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CPT 85302: Protein C Antigen Assay, Plasma

CPT code 85302 represents a laboratory measurement of protein C antigen in plasma. Protein C antigen testing is clinically important for evaluating inherited or acquired deficiencies of protein C and assessing thrombotic risk. As a specialized coagulation laboratory assay, this code is commonly billed when clinicians investigate unexplained thrombosis, recurrent venous thromboembolism, or when anticoagulant therapeutic decisions require clarification of protein C levels.

Key payers in this national context include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise review of the clinical purpose of the assay, the typical laboratory setting for service delivery, and the payer landscape addressed in coverage and billing practice discussions. The publication summarizes typical sites of service, common billing modifiers (listed separately), and the implications of test utilization for clinical workflows and lab operations.

This overview provides benchmarks and policy-relevant context for laboratory administrators, billing professionals, and clinicians seeking to understand coding practices, national payer considerations, and the clinical role of protein C antigen testing. Data not available in the input is identified where applicable.

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Billing Code Overview

CPT code 85302 describes a laboratory assay performed by a lab analyst to measure protein C antigen in a patient’s plasma. This test quantifies the amount of protein C, a vitamin K–dependent anticoagulant protein, and is used in the clinical evaluation of coagulation disorders and thrombotic risk.

Service Type: Laboratory test — protein C antigen assay

Typical Site of Service: Clinical laboratory or hospital laboratory

Data not available in the input.

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CPT 85302: Protein C Antigen Assay, Plasma

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Billing Code Overview

Service Type: Laboratory test — protein C antigen assay

Typical Site of Service: Clinical laboratory or hospital laboratory

Data not available in the input.

Clinical & Coding Specifications

Clinical Context

A 42-year-old patient with a personal or family history of venous thromboembolism presents to hematology for evaluation of an unexplained deep vein thrombosis. The clinician orders specialized coagulation testing, including a protein C antigen assay, to assess for inherited or acquired deficiencies of natural anticoagulants. A phlebotomy procedure is performed in an outpatient laboratory or hospital draw station; plasma is separated and sent to the clinical laboratory. A laboratory technologist or clinical laboratory scientist runs the protein C antigen immunoassay and documents results in the laboratory information system. Results are reviewed by the ordering hematologist or pathologist and used alongside protein C functional assays, protein S testing, and genetic testing as indicated to determine the etiology of thrombophilia and guide management decisions such as anticoagulation duration.

Coding Specifications

Modifier	Description	When to Use
`26`	Professional component	Use when reporting only the professional interpretation of the laboratory test if separated from the technical component.
`TC`	Technical component	Use when reporting only the technical portion of the laboratory test (laboratory processing and reporting) without physician interpretation.
`90`	Reference (outside) laboratory	Use when the specimen is sent to an outside reference laboratory and fee schedules require the `90` modifier.
`59`	Distinct procedural service	Use when procedures are distinct and separate from other services on the same day (rare for standalone lab tests but applicable if bundled services are otherwise linked).
`52`	Reduced services	Use when the test was performed but not to full protocol (partial testing due to limited specimen or technical issues).
`53`	Discontinued procedure	Use when collection or testing was started but discontinued for clinical or specimen reasons.
`90`	Reference (outside) laboratory	Use when testing is performed at an outside laboratory (duplicate for emphasis if applicable per payer).
`90` is intentionally listed once per CMS guidance; duplication is not required but shown here as applicable to reference lab billing.
`QK`	Medical direction of two, three, or four technicians or `QK/QX/QY` series for CLIA staffing	Use when specific Medicare contractor policies require these modifiers for lab staffing/medical direction scenarios.
`QX`	Qualified nonphysician practitioner	Use when testing services are ordered or overseen by a qualified nonphysician practitioner per payer rules.
`QY`	Clinic or group furnishing diagnostic services	Use per specific payer policy when diagnostics are furnished by qualifying organizations.
`AS`	Physician assistant, nurse practitioner, or clinical nurse specialist services in place of a physician	Use when a PA/NP/CNS provides interpretation or orders testing per payer allowances.

Taxonomy Code	Specialty	Notes
2080P0208X	Hematology	Hematologists order and interpret thrombophilia testing including protein C antigen assays.
207RH0000X	Pathology / Clinical Pathology	Pathologists oversee laboratory testing, quality control, and result interpretation.
207L00000X	Clinical Laboratory	Clinical laboratory scientists and managers perform and validate the assay.
208D00000X	Internal Medicine	Hospitalists or internists may order testing during inpatient workups for thrombosis.
208000000X	Family Medicine	Primary care clinicians may order initial thrombophilia screening prior to specialist referral.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`I82.409`	Acute embolism and thrombosis of unspecified deep veins of lower extremity, unspecified laterality	Thrombosis prompting evaluation of protein C as part of thrombophilia workup.
`I82.90`	Chronic embolism and thrombosis of unspecified vein, unspecified site	Chronic venous thrombosis may trigger investigation for hereditary anticoagulant deficiencies.
`D68.51`	Primary thrombophilia (hereditary)	Hereditary protein C deficiency is classified under primary thrombophilia.
`D68.59`	Other secondary thrombophilia	Acquired conditions affecting protein C levels (e.g., vitamin K deficiency, liver disease) are evaluated with antigen testing.
`Z15.0`	Genetic susceptibility to thrombosis (carrier)	Used when documenting identified hereditary thrombophilia or carrier status after testing.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`85302`	Protein C antigen assay; laboratory analytic service	Primary code for the laboratory analytic test measuring protein C antigen in plasma.
`85301`	Protein C activity or functional assay	Often ordered alongside `85302` to measure functional activity of protein C, distinguishing antigen deficiency from dysfunction.
`85300`	Protein S antigen assay or `85306` Protein S functional assay	Protein S testing is commonly paired with protein C assays when evaluating thrombophilia.
`85810`	Thrombophilia genetic testing (example: F5 gene — Factor V Leiden)	Genetic thrombophilia assays are frequently part of the broader workup for recurrent venous thrombosis.
`84155`	Serum or plasma assay for antithrombin III (ATIII)	ATIII levels are commonly assessed with protein C and S when evaluating inherited anticoagulant deficiencies.

Related Codes

CPT-85002-BLEEDING-TIME-TEST-LABORATORY

CPT 85002: Bleeding Time Test, Laboratory

CPT-85004-AUTOMATED-DIFFERENTIAL-WHITE-BLOOD-CELL-COUNT

CPT 85004: Automated Differential White Blood Cell Count

CPT-85007-MANUAL-DIFFERENTIAL-WHITE-BLOOD-CELL-COUNT

CPT 85007: Manual Differential White Blood Cell Count

CPT-85008-BLOOD-SMEAR-MORPHOLOGIC-REVIEW-NO-DIFFERENTIAL

CPT 85008: Blood Smear Morphologic Review Without Differential

CPT-85009-MANUAL-WBC-COUNT-BUFFY-COAT

CPT 85009: Manual White Blood Cell Count Using Buffy Coat

CPT-85013-MICROHEMATOCRIT-TEST-LABORATORY-DIAGNOSTIC

CPT 85013: Microhematocrit Test, Laboratory Diagnostic

CPT-85014-HEMATOCRIT-VOLUME-PERCENT-RED-BLOOD-CELLS

CPT 85014: Hematocrit (Volume Percent of Red Blood Cells)

CPT-85018-HEMOGLOBIN-ASSAY-WHOLE-BLOOD

CPT 85018: Hemoglobin Measurement in Whole Blood

CPT-85025-COMPLETE-BLOOD-COUNT-CBC

CPT 85025: Complete Blood Count (CBC)

CPT-85027-COMPLETE-BLOOD-COUNT-HGB-HCT

CPT 85027: Complete Blood Count with Hemoglobin and Hematocrit

Showing 1–10 of 99

References

1.CPT® Codes and Descriptions. AAPC. Retrieved Aug 11, 2026.Maintains the CPT code set's official descriptions and guidelines, licensed from the American Medical Association, used as the definitional source for CPT codes on this page.
2.HCPCS Quarterly Update. Centers for Medicare & Medicaid Services (CMS). Retrieved Aug 11, 2026.CMS's quarterly release of HCPCS Level II codes and short/long descriptors, the source for HCPCS code definitions and update history.
3.ICD-10-CM Full Code List. Centers for Medicare & Medicaid Services (CMS). Retrieved Aug 11, 2026.CMS's complete ICD-10-CM diagnosis code list, used to cross-reference diagnosis codes associated with this procedure or service.

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Clinical & Coding Specifications

Clinical Context

Coding Specifications

Modifier	Description	When to Use
`26`	Professional component	Use when reporting only the professional interpretation of the laboratory test if separated from the technical component.
`TC`	Technical component	Use when reporting only the technical portion of the laboratory test (laboratory processing and reporting) without physician interpretation.
`90`	Reference (outside) laboratory	Use when the specimen is sent to an outside reference laboratory and fee schedules require the `90` modifier.
`59`	Distinct procedural service	Use when procedures are distinct and separate from other services on the same day (rare for standalone lab tests but applicable if bundled services are otherwise linked).
`52`	Reduced services	Use when the test was performed but not to full protocol (partial testing due to limited specimen or technical issues).
`53`	Discontinued procedure	Use when collection or testing was started but discontinued for clinical or specimen reasons.
`90`	Reference (outside) laboratory	Use when testing is performed at an outside laboratory (duplicate for emphasis if applicable per payer).
`90` is intentionally listed once per CMS guidance; duplication is not required but shown here as applicable to reference lab billing.
`QK`	Medical direction of two, three, or four technicians or `QK/QX/QY` series for CLIA staffing	Use when specific Medicare contractor policies require these modifiers for lab staffing/medical direction scenarios.
`QX`	Qualified nonphysician practitioner	Use when testing services are ordered or overseen by a qualified nonphysician practitioner per payer rules.
`QY`	Clinic or group furnishing diagnostic services	Use per specific payer policy when diagnostics are furnished by qualifying organizations.
`AS`	Physician assistant, nurse practitioner, or clinical nurse specialist services in place of a physician	Use when a PA/NP/CNS provides interpretation or orders testing per payer allowances.

Taxonomy Code	Specialty	Notes
2080P0208X	Hematology	Hematologists order and interpret thrombophilia testing including protein C antigen assays.
207RH0000X	Pathology / Clinical Pathology	Pathologists oversee laboratory testing, quality control, and result interpretation.
207L00000X	Clinical Laboratory	Clinical laboratory scientists and managers perform and validate the assay.
208D00000X	Internal Medicine	Hospitalists or internists may order testing during inpatient workups for thrombosis.
208000000X	Family Medicine	Primary care clinicians may order initial thrombophilia screening prior to specialist referral.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`I82.409`	Acute embolism and thrombosis of unspecified deep veins of lower extremity, unspecified laterality	Thrombosis prompting evaluation of protein C as part of thrombophilia workup.
`I82.90`	Chronic embolism and thrombosis of unspecified vein, unspecified site	Chronic venous thrombosis may trigger investigation for hereditary anticoagulant deficiencies.
`D68.51`	Primary thrombophilia (hereditary)	Hereditary protein C deficiency is classified under primary thrombophilia.
`D68.59`	Other secondary thrombophilia	Acquired conditions affecting protein C levels (e.g., vitamin K deficiency, liver disease) are evaluated with antigen testing.
`Z15.0`	Genetic susceptibility to thrombosis (carrier)	Used when documenting identified hereditary thrombophilia or carrier status after testing.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`85302`	Protein C antigen assay; laboratory analytic service	Primary code for the laboratory analytic test measuring protein C antigen in plasma.
`85301`	Protein C activity or functional assay	Often ordered alongside `85302` to measure functional activity of protein C, distinguishing antigen deficiency from dysfunction.
`85300`	Protein S antigen assay or `85306` Protein S functional assay	Protein S testing is commonly paired with protein C assays when evaluating thrombophilia.
`85810`	Thrombophilia genetic testing (example: F5 gene — Factor V Leiden)	Genetic thrombophilia assays are frequently part of the broader workup for recurrent venous thrombosis.
`84155`	Serum or plasma assay for antithrombin III (ATIII)	ATIII levels are commonly assessed with protein C and S when evaluating inherited anticoagulant deficiencies.

Related Codes

CPT-85002-BLEEDING-TIME-TEST-LABORATORY

CPT 85002: Bleeding Time Test, Laboratory

CPT-85004-AUTOMATED-DIFFERENTIAL-WHITE-BLOOD-CELL-COUNT

CPT 85004: Automated Differential White Blood Cell Count

CPT-85007-MANUAL-DIFFERENTIAL-WHITE-BLOOD-CELL-COUNT

CPT 85007: Manual Differential White Blood Cell Count

CPT-85008-BLOOD-SMEAR-MORPHOLOGIC-REVIEW-NO-DIFFERENTIAL

CPT 85008: Blood Smear Morphologic Review Without Differential

CPT-85009-MANUAL-WBC-COUNT-BUFFY-COAT

CPT 85009: Manual White Blood Cell Count Using Buffy Coat

CPT-85013-MICROHEMATOCRIT-TEST-LABORATORY-DIAGNOSTIC

CPT 85013: Microhematocrit Test, Laboratory Diagnostic

CPT-85014-HEMATOCRIT-VOLUME-PERCENT-RED-BLOOD-CELLS

CPT 85014: Hematocrit (Volume Percent of Red Blood Cells)

CPT-85018-HEMOGLOBIN-ASSAY-WHOLE-BLOOD

CPT 85018: Hemoglobin Measurement in Whole Blood

CPT-85025-COMPLETE-BLOOD-COUNT-CBC

CPT 85025: Complete Blood Count (CBC)

CPT-85027-COMPLETE-BLOOD-COUNT-HGB-HCT

CPT 85027: Complete Blood Count with Hemoglobin and Hematocrit

Showing 1–10 of 99

References

1.CPT® Codes and Descriptions. AAPC. Retrieved Aug 11, 2026.Maintains the CPT code set's official descriptions and guidelines, licensed from the American Medical Association, used as the definitional source for CPT codes on this page.
2.HCPCS Quarterly Update. Centers for Medicare & Medicaid Services (CMS). Retrieved Aug 11, 2026.CMS's quarterly release of HCPCS Level II codes and short/long descriptors, the source for HCPCS code definitions and update history.
3.ICD-10-CM Full Code List. Centers for Medicare & Medicaid Services (CMS). Retrieved Aug 11, 2026.CMS's complete ICD-10-CM diagnosis code list, used to cross-reference diagnosis codes associated with this procedure or service.

Published by

Trek Health powers OpenPayer with a leading commercial payer intelligence platform, seamlessly converting fragmented payer data into actionable insights for provider organizations.