CPT® Code 85610

Quick Reference: CPT 85610

• Definition: Prothrombin Time (PT) test. Includes the calculated INR result. Do not bill INR separately.
• Coverage: Covered for warfarin monitoring, bleeding disorders, and liver disease. Routine pre-op screening is generally not covered without risk factors.
• Medicare Rule: For warfarin, code Z79.01 as primary, followed by the condition (e.g., Atrial Fibrillation). Stable patients are limited to testing every 2-3 weeks.
• Point-of-Care: Must bill as 85610-QW if using a CLIA-waived device (e.g., fingerstick) in the office.
• Reimbursement: Approx. $4.29 (Medicare National Avg), paid under the Clinical Lab Fee Schedule.

CPT code 85610 refers to the Prothrombin Time (PT) test, a clinical laboratory test that measures how long it takes blood plasma to clot. In practice, a sample of the patient's plasma is mixed with reagents (such as calcium and tissue factor) and the clotting time is recorded.

The PT/INR primarily evaluates the extrinsic (tissue factor-dependent) and common coagulation pathways. It is commonly used to assess anticoagulation therapy (especially warfarin), which affects vitamin K-dependent clotting factors.

Importantly, when a PT test is performed, the International Normalized Ratio (INR) is a calculated value and is not separately billable; it is considered part of the PT test itself (i.e., you should not code an INR separately from CPT 85610).

Common Indications for PT/INR and Associated ICD-10 Codes

Medical necessity for CPT 85610 must be supported by an appropriate diagnosis code on the claim, or insurers will deny coverage. Common indications include:

• Monitoring of Anticoagulation: For patients on warfarin (Coumadin), Medicare instructs providers to use Z79.01 (Long term current use of anticoagulants) as the primary diagnosis. This should be paired with the underlying condition code, such as:
  • Atrial fibrillation (e.g., I48.91, I48.20).
  • History of venous thromboembolism/DVT (e.g., Z86.711, Z86.718).
  • Mechanical heart valve (Z95.2).
• Bleeding or Clotting Disorders: For patients with abnormal bruising, bleeding, or thrombosis. Codes include:
  • Coagulation defect, unspecified (D68.9).
  • Disseminated intravascular coagulation (D65).
  • Abnormal coagulation profile (R79.1).
• Liver Disease and Vitamin K Deficiency: Since clotting factors are produced in the liver and are Vitamin K dependent, indicated codes include:
  • Unspecified cirrhosis of liver (K74.60).
  • Vitamin K deficiency (E56.1).
• Pre-Operative Assessment: Medicare only covers pre-op PT testing if specific risk factors exist (e.g., history of bleeding). Routine screening using Z01.812 is generally not covered without an additional risk diagnosis.

Medicare Coverage (NCD 190.17) and Frequency Limitations

Medicare coverage is strictly governed by National Coverage Determination (NCD) 190.17. Key rules include:

• Frequency Limits: For patients on stable warfarin therapy, testing is typically covered every 2-3 weeks. More frequent testing (e.g., weekly) requires documentation of instability or dose changes.
• ESRD Patients: Testing more often than weekly for ESRD patients on heparin requires documentation of necessity beyond ESRD alone.
• Acute Evaluation: If an initial test is normal, repeat testing is not covered unless the clinical status changes.
• Non-Covered: Medicare does not pay for routine screening or home self-testing under CPT 85610 (home testing uses G0250/93793).

Medicaid and Commercial Payer Considerations

Medicaid: Most states follow Medicare's medical necessity principles. For example, New York Medicaid lists 85610 as covered when medically necessary. Reimbursement is often lower (e.g., $3-$5).

Commercial: Plans like UnitedHealthcare and Blue Cross typically cover PT/INR for the same indications (AFib, liver disease, etc.). However, Medicare Advantage plans will strictly follow the NCD rules. Always verify specific payer policies regarding pre-op testing.

Documentation and Billing Requirements

To withstand audits and ensure payment:

• Order and Signature: There must be a signed physician order for the test.
• Diagnosis Linking: The claim diagnosis must match the documented indication. Do not use a "routine exam" code for a diagnostic test.
• CLIA Number: Claims must include the CLIA number. For waived tests, this allows payment for 85610-QW.
• ABN: For Original Medicare, obtain an Advance Beneficiary Notice (ABN) if you suspect the test (e.g., routine pre-op) will be denied. Use modifier GA if an ABN is on file.
• Bill the Correct Entity: Only the entity performing the test bills 85610. If you draw blood and send it to LabCorp, LabCorp bills 85610; you bill only the draw fee (36415).

Point-of-Care (POC) Testing vs. Laboratory Testing

Coding differs based on the setting:

• CLIA Waived (Office/Clinic): If using a fingerstick device (e.g., CoaguChek), you must append Modifier QW (85610-QW). This tells Medicare it was a waived test.
• Reagents/Supplies: You cannot bill separately for test strips or cartridges; these costs are included in the 85610 reimbursement.
• Home Self-Testing: Do not use 85610 for patients testing themselves at home. Use codes G0250 (review) or 93793 (management) instead.

Related Tests and Bundling Considerations

INR vs PT: INR is a calculation included in 85610. Never bill a separate code for INR.

PTT (85730): Can be billed with 85610 if medically necessary (e.g., heparin + warfarin). There is no NCCI edit bundling these two.

Specimen Collection:

• Venous (36415): Billable if drawing venous blood to send out.
• Capillary (36416): Generally bundled into 85610 by Medicare and not separately payable.

The 99211 Issue: CMS considers the work of drawing blood and running the test to be included in the lab code. Do not bill 99211 for a simple INR check where no separate evaluation occurs. A CMS compliance officer stated that "all work performed by the nurse is part of the blood draw and is encompassed in that code."

Proper Use of Modifiers

Reimbursement and RVU Benchmarks

CPT 85610 has no global period and is paid under the Clinical Laboratory Fee Schedule (CLFS), not the Physician Fee Schedule.

• Medicare Allowable (2025/2026): Approximately $4.29 per test. This has decreased from ~$5.39 in previous years due to PAMA adjustments.
• RVUs: 0.00 Work RVU (no physician work component).
• Medicaid Rates: Varies by state (e.g., FL ~$2.31, NY ~$3.95).
• Commercial Rates: Typically range from $4.00 to $7.00.

Real-World Clinical Billing Scenarios

Scenario 1: Routine Warfarin Clinic (Office)

Patient on warfarin for AFib comes in. Nurse performs fingerstick INR (2.5, stable). Physician briefly reviews and says "continue dose."

Bill: 85610-QW. Do NOT Bill: 99211 (bundled) or 36416. Diagnosis: Z79.01 + I48.91.

Scenario 2: High INR with Evaluation

Patient INR is 4.5; reports bleeding gums. Physician evaluates, examines, holds dose, and counsels (15 mins).

Bill: 85610-QW AND 99213-25. The E/M is justified by the significant separate evaluation and management of the complication.

Scenario 3: Repeat Testing Same Day

INR is 9.0 at 8 AM. Vitamin K given. Repeat test ordered at 1 PM shows INR 5.0.

Bill: Line 1: 85610-QW Line 2: 85610-QW-91 Modifier 91 indicates a medically necessary repeat, distinct from a duplicate error.

Scenario 4: Pre-Operative Clearance

Healthy 60-year-old for hernia repair. No risk factors. Surgeon orders PT/INR.

Coding: Medicare will likely deny this as not medically necessary. An ABN should be signed, and the claim billed with Modifier GA to transfer liability to the patient.

Decision Logic: When to Use Modifier QW

flowchart TD
    A[PT/INR Test Ordered] --> B{Where is the test performed?}
    B -->|Office/Clinic POC device| C{Is the device CLIA-waived?}
    B -->|Reference laboratory| D[Bill 85610 without QW]
    B -->|Patient home self-test| E[Do not use 85610 - Use G0250/93793]
    C -->|Yes| F[Bill 85610-QW]
    C -->|No| G[Bill 85610 - Ensure moderate/high complexity CLIA certificate]