Modifier 90- Reference (Outside) Laboratory

Identifies a clinical laboratory test that was referred by the billing laboratory to an outside laboratory where the test was actually performed.

What Is Modifier 90?

Modifier 90 identifies a reference or outside laboratory service. It is used when an independent clinical laboratory receives an ordered test but refers the specimen to another laboratory to perform the actual testing.

The billing laboratory reports Modifier 90 on the referred laboratory test to indicate that the service was performed by a different reference laboratory.

For Medicare referred laboratory claims, the performing reference laboratory's CLIA number and address must be reported as required. CMS can return a referred test claim as unprocessable when required reference-laboratory information is missing.

When to Use Modifier 90

Referred Laboratory Test: The billing laboratory sends a specimen to another laboratory for testing.

Outside Laboratory Performs Test: The laboratory submitting the claim did not actually perform the test identified with Modifier 90.

Independent Laboratory Billing: Medicare referred-laboratory billing requirements are generally applicable to independently billing clinical laboratories.

CLIA Requirements Met: The performing laboratory has the appropriate CLIA certification for the testing performed.

Reference Laboratory Identified: Required information about the laboratory that actually performed the test is included on the claim.

When NOT to Use Modifier 90

Billing Laboratory Performs Test: Do not append Modifier 90 when the billing laboratory actually performs the test.

No Referral Occurred: Modifier 90 should not be used merely because another organization handled or transported the specimen.

Professional or Technical Component: Modifier 90 does not identify professional or technical components of diagnostic services.

Repeat Laboratory Test: Modifier 91 may be appropriate when a qualifying clinical diagnostic laboratory test must be repeated for the same patient.

CLIA-Waived Test: Modifier QW serves a different purpose by identifying certain CLIA-waived laboratory tests.

Billing Example

An independent laboratory receives an order for multiple clinical laboratory tests.

The laboratory performs some tests itself but sends one specimen to a reference laboratory because it does not perform that particular test.

The billing laboratory reports the referred test with Modifier 90.

The claim must also contain the required information identifying the reference laboratory that actually performed the testing. CMS specifically uses Modifier 90 at the line-item level to identify referred tests.

Documentation Requirements

Ordering Information: Maintain the physician or qualified practitioner's order for the laboratory testing.

Referred Test: Identify which test was sent to an outside laboratory.

Performing Laboratory: Document the laboratory that actually performed the test.

CLIA Number: Report the performing reference laboratory's applicable CLIA number as required.

Laboratory Address: Include the required address identifying where the referred test was actually performed.

Test Results: Maintain appropriate laboratory results and supporting records for the billed service.

Billing and Claim Considerations

Modifier 90 is important because it tells Medicare that the billing laboratory and performing laboratory are different for the identified test.

CMS requires referred tests reported with Modifier 90 to contain information identifying the performing laboratory, including its CLIA number and address. Claims can be returned as unprocessable when required information is absent.

For electronic claims containing both tests performed by the billing laboratory and tests referred elsewhere, Modifier 90 identifies the individual referred-test line items.

Providers should also verify current CLIA requirements and Medicare Clinical Laboratory Fee Schedule policies applicable to the service. CMS continues to maintain and update the CLFS and laboratory billing requirements in 2026.

Common Billing Mistakes

Using 90 on In-House Testing: Modifier 90 should identify testing actually performed by an outside reference laboratory.

Missing CLIA Number: Failing to report the performing laboratory's required CLIA information can prevent claim processing.

Missing Reference Laboratory Address: Medicare requires identification of where the referred testing was performed.

Incorrect Performing Laboratory: Reporting information for the billing laboratory instead of the laboratory that performed the referred test can cause claim problems.

Using 90 for Repeat Testing: Modifier 91 addresses qualifying repeat clinical diagnostic laboratory testing and serves a different purpose.

Assuming 90 Establishes Coverage: Modifier 90 identifies a referred test; it does not independently establish medical necessity or Medicare coverage.

Common Denial Reasons

Reference Laboratory Not Identified: Required performing-laboratory information is missing.

Invalid or Missing CLIA Information: The claim does not contain the appropriate CLIA number for the laboratory performing the test.

Modifier Used Incorrectly: The billing laboratory actually performed the test.

Laboratory Certification Issue: The performing laboratory does not meet applicable CLIA requirements for the testing reported.

Coverage Requirements Not Met: The underlying laboratory test does not satisfy applicable Medicare coverage requirements.

Claim Information Inconsistent: The reported billing, performing, or reference laboratory information conflicts with the service reported.

Modifier 90 vs. Modifier 91

Modifier 90: Identifies a laboratory test referred to an outside laboratory for performance.

Modifier 91: Identifies a qualifying clinical diagnostic laboratory test repeated for the same patient when additional results are medically necessary.

Modifier 90 describes where the test was performed, while Modifier 91 describes why a laboratory test was repeated.

Frequently Asked Questions

Does Modifier 90 mean the billing laboratory performed the test?
No. Modifier 90 identifies a test referred to an outside laboratory that actually performed the testing.

What information does Medicare require for a referred laboratory test?
Medicare requires the referred service to be properly identified and requires information about the reference laboratory, including the applicable CLIA number and the address where the test was performed.