CPT 85610: Prothrombin Time (PT/INR)
CPT 85610 reports a prothrombin time (PT), the clotting test used to monitor warfarin therapy; the INR is a calculated value reported with it and is not separately billable. It is a clinical laboratory service priced under the Medicare Clinical Laboratory Fee Schedule (CLFS), not the Physician Fee Schedule, so it carries no PFS dollar amount.
- Code type
- Clinical laboratory test (coagulation)
- Priced under
- Medicare Clinical Laboratory Fee Schedule (CLFS), not the PFS
- Components
- PT clotting time; INR calculated, not separately billable
- Frequency notes
- Stable warfarin: generally every 2-3 weeks; more often needs documentation
What is CPT 85610 used for?
CPT 85610 reports a prothrombin time (PT) — a coagulation test measuring how long plasma takes to clot after calcium and tissue factor are added. Its most common use is monitoring warfarin (Coumadin) anticoagulation, where the associated INR guides dosing. It is also used to evaluate bleeding disorders and liver-related clotting factor deficiencies.
The test is defined as a single coagulation measurement; the INR that clinicians actually track is derived from the PT result.
How much does Medicare pay for 85610?
On the Physician Fee Schedule, 85610 shows status indicator X — statutory exclusion, so no RVUs and no PFS dollar. That is correct: lab tests are not paid on the PFS.
Payment comes from the Medicare Clinical Laboratory Fee Schedule (CLFS), priced with national limitation amounts and gapfill/crosswalk methodology. The Medicare fee calculator PFS lookup returns nothing — reference the current CLFS rate from your MAC or payer fee file.
Is the INR billed with 85610?
No — this is a frequent billing error. The INR is a calculated normalization of the prothrombin time, not a separate assay. It is inherent to 85610 and carries no additional code or charge.
- PT performed → report 85610.
- INR reported with it → included, not separately billable.
- Do not add a second code to represent the INR.
How do frequency and medical necessity apply?
Example: a patient on stable warfarin is tested every 4 weeks, then weekly for a month after a dose increase. The stable-phase testing fits the every-2-to-3-week norm, and the weekly testing is supported because the note documents the dose change and instability.
- Stable warfarin → generally every 2-3 weeks.
- Dose change or instability → more frequent testing, when documented.
- Report the supporting diagnosis (commonly Z79.01) per the PT NCD/LCD.
High-volume anticoagulation testing can hit MUE unit limits, so watch units per day and tie each order to payer medical-necessity support. Related coagulation and hematology testing includes 85025.
Frequently asked questions
Because 85610 is a clinical laboratory test, not a physician service. On the PFS it carries status indicator X (statutory exclusion) and is never paid there. Payment comes from the Clinical Laboratory Fee Schedule (CLFS) using national limitation amounts and gapfill/crosswalk pricing. The PFS $0 is expected, not an error.
No. The International Normalized Ratio (INR) is a calculated value derived from the prothrombin time, not a separate test. It is reported as part of 85610 and is not separately billable. Reporting an additional code for the INR alongside 85610 is not appropriate.
For a patient on stable warfarin therapy, testing is ordinarily not necessary more often than about every two to three weeks. More frequent testing (for example, weekly during dose changes or instability) is covered when the record documents the clinical reason. Medicare typically expects Z79.01 (long-term anticoagulant use) as the supporting diagnosis.
Sources & further reading
Reviewed by the ImmediCare Solutions RCM team
Certified billers and coders handling claims across 50+ specialties nationwide. This entry is reviewed against current payer policy and CMS rules. Last review: Jul 5, 2026.
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