Summary & Overview
CPT 80186: Quantitative Assay for Free Phenytoin
CPT code 80186 specifies a quantitative laboratory assay to measure the free (unbound) concentration of phenytoin, an anticonvulsant drug. Free phenytoin measurement is clinically important because only the unbound fraction is pharmacologically active; results guide dose adjustments in patients with altered protein binding, critical illness, or interacting medications. Nationally, accurate reporting of this laboratory procedure affects clinical decision-making, quality measurement for therapeutic drug monitoring, and lab reimbursement workflows.
Key payers addressed in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of the clinical context for ordering free phenytoin testing, typical sites of service where the assay is performed, and the common modifiers used with the service line. The publication summarizes billing and coding considerations relevant to labs and ordering clinicians and provides benchmarking and policy context where available. Data not available in the input will be noted explicitly.
This summary equips billing managers, laboratorians, and clinical leaders with the essential information needed to classify CPT code 80186, understand its clinical purpose, and locate further details on payer coverage and coding practice.
Billing Code Overview
CPT code 80186 describes a laboratory procedure in which a lab analyst performs the technical testing to quantitate the amount of free phenytoin in a patient specimen. Phenytoin is an anticonvulsant medication, and measuring free (unbound) phenytoin concentration informs clinical management of seizure disorders and dosing adjustments when protein binding is altered.
Service type: Therapeutic drug monitoring / laboratory quantitative assay
Typical site of service: Clinical laboratory or hospital laboratory
Clinical & Coding Specifications
Clinical Context
A typical patient is an adult with epilepsy or seizure disorder on phenytoin therapy who presents for therapeutic drug monitoring due to breakthrough seizures, suspected toxicity, or changes in clinical status (renal or hepatic dysfunction, new interacting medications, or altered albumin levels). An outpatient phlebotomy encounter or inpatient laboratory order is placed for a free (unbound) phenytoin level to assess the pharmacologically active fraction. The clinician orders 80186 when measurement of free phenytoin concentration is needed to guide dose adjustment. The specimen (serum or plasma) is collected, labeled, and sent to the clinical laboratory. The laboratory analyst performs the technical assay to quantitate free phenytoin, documents the result in the laboratory information system, and the interpreting clinician reviews the numeric concentration with the patient chart to adjust dosing or investigate toxicity. Typical sites of service include hospital inpatient laboratories, outpatient hospital labs, and independent clinical reference laboratories.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
26 | Professional component | When reporting the physician interpretation component separate from the technical lab component (rare for lab-only 80186). |