 = Same lab test, same patient, same day, medically necessary repeat result.”
The key question is not simply whether the laboratory test was performed twice. The reason for repeating the test matters.
The 4 Core Rules of Modifier 91
Check all four before reporting the modifier.
Same Patient
The repeated laboratory test is performed for the same patient.
Same Day
The repeat test occurs on the same date of service.
Medically Necessary
The repeat result is needed during the patient’s treatment.
Subsequent Result
The repeat test is performed to obtain another medically necessary reportable result.
CMS Rule
CMS states that Modifier 91 is for repeat laboratory testing on the same day when multiple results are medically necessary. It should not be used when testing is repeated merely to confirm the initial result or because of testing problems.
When Should Modifier 91 Be Used?
The clinical reason for the repeat test is the most important consideration.
Treatment Monitoring
A laboratory test is repeated during treatment because another result is needed to monitor the patient’s condition.
Multiple Clinical Results
The provider needs additional results during the same day to guide ongoing clinical management.
Repeat Component Test
A component of a laboratory panel is repeated as a medically reasonable and necessary service on the same date.
Subsequent Values
Additional reportable values are required during the course of treatment.
Sequential Monitoring
The patient’s condition requires laboratory monitoring at different points during the same day.
Separate Reportable Result
The second test produces a clinically needed result rather than simply repeating the first test because of a technical problem.
When Should Modifier 91 NOT Be Used?
These situations are especially important for denial prevention.
Confirming Initial Results
Do not use Modifier 91 when the test is repeated simply to confirm the initial result.
Laboratory Error
Do not use it when the original test needs to be rerun because of a laboratory error.
Specimen Problem
Do not use it merely because there was a problem with the specimen or equipment.
Quality Control
Internal quality-control reruns are not the intended use of Modifier 91.
One Reportable Result
Do not use the modifier when only one normal, reportable result is required.
Test Series Already Described
Do not use Modifier 91 when another CPT code already describes the series of test results.
CMS Warning
CMS specifically excludes repeat testing performed to confirm initial results, because of specimen or equipment problems, or when a normal one-time reportable result is all that is required.
Modifier 91 Decision Workflow
Use this workflow before appending Modifier 91.
Modifier 90 vs Modifier 91
These two laboratory modifiers address completely different situations.
| Feature | Modifier 90 | Modifier 91 |
|---|---|---|
| Main purpose | Identifies a referred/outside laboratory service. | Identifies an appropriate repeat clinical diagnostic laboratory test. |
| Key issue | Who performed the test? | Why was the test repeated? |
| Same day requirement | Not the defining concept. | Yes. |
| Medical necessity | Still subject to coverage requirements. | Critical to appropriate use. |
| Laboratory error | Not the defining issue. | Do not use Modifier 91 for this reason. |
| Basic memory trick | Outside laboratory. | Repeat laboratory result. |
Modifier 91 vs Modifier 59
One of the most important distinctions for laboratory billing.
Modifier 91
- Repeat clinical diagnostic laboratory test.
- Same patient.
- Same day.
- Medically necessary subsequent result.
- Used for appropriate repeat laboratory testing.
Modifier 59
- Identifies a distinct procedural service.
- Different clinical circumstances must support distinctness.
- Can involve separate encounters, sites, structures or other circumstances depending on the service.
- Not simply a replacement for Modifier 91.
- Must be supported by appropriate documentation.
CMS NCCI Guidance
CMS identifies Modifier 91 among the modifiers that may be used under appropriate circumstances with NCCI edits. CMS separately describes Modifier 91 as the appropriate modifier for repeating a component laboratory test from a panel when the repeat is medically reasonable and necessary on the same date.
Modifier 91 vs Modifier 76
| Modifier | Meaning | Typical Service | Key Question |
|---|---|---|---|
| 76 | Repeat procedure/service by same physician or other qualified healthcare professional. | General procedures/services. | Was the procedure repeated by the same provider? |
| 77 | Repeat procedure/service by another physician or other qualified healthcare professional. | General procedures/services. | Was the procedure repeated by another provider? |
| 91 | Repeat clinical diagnostic laboratory test. | Laboratory testing. | Was the laboratory test repeated for a medically necessary subsequent result? |
AR Tip
Do not automatically replace Modifier 91 with Modifier 76. Determine whether the service is a clinical diagnostic laboratory test and then determine why it was repeated.
Modifier 91 and Laboratory Panels
A common CMS example involves repeating one component of a laboratory panel.
Example
A laboratory performs all tests included in an applicable laboratory panel.
During the same date of service, the provider needs one of the component tests repeated because another medically necessary result is required during treatment.
The applicable component CPT code may be reported again with Modifier 91 when the CMS requirements are met.
Coding Concept
Panel:
Laboratory PanelRepeat component:
Component CPT + 91CMS specifically gives this type of scenario as an example of appropriate Modifier 91 use.
Modifier 91 Billing Examples
Practical examples for billers, coders and AR callers.
A patient is receiving treatment and the provider needs another laboratory value later the same day to guide ongoing care.
CPT CPT-91The second laboratory service may be reported with Modifier 91 when the applicable requirements are satisfied.
The laboratory performs a complete panel. One component must be repeated later that day to obtain a medically necessary subsequent value.
Panel CPT Component CPT-91CMS specifically identifies this type of repeat component testing as an example for Modifier 91.
The laboratory repeats the test because the first specimen was compromised or there was a testing problem.
CPTDo not use Modifier 91 merely to report the technical rerun.
The first laboratory result is unexpected and the laboratory repeats the test only to confirm the original result.
CPTModifier 91 should not be used when the test is rerun merely to confirm the initial result.
Multiple laboratory values are needed during the same day’s treatment because the patient’s condition is changing.
CPT CPT-91Modifier 91 may be appropriate when the repeated testing produces medically necessary subsequent reportable results.
Another CPT code already describes a series of laboratory results.
Series CPTDo not automatically report repeated individual tests with Modifier 91 when another code already describes the series.
Common Modifier 91 Denials
Common claim problems involving repeat clinical diagnostic laboratory testing.
Modifier Not Supported
The documentation does not establish that the repeat laboratory test was medically necessary.
Repeat Test Not Allowed
The payer does not consider the reported repeat laboratory service separately payable under the submitted circumstances.
Confirmation Testing
The second test was performed only to confirm the initial result.
Laboratory Error
The second test was required because of specimen, equipment or laboratory testing problems.
Duplicate Service
The payer considers the repeated laboratory service duplicative rather than a medically necessary subsequent test.
Wrong Modifier
Another modifier or coding approach may be applicable depending on the actual circumstances.
Missing Documentation
The medical record does not support why the repeat test was required.
Frequency Limitation
The payer’s policy may limit the number or frequency of separately payable laboratory services.
Medical Necessity
The diagnosis or clinical documentation does not support the repeat service under the payer’s policy.
AR Caller Workflow for Modifier 91 Denials
A practical denial-management workflow for repeat laboratory services.
Review ERA / EOB
Identify the exact denial message, CARC, RARC, adjustment and affected laboratory claim line.
Identify the CPT / HCPCS
Determine which laboratory test was billed more than once.
Verify Date of Service
Confirm whether the repeated laboratory services occurred on the same date.
Compare Claim Lines
Review the original line and repeated line, including units and modifiers.
Verify Modifier 91
Determine whether the second laboratory service has Modifier 91 and whether the clinical circumstances support it.
Determine Why It Was Repeated
Find out whether the repeat was medically necessary for treatment, a confirmation test, QC rerun, specimen issue or another reason.
Review Medical Documentation
Verify that the medical record supports the clinical reason for obtaining another result.
Check Laboratory Policy
Review Medicare, MAC, Medicaid, Medicare Advantage or commercial payer requirements.
Check NCCI / MUE
Determine whether an NCCI edit or MUE affects the service and whether the applicable modifier rules support separate reporting.
Identify Root Cause
Determine whether the denial resulted from coding, documentation, medical necessity, frequency, duplicate billing or payer policy.
Correct or Appeal
Submit a corrected claim or appeal only when supported by the payer’s requirements and documentation.
Document Follow-Up
Record payer instructions, representative name, reference number, filing deadline and next action.
AR Caller Script for Modifier 91
Questions to ask the payer when investigating a repeat laboratory denial.
“I’m calling regarding a clinical laboratory claim where the repeat laboratory service was submitted with Modifier 91.”
“Could you please provide the exact denial reason and the applicable CARC and RARC codes?”
“Can you confirm whether the denial is related to Modifier 91 or to the laboratory service itself?”
“Can you confirm whether your policy allows this laboratory test to be reported more than once on the same date of service?”
“Can you confirm what documentation is required to support the medical necessity of the repeat test?”
“Does your policy distinguish between medically necessary repeat testing and testing performed to confirm an initial result?”
“Can you confirm whether an NCCI or MUE edit affected this claim line?”
“If a corrected claim is required, could you please confirm the exact correction and filing deadline?”
“If an appeal is appropriate, what documentation should be submitted?”
“May I have the call reference number for our records?”
Documentation Checklist
What the billing and AR team should be able to establish.
First Test
Identify the original laboratory test and result.
Repeat Test
Identify the second laboratory test and its result.
Same Date
Verify that the repeat service occurred on the same date of service.
Clinical Reason
Document why the subsequent laboratory result was needed.
Medical Necessity
Establish that the repeat test was necessary during the course of treatment.
Reportable Result
Establish that the repeat test generated a clinically needed subsequent reportable result.
Root Causes of Modifier 91 Denials
Wrong Modifier
Modifier 91 was reported when the circumstances do not support it.
Confirmation Test
The laboratory repeated the test only to confirm the first result.
Technical Rerun
The second test resulted from a specimen, equipment or laboratory problem.
No Medical Necessity
The record does not support why another result was required.
Duplicate Service
The payer considers the second test duplicative.
Frequency Issue
The payer’s policy limits the frequency or number of separately payable tests.
Wrong CPT
Another CPT code may describe the laboratory testing series.
Missing Documentation
The claim does not have adequate support for the repeat service.
Modifier 91 and Medicare NCCI
Modifier 91 can have an important role when laboratory edits are involved.
NCCI Considerations
- Review the applicable NCCI PTP edit.
- Check whether the same CPT/HCPCS code appears on multiple claim lines.
- Determine the MUE Adjudication Indicator when applicable.
- Confirm that Modifier 91 is clinically supported.
- Review the current NCCI Policy Manual.
CMS NCCI Guidance
CMS identifies Modifier 91 as one of the modifiers that can be used under appropriate circumstances with NCCI edits. CMS also explains that when an applicable MUE is adjudicated as a claim line edit, certain modifiers can allow separate lines to be adjudicated separately.
However, the existence of a modifier does not automatically justify its use. The clinical circumstances must support the modifier.
Modifier 91 and MUEs
What AR Should Know
A Medically Unlikely Edit, or MUE, represents a maximum number of units of a service that a provider would typically report for the same beneficiary on the same date of service under the applicable MUE methodology.
CMS explains that when a HCPCS/CPT code has an MUE adjudicated as a claim-line edit, certain appropriate modifiers—including 91—may allow separate claim lines to be adjudicated separately. This does not mean every repeat service should receive Modifier 91.
Modifier 91 and Medicaid
Medicaid Review Checklist
- Identify the state Medicaid program.
- Verify whether Modifier 91 is recognized.
- Review the state Medicaid laboratory policy.
- Check Medicaid NCCI requirements.
- Verify frequency and medical-necessity requirements.
- Check the applicable Medicaid MCO policy.
Important
Medicaid requirements can vary by state. Medicaid managed-care organizations may also have additional billing policies.
CMS notes that Medicaid NCCI policies can apply Modifier 91 in appropriate laboratory circumstances, but providers should also check the applicable state Medicaid agency requirements.
Modifier 91 and Commercial Insurance
Verify Payer Policy
Confirm whether the payer recognizes Modifier 91 and under what circumstances.
Verify Medical Necessity
Determine whether the payer requires documentation supporting the repeat test.
Check Frequency
Review payer-specific frequency limitations.
Check NCCI
Determine whether an applicable coding edit affected payment.
Review Documentation
Confirm the clinical reason for the subsequent laboratory result.
Follow Appeal Rules
Use the payer’s current corrected-claim and appeal process.
How to Prevent Modifier 91 Denials
Capture Clinical Reason
Document why another laboratory result was needed.
Confirm Same Day
Verify the repeated service occurred on the same date of service.
Avoid Confirmation Reruns
Do not use Modifier 91 simply to confirm the first result.
Avoid QC Reruns
Separate internal quality-control testing from medically necessary patient testing.
Check CPT Series
Confirm another CPT code does not already describe the testing series.
Audit Denials
Track Modifier 91 denials by payer, CPT, reason and root cause.
Modifier 91 Quick Cheat Sheet
- Modifier 91 = Repeat Clinical Diagnostic Laboratory Test.
- The test is repeated for the same patient.
- The repeat test occurs on the same date of service.
- Another medically necessary reportable result is required.
- The repeat test is part of the patient’s course of treatment.
- Do not use Modifier 91 for laboratory errors.
- Do not use it for quality-control reruns.
- Do not use it merely to confirm the initial result.
- Do not use it when only one normal reportable result is needed.
- Check whether another CPT code already describes the testing series.
- Check applicable NCCI and MUE rules.
- Medical necessity documentation may be required.
- Modifier 91 does not guarantee payment.
- Always verify the applicable payer policy before appealing.
Before You Bill Modifier 91
- Confirm the laboratory test was performed more than once.
- Confirm the same patient was tested.
- Confirm the same date of service.
- Confirm the repeat was medically necessary.
- Confirm another reportable result was needed.
- Confirm the repeat was not caused by laboratory error.
- Confirm it was not simply a confirmation test.
- Confirm it was not a quality-control rerun.
- Check whether another CPT describes the test series.
- Review applicable NCCI/MUE rules.
- Verify payer-specific requirements.
- Maintain documentation supporting the repeat service.
Modifier 91 FAQs
What is Modifier 91?
Modifier 91 identifies an appropriate repeat clinical diagnostic laboratory test performed on the same patient on the same day when another medically necessary reportable result is required.
What does Modifier 91 mean in medical billing?
It means an appropriate laboratory test was repeated on the same day to obtain a subsequent medically necessary result.
Can Modifier 91 be used on the same day?
Yes. Same-day repeat laboratory testing is the specific circumstance addressed by Modifier 91.
Can Modifier 91 be used for the same patient?
Yes. The modifier applies when the same laboratory test is performed more than once on the same patient on the same day under the applicable circumstances.
Can Modifier 91 be used to confirm an initial result?
No. CMS specifically says Modifier 91 must not be used when testing is repeated to confirm the initial result.
Can Modifier 91 be used for laboratory errors?
No. Modifier 91 should not be used when the test is rerun because of laboratory, specimen or equipment problems.
Can Modifier 91 be used for quality control?
No. Quality-control reruns are not the intended use of Modifier 91.
Can Modifier 91 be used when only one result is needed?
No. CMS excludes situations where a normal one-time reportable result is all that is required.
Can Modifier 91 be used with a laboratory panel?
When a laboratory performs all tests in an applicable panel and one component is repeated as a medically reasonable and necessary service on the same date, CMS states that the component CPT code may be reported with Modifier 91.
What is the difference between Modifier 91 and Modifier 90?
Modifier 90 identifies a referred or outside laboratory service. Modifier 91 identifies an appropriate repeat clinical diagnostic laboratory test.
What is the difference between Modifier 91 and Modifier 59?
Modifier 91 addresses appropriate repeat clinical diagnostic laboratory testing. Modifier 59 identifies a distinct procedural service. They should not be treated as interchangeable modifiers.
What is the difference between Modifier 91 and Modifier 76?
Modifier 76 is used for an appropriate repeat procedure or service by the same physician or other qualified healthcare professional. Modifier 91 specifically addresses repeat clinical diagnostic laboratory testing.
Does Modifier 91 guarantee payment?
No. Modifier 91 only identifies the applicable repeat laboratory circumstance. Coverage, medical necessity, frequency limits, NCCI edits and payer policy still apply.
Can Modifier 91 bypass an NCCI edit?
Modifier 91 is among the modifiers that CMS recognizes for appropriate NCCI circumstances, but it must be clinically supported. It should never be added simply to bypass an edit.
What should an AR caller check first?
Start with the ERA/EOB denial reason, then review the CPT, date of service, claim lines, Modifier 91, clinical reason for the repeat test, documentation and payer policy.
What documentation supports Modifier 91?
Documentation should support that the repeat test was clinically necessary during treatment and that another reportable result was needed. The exact documentation requirements can vary by payer and policy.
Can Modifier 91 be used for a test repeated because of a bad specimen?
Generally no. CMS specifically excludes repeat testing caused by testing problems involving specimens or equipment.
What if another CPT code already describes a series of results?
Modifier 91 should not be used to separately report repeated tests when another code already describes the testing series.
Does Medicare use Modifier 91 for laboratory tests?
Yes. CMS Medicare guidance specifically addresses Modifier 91 for laboratory tests paid under the Clinical Laboratory Fee Schedule when the applicable repeat-testing circumstances are met.
Where can I find official CMS laboratory guidance?
CMS maintains a Clinical Labs Information Center containing Medicare Claims Processing Manual laboratory guidance, NCCI resources, Clinical Laboratory Fee Schedule information and other laboratory billing resources.
Official CMS References
Use official CMS resources when validating Modifier 91 and laboratory billing requirements.
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