80185

Phenytoin; Total

Phenytoin (commonly known by the brand name Dilantin) is a first-generation antiepileptic medication primarily utilized for the management and prevention of generalized tonic-clonic seizures and complex partial seizures. CPT code 80185 refers specifically to the quantitative measurement of total phenytoin concentration in a patient's serum or plasma. This therapeutic drug monitoring (TDM) is clinically essential because phenytoin possesses a narrow therapeutic index, usually targeted between 10 and 20 mcg/mL. One of the most critical aspects of phenytoin pharmacology is its non-linear, or zero-order, Michaelis-Menten kinetics at therapeutic levels. This means that once the liver's metabolic enzymes are saturated, even a small increase in the administered dose can lead to a disproportionately large and potentially toxic increase in serum concentration. Furthermore, phenytoin is highly protein-bound, primarily to albumin. CPT 80185 measures the total amount of the drug, which includes both the protein-bound portion and the free (active) portion. Clinicians must exercise caution when interpreting 80185 results in patients with hypoalbuminemia, chronic kidney disease, or those taking other highly protein-bound drugs, as the total level may appear normal while the free fraction is toxic. The laboratory procedure for 80185 typically employs automated immunoassays, such as enzyme-multiplied immunoassay technique (EMIT) or fluorescence polarization immunoassay (FPIA), though liquid chromatography-tandem mass spectrometry (LC-MS/MS) may be used for higher specificity. Monitoring is indicated during the initiation of therapy, following dosage adjustments, or when clinical signs of toxicity—such as nystagmus, ataxia, or cognitive impairment—are present.

Clinical Indications

  • Management of generalized tonic-clonic seizures
  • Management of complex partial seizures
  • Prevention and treatment of seizures following neurosurgery
  • Monitoring dosage adjustments in patients with epilepsy
  • Assessment of patient compliance with medication regimen
  • Evaluation of suspected phenytoin toxicity (e.g., nystagmus, ataxia, lethargy)
  • Monitoring patients with altered protein binding (e.g., pregnancy, renal failure, liver disease)
  • Management of status epilepticus after initial stabilization
  • Investigation of breakthrough seizures despite therapy

Procedure Steps

  1. Verify the patient's identity and confirm the timing of the last phenytoin dose (ideally at trough levels).
  2. Perform venipuncture to collect a blood sample in a plain red-top or green-top (heparin) tube, avoiding separator gels that may absorb the drug.
  3. Centrifuge the specimen to separate the serum or plasma from the cellular components.
  4. Aliquoted serum/plasma is introduced into the automated analyzer.
  5. The laboratory performs a quantitative assay (e.g., Immunoassay, HPLC, or LC-MS/MS) to determine the total phenytoin concentration.
  6. The analyzer calculates the concentration based on a multi-point calibration curve.
  7. The results are reviewed by laboratory personnel for technical accuracy.
  8. The final quantitative value is reported in micrograms per milliliter (mcg/mL) to the requesting clinician.

Coding Guidelines

  • Report 80185 for the quantitative measurement of total phenytoin.
  • If the laboratory measures the 'free' or 'unbound' phenytoin fraction instead of or in addition to the total, use CPT code 80186.
  • If multiple drug levels are performed on the same day to evaluate peak and trough or due to acute toxicity management, append modifier 91 (Repeat clinical diagnostic laboratory test) to the subsequent codes.
  • Do not use 80185 for qualitative drug screening; it is specifically for therapeutic monitoring (quantitative).
  • This code should not be bundled into a general chemistry panel; it is a distinct therapeutic drug assay.
  • Ensure the documentation supports medical necessity, such as a diagnosis of epilepsy or symptoms of toxicity.

Associated ICD-10 Codes