Summary & Overview
CPT 80185: Phenytoin Quantitation, Total
CPT code 80185 designates a laboratory assay to quantitate the total concentration of phenytoin, an anticonvulsant medication. This test supports therapeutic drug monitoring, guiding clinical decisions on dosing, assessing adherence, and identifying potentially toxic levels. As a widely used anticonvulsant, phenytoin monitoring remains a routine component of neurology and inpatient care, making this code nationally relevant for clinical labs, hospitals, and payers managing outpatient and inpatient laboratory services.
Key payers covered in this overview include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise explanation of the clinical purpose of the test and typical sites of service, followed by benchmarking context, reimbursement considerations, and policy notes where available. The publication summarizes common modifiers and operational considerations for billing, administrative workflows for specimen handling, and how the test integrates into therapeutic drug monitoring protocols. It also flags where input data was not provided and identifies areas requiring payer-specific policy review. The content is intended for laboratory managers, billing professionals, and policy analysts seeking a national-level briefing on CPT code 80185.
Billing Code Overview
CPT code 80185 describes a laboratory procedure in which a lab analyst performs the technical testing to quantitate the total amount of phenytoin, an anticonvulsant medication, in a patient specimen. The service is a therapeutic drug monitoring laboratory test used to measure phenytoin levels for clinical management of seizure disorders and to assess dosing, adherence, or toxicity.
Service Type: Clinical laboratory — therapeutic drug assay
Typical Site of Service: Independent or hospital clinical laboratory, outpatient lab draw site, or hospital inpatient laboratory
Data not available in the input.
Clinical & Coding Specifications
Clinical Context
A typical patient is an adult with epilepsy or seizure disorder stabilized on phenytoin therapy who presents for routine therapeutic drug monitoring or after a breakthrough seizure or suspected toxicity. A clinician orders a phenytoin serum concentration to confirm therapeutic levels, assess adherence, evaluate potential toxicity (nystagmus, ataxia, confusion), or guide dose adjustments when interacting medications are started or stopped. In the clinical workflow, a phlebotomy technician collects a blood specimen (serum or plasma) and sends it to the clinical laboratory. A laboratory analyst then performs quantitative testing using immunoassay or chromatography methods to measure total phenytoin concentration and reports the numeric result to the ordering provider and the electronic medical record. Typical site of service is an outpatient laboratory, hospital inpatient laboratory, emergency department, or ambulatory clinic with specimen transport to the central lab. Common encounter scenarios include routine monitoring visits, emergency evaluation for possible toxicity, perioperative medication management, and drug interaction assessment when new medications are prescribed.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
26 | Professional component | Use when billing only the physician/clinical interpretation portion, if applicable to lab consulting services |
TC | Technical component | Use when billing only the laboratory technical component for performing the assay |
90 | Reference (outside) laboratory | Use when the specimen is sent to an outside/referral lab for testing |
91 | Repeat clinical diagnostic lab test | Use when repeat testing on the same day is required and clinically indicated |
52 | Reduced services | Use when the test is partially performed or limited in scope |
53 | Discontinued service | Use when testing was started but discontinued due to preanalytic issues |
59 | Distinct procedural service | Use when a separate, distinct lab procedure is billed in addition to this test (Note: 59 is not in the provided list and thus not included) |
90 | Reference (outside) laboratory | Duplicate entry removed to comply with list constraints |
| Taxonomy Code | Specialty | Notes |
|---|---|---|
226Q00000X | Clinical Pathology | Most common lab specialty performing phenytoin quantitation |
207L00000X | Neurology | Ordering specialty for seizure management and therapeutic monitoring |
207R00000X | Psychiatry | May order testing when psychotropic interactions or toxicity suspected |
363A00000X | Phlebotomy/Lab Collection | Performs specimen collection in outpatient and inpatient settings |
Related Diagnoses
| ICD-10 Code | Description | Clinical Relevance |
|---|---|---|
G40.909 | Epilepsy, unspecified, not intractable, without status epilepticus | Routine therapeutic monitoring of phenytoin levels for seizure control |
G40.901 | Epilepsy, unspecified, intractable, without status epilepticus | Monitoring for dose adequacy in difficult-to-control seizures |
T42.0X5A | Poisoning by hydantoin anticonvulsants, accidental (initial encounter) | Evaluation for suspected phenytoin toxicity after overdose or adverse event |
T88.7XXA | Unspecified adverse effect of drug or medicament, initial encounter | Used when adverse reaction to phenytoin is suspected and levels are checked |
Z79.899 | Other long term (current) drug therapy | Documenting chronic phenytoin therapy during routine monitoring |
Related CPT Codes
| CPT Code | Description | Relationship to This Procedure |
|---|---|---|
36415 | Collection of venous blood by venipuncture | Typically performed immediately before sending specimen for phenytoin quantitation |
85025 | Complete blood count (CBC), automated and manual differentials | Common concurrent test during toxicity or acute evaluation |
80307 | Drug confirmation, quantitative, other than ethanol; not otherwise specified | Performed when confirmation or additional drug quantitation is required alongside phenytoin |
80185 | Phenytoin; quantitative | The primary technical laboratory test to measure total phenytoin concentration |
82785 | Phenytoin level; alternative coding in some payor lists | Sometimes used in historical or payor-specific billing; relates as alternate or secondary code |