Modifier 90 (Reference (Outside) Laboratory) is used to identify laboratory tests that are performed by a laboratory other than the billing provider or billing laboratory.

A provider may collect a specimen but lack the equipment, certification, or expertise to perform a specialized test. In that situation, the specimen is sent to a reference (outside) laboratory, which performs the testing. Modifier 90 identifies those referred laboratory services when billing is permitted under Medicare or payer policy. Medicare allows referral laboratory billing only in limited situations, generally for independently billing clinical laboratories. In most cases, the laboratory that actually performs the test should bill Medicare directly.

Modifier Number

Modifier Name

Reference (Outside) Laboratory

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 90 tells the payer:

“This laboratory test was not performed by the billing provider. The specimen was sent to another qualified laboratory that actually performed the testing.”

Purpose of Modifier 90

Modifier 90 is used to:

Identify laboratory tests performed by an outside reference laboratory.
Distinguish referred testing from tests performed in-house.
Support appropriate claims processing when referral billing is allowed.
Identify the performing laboratory.
Comply with Medicare and payer billing requirements.

Understanding Modifier 90

A reference laboratory is a laboratory that receives a specimen from another laboratory or provider and performs specialized testing.

Examples include:

Genetic testing.
Molecular pathology.
Specialized microbiology.
Toxicology.
Rare chemistry tests.
Esoteric laboratory testing.

Modifier 90 does not mean the provider performed the laboratory service. Instead, it indicates another qualified laboratory performed the test.

When to Use Modifier 90

Modifier 90 is appropriate when:

A specimen is sent to an outside laboratory.
The outside laboratory performs the testing.
Referral laboratory billing is permitted by the payer.
Documentation identifies the performing laboratory.
Medicare or commercial payer requirements are satisfied.

Common Examples

✔ Genetic testing referred to a specialty laboratory.

✔ Molecular pathology testing.

✔ Specialized infectious disease testing.

✔ Toxicology testing.

✔ Cytogenetic testing.

When NOT to Use Modifier 90

Do not use Modifier 90 when:

The billing laboratory performs the test itself.
Billing venipuncture or specimen collection codes (e.g., 36415).
Billing handling or transportation fees.
The performing laboratory is billing the payer directly.
The payer prohibits pass-through or referral laboratory billing.

Many commercial insurers discourage or prohibit pass-through billing and require the performing laboratory to submit the claim directly.

Medicare Rules

Medicare requires:

Modifier 90 only for eligible referral laboratory services.
The reference laboratory’s CLIA number must be reported on the claim when applicable.
The billing laboratory’s CLIA number must also be included.
The performing laboratory must be identified.
Modifier 90 is appended only to the referred laboratory test line item.
Referral laboratory billing is generally limited to independently billing clinical laboratories.

Commercial Insurance Rules

Commercial payer policies vary.

Many insurers:

Require the performing laboratory to bill directly.
Do not reimburse pass-through laboratory billing.
Require documentation identifying the reference laboratory.
Review CLIA certification.
May deny claims billed incorrectly with Modifier 90.

Documentation Requirements

Documentation should include:

Ordering provider.
Date specimen collected.
Date specimen sent.
Test ordered.
CPT® code.
Name of the reference laboratory.
Performing laboratory CLIA number (when required).
Billing laboratory CLIA number.
Test results.
Medical necessity.

Real Billing Examples

Example 1 – Genetic Testing

A family physician orders hereditary cancer genetic testing.

The specimen is collected in the physician’s office and sent to a specialty molecular laboratory.

The laboratory performing the test is identified with Modifier 90, if referral billing is permitted.

Example 2 – Toxicology

An independent laboratory receives a urine specimen.

Because it cannot perform advanced toxicology testing, the specimen is referred to another certified laboratory.

The referred laboratory service is billed with Modifier 90 when Medicare referral billing requirements are met.

Example 3 – Molecular Pathology

A regional laboratory sends a molecular pathology specimen to a national reference laboratory.

Modifier 90 identifies the referred testing on the applicable claim line.

Example 4 – Incorrect Use

A physician bills:

36415-90

This is incorrect because Modifier 90 applies only to laboratory testing—not specimen collection or venipuncture services.

CMS-1500 Claim Example

Field	Example
CPT Code	80074-90
Modifier	90
Diagnosis Pointer	Appropriate ICD-10-CM diagnosis
CLIA Number	Performing laboratory CLIA
Units	1

Billing Example

Provider	CPT® Code
Independent Laboratory	80074-90

Common Denial Reasons

Modifier 90 billed on non-laboratory services.
Missing CLIA information.
Performing laboratory billed separately.
Pass-through billing not allowed by the payer.
Missing documentation identifying the reference laboratory.
Billing provider performed the test but incorrectly appended Modifier 90.

How to Correct the Denial

Review the EOB or ERA.
Confirm the service was performed by a reference laboratory.
Verify that referral billing is permitted by the payer.
Ensure both billing and performing laboratory information is reported correctly.
Submit CLIA information and supporting documentation.
Appeal if documentation supports appropriate use.

Coding Tips

Modifier 90 applies only to laboratory test codes.
Never append Modifier 90 to venipuncture or specimen collection codes.
Confirm whether the payer allows referral laboratory billing.
Ensure the correct CLIA numbers are reported.
Document the name of the performing laboratory.

Modifier 90 vs Modifier 91

Modifier	Purpose
90	Test performed by an outside reference laboratory.
91	Repeat clinical diagnostic laboratory test performed on the same day.

Modifier 90 vs Modifier 59

Modifier	Purpose
90	Identifies the performing reference laboratory.
59	Identifies a distinct procedural service and is not used to identify outside laboratory testing.

Frequently Asked Questions (FAQs)

Q1. What is Modifier 90?

Answer: Modifier 90 identifies laboratory tests performed by a reference (outside) laboratory rather than by the billing provider or laboratory.

Q2. Can Modifier 90 be used on venipuncture CPT® 36415?

Answer: No. Modifier 90 is not valid for specimen collection or venipuncture services.

Q3. Who usually bills Medicare for reference laboratory testing?

Answer: In most Medicare situations, the laboratory that actually performs the test bills Medicare directly. Referral laboratory billing is generally limited to independently billing clinical laboratories meeting CMS requirements.

Q4. Is Modifier 90 accepted by all commercial payers?

Answer: No. Many commercial insurers prohibit pass-through billing and require the performing laboratory to submit the claim directly. Always verify payer-specific policies.

AR Caller Tips

When following up on a denied Modifier 90 claim:

Verify whether the payer allows referral laboratory billing.
Confirm the performing laboratory’s CLIA number is present.
Ensure the laboratory test—not specimen collection—was billed with Modifier 90.
Request claim review if all documentation supports correct billing.
Record the payer representative’s name, reference number, and appeal instructions.

Interview Questions

Question 1

What is Modifier 90?

Answer: Modifier 90 identifies laboratory tests performed by an outside reference laboratory.

Question 2

Can a physician append Modifier 90 to 36415?

Answer: No. Modifier 90 applies only to laboratory testing, not specimen collection or venipuncture.

Question 3

What information is important when billing Modifier 90?

Answer: The performing laboratory, applicable CLIA information, medical necessity, and compliance with payer-specific referral laboratory billing rules.

Practice Scenario

Scenario

An independent laboratory receives a physician’s order for a complex molecular pathology test. Because the laboratory is not equipped to perform the test, it sends the specimen to a nationally accredited reference laboratory. The billing laboratory reports the referred test according to Medicare referral laboratory requirements, including the performing laboratory’s CLIA information.

Question

Should Modifier 90 be reported?

Answer

Yes. Modifier 90 is appropriate because the laboratory test was performed by an outside reference laboratory and the claim meets Medicare referral laboratory billing requirements.

Related Modifiers

Modifier 91 – Repeat Clinical Diagnostic Laboratory Test
Modifier 59 – Distinct Procedural Service
Modifier 99 – Multiple Modifiers

Common Billing Mistakes

Billing specimen collection with Modifier 90.
Omitting required CLIA information.
Billing when the performing laboratory should bill directly.
Using Modifier 90 on non-laboratory CPT® codes.
Failing to verify payer-specific pass-through billing rules.

Key Takeaways

Modifier 90 identifies laboratory tests performed by an outside reference laboratory.
It should be appended only to eligible laboratory test CPT® codes.
Medicare referral laboratory billing is limited and requires compliance with CLIA and CMS billing rules.
Many commercial payers prohibit pass-through billing.
Proper documentation and accurate reporting help reduce denials.

References

CMS Medicare Claims Processing Manual, Chapter 16 – Laboratory Services.
Noridian Medicare – Modifier 90 Guidance.
Bridgespan Health – Modifier 90 Reimbursement Policy.
AMA CPT® Professional Edition (licensed codebook).

Conclusion

Modifier 90 is an important laboratory modifier used to identify tests performed by an outside reference laboratory. Proper reporting requires understanding Medicare referral laboratory rules, documenting the performing laboratory, complying with CLIA requirements, and verifying payer-specific billing policies. Accurate use of Modifier 90 helps ensure compliant laboratory billing, minimizes denials, and supports appropriate reimbursement.

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.