CPT 85301: Antithrombin III Antigen Assay in Plasma

CPT code 85301 represents a laboratory immunoassay for antithrombin III antigen in plasma, used to detect and quantify antithrombin III protein levels relevant to coagulation and thrombophilia evaluation. Nationally, this assay supports diagnosis and monitoring of bleeding and clotting disorders and informs clinical decisions about anticoagulation management and hereditary deficiency assessment. Coverage and payment for this laboratory service affect hospital and independent laboratory workflows and laboratory testing utilization patterns.

Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of clinical context for the assay, typical sites of service, common modifiers and billing considerations, and gaps where input data were not provided. The publication outlines typical coding usage, expected service line placement for laboratory claims, and the role of this antigen assay in diagnostic pathways.

This brief provides national-level context rather than state-specific policy. Data not available in the input are identified explicitly where applicable, and clinical background is summarized to support coding and billing clarity around CPT code 85301.

Billing Code Overview

CPT code 85301 describes a laboratory immunoassay performed by a lab analyst to detect antithrombin III antigen in patient plasma using an antigen/antibody reaction. The test identifies and quantifies antithrombin III protein levels, which are relevant to coagulation disorders and thrombophilia evaluation.

Service type: Clinical laboratory immunoassay (antigen detection)
Typical site of service: Clinical laboratory or hospital laboratory setting

Clinical Context

A typical patient is a 45-year-old individual with a personal or family history of thrombotic events, recurrent venous thromboembolism, or an abnormal clotting profile on routine screening. The clinician orders 85301 to measure antithrombin III antigen levels by an antigen/antibody assay to evaluate for inherited or acquired antithrombin deficiency. The phlebotomy team collects a citrate plasma sample and sends it to the clinical laboratory. The laboratory analyst performs the antigen/antibody reaction, documents assay controls and calibration, and reports quantitative antigen concentration. Results are reviewed by the ordering provider (hematologist, primary care physician, or vascular medicine specialist) to correlate with antithrombin activity assays, clinical history, anticoagulant therapy (e.g., heparin), and other coagulation studies. Common clinical workflows include pre-analytical verification of sample integrity, running the 85301 assay on automated immunoassay instrumentation, validating results with quality controls, and transmitting the report to the electronic health record for provider interpretation and follow-up management.

Coding Specifications

Modifier	Description	When to Use
`26`	Professional Component	Use when billing only the professional interpretation component if instrumentation (technical) is billed separately.

Taxonomy Code	Specialty	Notes
`207RH0000X`	Hematology	Hematologists commonly order and interpret `85301` for thrombotic disorders.
`208D00000X`	Pathology	Pathologists/directors oversee laboratory testing quality and interpretation.
`207L00000X`	Internal Medicine	Primary care internists often initiate evaluation for thrombosis risk and order antithrombin testing.
`208000000X`	Clinical Laboratory	Clinical laboratory scientists and directors perform and validate assays.
`2080S0012X`	Hospital Laboratory	Hospital-based lab specialists manage inpatient coagulation testing workflows.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`D68.51`	Antithrombin III deficiency	Direct indication for measuring antithrombin antigen to confirm inherited or acquired deficiency.
`I82.40`	Acute embolism and thrombosis of unspecified deep veins of lower limb	Thrombotic event prompting evaluation of thrombophilia including antithrombin levels.
`I26.99`	Other pulmonary embolism without acute cor pulmonale	Pulmonary embolism often triggers thrombophilia testing, including antithrombin antigen and activity.
`D68.9`	Coagulation defect, unspecified	Used when a coagulation abnormality is suspected and antithrombin testing is part of the diagnostic workup.
`O99.83`	Coagulation defects in the mother, antepartum condition	In pregnant patients with thrombosis or recurrent pregnancy loss, antithrombin testing may be indicated.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`85301`	Antithrombin III; antigen assay by immunologic technique (e.g., antigen/antibody reaction)	Primary test for quantifying antithrombin antigen concentration in plasma.
`85302`	Antithrombin III; functional assay	Commonly ordered alongside `85301` to compare antigen level to functional activity and distinguish type I vs type II deficiencies.
`85730`	Thrombin time or reptilase time	Performed in some coagulation workups to assess fibrinogen function and presence of inhibitors when evaluating bleeding or thrombotic risk.
`85610`	Prothrombin time; INR reported with PT	Frequently obtained in parallel to evaluate overall coagulation status, especially in patients on warfarin or with suspected coagulopathy.
`36415`	Collection of venous blood by venipuncture	Associated procedural code for specimen collection required before performing `85301`.

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Summary & Overview