Summary & Overview

CPT 85303: Protein C Activity Assay

CPT code 85303 represents a laboratory assay for measuring protein C activity in patient plasma. This test is used to assess the functional status of protein C, an anticoagulant protein with clinical relevance for patients evaluated for thrombophilia, unexplained thrombosis, or coagulopathy. Nationally, accurate reporting of 85303 supports clinical decision-making, appropriate use of anticoagulant therapies, and proper laboratory service reimbursement.

Key payers addressed include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of clinical context and testing indications, typical sites of service, and commonly used billing modifiers. The publication also outlines payer coverage considerations and coding guidance relevant to laboratory and hospital billing workflows.

This summary prepares clinicians, laboratory billing staff, and health policy analysts to understand where 85303 fits within diagnostic testing for coagulation disorders, what sites perform the service, and which major national payers commonly cover such laboratory assays. Data not available in the input is noted where applicable.

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Billing Code Overview

CPT code 85303 describes a laboratory assay in which a lab analyst measures protein C activity in a patient’s plasma. This test evaluates the functional activity of protein C, a vitamin K–dependent anticoagulant protein important in the regulation of blood clotting.

Service type: Laboratory diagnostic test measuring protein C activity
Typical site of service: Clinical laboratory or hospital laboratory setting

Clinical & Coding Specifications

Clinical Context

A typical patient is an adult referred to the hematology laboratory for evaluation of a suspected inherited or acquired thrombophilia after an unprovoked venous thromboembolism or recurrent pregnancy loss. The clinician orders a protein C activity assay (CPT 85303) to assess functional protein C level as part of a thrombophilia panel. The patient presents to an outpatient phlebotomy collection area or hospital inpatient unit; blood is drawn into citrated tubes, centrifuged to obtain platelet-poor plasma, and transported to the clinical laboratory under proper temperature and time constraints. The laboratory analyst performs the functional chromogenic or clot-based assay, documents quality control, and reports results to the ordering provider. Results are interpreted alongside activated protein C resistance, protein S testing, factor V Leiden, and anticoagulant medication status to guide diagnostic conclusions and subsequent management decisions by the ordering clinician.

Coding Specifications

Modifier	Description	When to Use
`26`	Professional component	Use when billing only the interpretation/reporting portion performed by a pathologist or laboratory professional separate from the technical work.
`TC`	Technical component	Use when billing only the technical component (laboratory processing and testing) without the professional interpretation.
`90`	Reference laboratory	Use when the specimen is sent to an outside/reference laboratory for testing; indicates non-participating lab performed the test.
`91`	Repeat clinical diagnostic laboratory test	Use when the same test is repeated on the same day to obtain a valid result (e.g., specimen hemolysis or processing error).
`59`	Distinct procedural service	Use when this test is separately identifiable and billed in conjunction with other services that could be bundled.
`90`	Duplicate entry to emphasize external lab use	Use as above to indicate outside lab performed the technical component.
`90`	Duplicate prevented by payer rules; include once as applicable	Use only once per claim if required by payer.
`91`	Duplicate for repeated measurement due to clinical necessity	Use as above when clinically necessary repeats occur.
`22`	Increased procedural services	Use when there is substantially greater effort or complexity for processing or interpretation beyond typical expectations.
`52`	Reduced services	Use when the test is partially reduced or not completed as initially intended (e.g., insufficient sample).
`53`	Discontinued procedure	Use when the test was started but discontinued for reasons beyond control (rare for lab tests).
`90`	Multiple lab locations — reference lab indicated	Use to indicate the test was performed at an external laboratory location.
`91`	Single-analyte repeated testing due to instrument error	Use when a repeat run is performed because of instrument failure.

Taxonomy Code	Specialty	Notes
207RC0000X	Hematology	Laboratory hematologists and pathologists who interpret coagulation testing and oversee thrombophilia testing.
207RR0500X	Clinical Pathology	Clinical pathologists directing clinical laboratory operations and interpretation of coagulation assays.
207LP2900X	Laboratory Medicine	Medical laboratory specialists and directors responsible for test validation and reporting.
207L00000X	Anatomic and Clinical Pathology	Pathologists with combined anatomic and clinical responsibilities participating in interpretation and oversight.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`I82.4`	Embolism and thrombosis of deep veins of lower extremity	Deep vein thrombosis prompts thrombophilia evaluation including protein C activity testing.
`I26.9`	Pulmonary embolism without acute cor pulmonale	Pulmonary embolism routinely triggers evaluation for underlying hypercoagulable states.
`O09.82`	Supervision of pregnancy with history of pulmonary embolism	History of thrombotic events in pregnancy warrants evaluation of anticoagulant proteins including protein C.
`O26.893`	Other specified pregnancy-related conditions, first trimester	Recurrent pregnancy loss or thrombosis-related pregnancy complications lead to thrombophilia testing.
`D68.51`	Primary thrombophilia	Direct indication to evaluate protein C activity when inherited thrombophilia is suspected.
`D68.52`	Secondary thrombophilia	Acquired causes of hypercoagulability (e.g., antiphospholipid syndrome) may prompt protein C testing.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`85303`	Protein C activity assay	This is the index test for functional assessment of protein C activity in plasma.
`85320`	Protein S activity assay	Commonly ordered alongside protein C activity to evaluate combined thrombophilia causes and assess anticoagulant pathway function.
`85306`	Activated protein C resistance assay	Often performed with protein C activity to detect resistance consistent with factor V Leiden or other causes of APC resistance.
`81241`	Factor V (F5) gene analysis for factor V Leiden	Molecular test used when activated protein C resistance or clinical suspicion suggests factor V Leiden mutation; may follow abnormal functional tests.
`85610`	Prothrombin time (PT) or international normalized ratio (INR)	Routine coagulation screening test performed before or in parallel to thrombophilia testing to assess anticoagulant effect and clotting status.
`85730`	Thrombin time (TT)	Ancillary coagulation test sometimes used in the differential evaluation of abnormal coagulation assays or anticoagulant interference.