Modifier 91- Repeat Clinical Diagnostic Laboratory Test

Modifier 91 is used when the same clinical diagnostic laboratory test must be repeated on the same patient, on the same day, to obtain medically necessary subsequent test results.

What is Modifier 91?

Modifier 91 identifies a clinical diagnostic laboratory test that is legitimately repeated on the same patient on the same date of service because additional test results are medically necessary during the patient's treatment.

The modifier helps distinguish a medically necessary repeat test from accidental duplicate billing. For example, a patient's condition may require the same laboratory value to be measured multiple times during the day so the provider can evaluate a changing clinical condition or response to treatment.

Modifier 91 does not mean that any laboratory test performed twice can be separately reported. CMS states that it should not be used when a test is rerun because of equipment or specimen problems, quality-control issues, or merely to confirm an initial result when only one reportable result is required.

When to Use Modifier 91

Same Laboratory Test Repeated: The same clinical diagnostic laboratory test is performed more than once for the same patient on the same date of service.

Subsequent Results Are Medically Necessary: Each test produces a separately reportable result needed for the patient's diagnosis, treatment, monitoring, or clinical management.

Patient's Condition Requires Repeated Monitoring: A patient's changing clinical condition may require the same laboratory value to be measured at different points during the day.

Repeat Component of a Laboratory Panel: If an entire laboratory panel is performed and one component must later be repeated as a medically reasonable and necessary service on the same date, CMS allows the individual test code to be reported with Modifier 91 when appropriate.

Separate Reportable Results Are Produced: Each properly reported test should represent an actual laboratory service that generates a medically necessary result rather than an internal laboratory rerun.

When NOT to Use Modifier 91

Confirming an Initial Result: Modifier 91 should not be used when a laboratory repeats a test simply to verify or confirm the accuracy of the original result when only one reportable result is required.

Specimen Problems: A test repeated because the original specimen was contaminated, inadequate, mishandled, or otherwise unsuitable does not qualify simply because the laboratory performed the test again.

Equipment or Testing Errors: Modifier 91 should not be appended when testing must be repeated because laboratory equipment malfunctioned or another technical problem occurred.

Quality-Control Testing: Internal laboratory quality-control procedures do not represent separately reportable patient services and should not be reported using Modifier 91.

Codes Already Describing Multiple Tests: Modifier 91 should not be used when an existing procedure code already describes a series of test results, such as certain glucose tolerance or evocative/suppression testing.

Different Laboratory Tests: Modifier 91 is intended for a repeat of the same laboratory test. It is not needed simply because several different laboratory tests are performed on the same day.

Billing Example

A hospitalized patient is being treated for a condition requiring close monitoring of a specific laboratory value. The physician orders the same clinical laboratory test in the morning and again later that afternoon because a second result is medically necessary to evaluate the patient's response to treatment.

The first laboratory test is reported normally. The subsequent medically necessary repeat test may be reported using the same procedure code with Modifier 91 appended.

The medical record should demonstrate why another result was necessary and show that both tests generated separate reportable results used in the patient's clinical management.

CMS provides similar guidance for same-day repeat laboratory testing and specifically distinguishes it from duplicate testing performed because of technical problems.

Documentation Requirements

Medical Necessity for Repetition: Documentation should explain why another result from the same laboratory test was necessary on the same date of service.

Separate Test Results: Maintain the individual results from each laboratory test. The record should demonstrate that multiple reportable results were actually produced.

Timing of Testing: When relevant, record when each specimen was collected or each test was performed to help establish that separate medically necessary services occurred.

Clinical Reason for Repeat Testing: Document the patient's condition, treatment, monitoring requirement, or other clinical circumstance that required subsequent testing.

Provider Order: Maintain applicable orders supporting the laboratory testing and repeated service.

Laboratory Records: Laboratory documentation should correspond with the services submitted on the claim and support the use of Modifier 91.

Billing and Claim Considerations

Modifier 91 should generally be appended to the repeat laboratory service, rather than the initial test, to distinguish the subsequent medically necessary test from the original service.

CMS explains that Modifier 91 communicates that the repeat laboratory test was distinct or separate from other laboratory services performed that day and was necessary to obtain subsequent reportable test values.

Correct reporting is especially important because multiple claims for the same laboratory code on the same date can resemble duplicate billing. Modifier 91 helps explain why the repeated service should be separately considered when the clinical circumstances support it.

The modifier does not automatically override coverage requirements or guarantee reimbursement. The laboratory service must still satisfy applicable medical necessity, coding, frequency, and payer requirements.

Common Billing Mistakes

Using Modifier 91 for Every Duplicate Laboratory Code: Two identical laboratory codes appearing on the same claim do not automatically justify Modifier 91. There must be a medically necessary reason for obtaining multiple reportable results.

Reporting a Technical Rerun as a New Service: Repeating a test because the laboratory encountered an equipment, specimen, or processing problem does not create another separately reportable clinical service.

Using Modifier 91 to Confirm Results: A laboratory may repeat an unexpected result as part of its internal procedures, but that does not necessarily qualify as a separately billable repeat test.

Ignoring Existing Multiple-Test Codes: When a procedure code already describes a series of tests or multiple results, reporting individual repeats with Modifier 91 may result in incorrect billing.

Failing to Document Why the Test Was Repeated: Multiple results alone may not demonstrate medical necessity. The record should establish why subsequent testing was clinically required.

Confusing Modifier 91 With Modifier 59: Modifier 91 specifically addresses medically necessary repeat clinical diagnostic laboratory testing. Modifier 59 serves a broader distinct-procedural-service purpose and should not be substituted merely because the same laboratory code appears more than once.

Common Denial Reasons

Repeat Testing Is Not Medically Necessary: The documentation does not establish why another result from the same laboratory test was needed during the patient's treatment.

Claim Appears to Contain Duplicate Services: Multiple instances of the same laboratory code are submitted without proper modifier reporting or sufficient documentation establishing legitimate repeat testing.

Testing Was Repeated Due to an Error: Documentation indicates that the second test resulted from specimen problems, equipment failure, processing errors, or another technical issue rather than clinical necessity.

Repeat Test Was Only Confirmatory: The second test was performed merely to confirm the original result when only one reportable result was clinically required.

Incorrect Procedure or Modifier Reporting: Modifier 91 is appended to a service that is not an appropriate clinical diagnostic laboratory test or the claim does not follow payer-specific reporting requirements.

Frequency or Coverage Requirements Are Not Met: Even medically necessary repeat testing may be subject to coverage policies, frequency limitations, or other payer rules.

Modifier 91 vs. Modifier 76

Modifier 91 and Modifier 76 can both involve repeated services, but they should not be treated interchangeably.

Modifier 91 is specifically intended for medically necessary repeat clinical diagnostic laboratory tests performed on the same patient on the same day to obtain subsequent reportable results.

Modifier 76 identifies a repeat procedure or service performed by the same physician or other qualified healthcare professional and has broader application outside the specific repeat laboratory circumstances addressed by Modifier 91.

For qualifying repeat clinical diagnostic laboratory testing, Modifier 91 is the more specific modifier.

Related Modifiers

Modifier 76

Modifier 90

Modifier 92

Home > All Categories > Laboratory Modifiers > Modifier 91