Summary & Overview

CPT 86651: La Crosse Virus Antibody Immunoassay

CPT code 86651 denotes an immunoassay to detect antibodies to the California (La Crosse) virus in serum or cerebrospinal fluid, a diagnostic test used in suspected cases of viral encephalitis. Nationally, accurate laboratory identification of arboviral encephalitides supports clinical decision-making, public health surveillance, and appropriate use of hospital resources. The code is relevant to hospitals and clinical laboratories that manage patients with central nervous system infections.

Key payers considered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find an overview of clinical context for testing, typical sites of service, common billing modifiers, and payer coverage considerations where available. The publication summarizes what payers commonly consider when processing laboratory immunoassay claims, highlights coding and billing implications for hospital and reference laboratories, and outlines areas where policy updates or payer-specific rules can affect reimbursement and documentation requirements.

This resource is intended to inform laboratory administrators, billing professionals, and policy analysts about the clinical purpose of 86651, how it fits within laboratory service lines, and which stakeholders typically cover the service. Data not available in the input are noted where applicable.

AetnaBlue Cross Blue ShieldCigna HealthUnitedHealthcareMedicare86651CPTimmunoassaylaboratoryinfectious-diseaseencephalitisserologyCSF

Billing Code Overview

CPT code 86651 describes an immunoassay performed by a laboratory analyst to detect antibodies to California (La Crosse) virus in a patient’s serum or cerebrospinal fluid. The test is used in the evaluation of suspected viral encephalitis when La Crosse virus is a diagnostic consideration.

Service type: Infectious disease serology / immunoassay laboratory test

Typical site of service: Clinical laboratory or hospital laboratory; specimen sources are serum or cerebrospinal fluid (CSF).

Clinical & Coding Specifications

Clinical Context

A patient presents to the emergency department or an outpatient infectious disease clinic with acute onset fever, severe headache, nausea, altered mental status, and focal neurologic signs during late summer in a region endemic for La Crosse (California) encephalitis. The clinician orders serum and/or cerebrospinal fluid testing to evaluate for arboviral causes of encephalitis. A lumbar puncture may be performed; cerebrospinal fluid is sent to the laboratory. The laboratory performs an immunoassay to detect IgM and/or IgG antibodies to La Crosse (California) virus, and documents results in the laboratory information system. Results are communicated to the ordering provider for clinical correlation, infection control reporting, and public health notification as required.

Coding Specifications

Modifier	Description	When to Use
`26`	Professional component	Use when a separate physician interpretation of the immunoassay is reported distinct from the clinical laboratory technical component.
`TC`	Technical component	Use when billing only the laboratory technical component (instrumentation, supplies, technician time) for the immunoassay.
`90`	Reference (outside) laboratory	Use when the specimen is forwarded to an external reference laboratory that performs the immunoassay.
`91`	Repeat clinical diagnostic laboratory test	Use when the same immunoassay is repeated on the same day for validation or verification.
`59`	Distinct procedural service	Use when the immunoassay is billed in the same encounter as another distinct laboratory procedure not typically bundled.
`52`	Reduced services	Use if the immunoassay was partially performed or truncated and full service was not rendered.
`53`	Discontinued procedure	Use when the immunoassay was started but discontinued for patient or technical reasons before completion.
`78`	Unplanned return to OR by same physician following initial procedure (applicable rarely)	Rarely applicable; use if a specimen collection procedure required an unplanned return to operating room by the same physician.
`90`	Reference (outside) laboratory	Use when specimen is sent to an outside commercial or public health reference lab for testing.
`91`	Repeat clinical diagnostic laboratory test	Use for repeat testing for accuracy or quality control on the same day.

Taxonomy Code	Specialty	Notes
207RP0000X	Infectious Disease	Physicians who interpret and manage encephalitis testing and clinical correlation.
207L00000X	Pathology	Pathologists who oversee laboratory testing policies, result validation, and reporting.
364A00000X	Clinical Laboratory	Clinical laboratory scientists/medical technologists performing immunoassays.
2080P0207X	Neurology	Neurologists involved in diagnosis and management of encephalitis where testing is ordered.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`A83.0`	Mosquito-borne viral encephalitis	Directly relevant as a category for La Crosse (California) virus encephalitis.
`G04.90`	Encephalitis, unspecified, unspecified	Used when encephalitis is diagnosed but specific viral etiology not yet identified; immunoassay helps narrow cause.
`A92.8`	Other specified mosquito-borne viral fevers	Covers other mosquito-borne viral infections that may be included in differential testing.
`R50.9`	Fever, unspecified	Presents commonly with arboviral infections prompting serologic testing.
`R41.82`	Altered mental status, unspecified	A common presenting sign leading to CSF testing for encephalitis.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`86769`	Antibodies; arbovirus (e.g., West Nile, St. Louis), IgM, IgG or unspecified	Alternative or additional arboviral serology panels often ordered alongside testing for La Crosse virus.
`80053`	Comprehensive metabolic panel	Commonly ordered as part of the initial workup in patients with encephalitis to assess metabolic contributors and organ function.
`81000`	Urinalysis, by dipstick or reagent strip	Frequently ordered as a basic screening test in the acute evaluation of febrile or altered patients.
`36415`	Collection of venous blood by venipuncture	Procedure code for blood specimen collection used to obtain serum for immunoassay testing.
`62270`	Spinal puncture, lumbar (diagnostic)	Code for the lumbar puncture performed to obtain cerebrospinal fluid for encephalitis evaluation and immunoassay testing.