80164

Therapeutic Drug Monitoring of Valproic Acid, Total

Valproic acid (also known as dipropylacetic acid) is a widely utilized broad-spectrum anticonvulsant and mood-stabilizing medication. CPT code 80164 represents the laboratory procedure to quantify the total concentration of valproic acid in a patient's blood, which includes both the portion bound to plasma proteins (primarily albumin) and the unbound (free) fraction. Valproic acid is primarily used in the management of various seizure disorders, including absence seizures, generalized tonic-clonic seizures, and complex partial seizures. It is also a mainstay treatment for the manic episodes associated with bipolar disorder and is frequently prescribed for migraine headache prophylaxis. The drug has a relatively narrow therapeutic index, typically cited between 50 and 100 micrograms per milliliter (mcg/mL), although some patients may require levels up to 125 or 150 mcg/mL for clinical control. Monitoring total valproic acid levels is essential for clinical management because the drug's metabolism can be influenced by age, hepatic function, and concurrent medications that induce or inhibit cytochrome P450 enzymes. Because valproic acid is highly protein-bound, the total level reported by 80164 is the standard baseline measurement. However, in patients with low albumin levels (hypoalbuminemia), renal failure, or those taking other highly protein-bound drugs, the ratio of free to bound drug may shift significantly, potentially leading to toxicity even when the total level appears within the therapeutic range. Clinical signs of toxicity include lethargy, ataxia, tremor, and more severe complications like hyperammonemic encephalopathy, hepatotoxicity, or pancreatitis.

Clinical Indications

  • Management and dose titration of epilepsy and seizure disorders
  • Monitoring of therapeutic levels for bipolar disorder treatment
  • Prophylactic management of migraine headaches
  • Assessment of patient compliance with the prescribed medication regimen
  • Evaluation of suspected valproic acid toxicity (e.g., altered mental status, vomiting)
  • Monitoring for drug-drug interactions when adding or removing concomitant medications
  • Regular surveillance in patients with hepatic or renal impairment

Procedure Steps

  1. Collection of venous blood through standard venipuncture techniques
  2. Placement of the sample in a serum separator tube (SST) or a plain red-top tube
  3. Allowing the blood to clot at room temperature if a serum tube is used
  4. Centrifugation of the specimen to separate the serum or plasma from the cellular components
  5. Aliquoting the serum or plasma into a secondary transport container if necessary
  6. Analysis of the specimen using quantitative immunoassay techniques, such as Enzyme-Multiplied Immunoassay Technique (EMIT) or Fluorescence Polarization Immunoassay (FPIA)
  7. Calculation and verification of the total valproic acid concentration in mcg/mL
  8. Documentation and reporting of the results to the ordering physician

Coding Guidelines

  • Code 80164 is used for the quantitative measurement of total valproic acid.
  • If only the 'free' or unbound portion of valproic acid is measured, use CPT code 80165 instead.
  • Therapeutic drug assays (80100-80299) are performed to monitor a specific drug's level for safety and efficacy; they are not screening tests.
  • Do not report 80164 more than once per day for the same patient unless clinical circumstances necessitate multiple timed draws (e.g., peak and trough levels), though typically one trough level is standard.
  • Ensure that the diagnosis code (ICD-10) reflects the medical necessity for monitoring a chronic medication.

Associated ICD-10 Codes