80175

Lamotrigine

CPT code 80175 describes the quantitative laboratory analysis of lamotrigine, an extensively utilized phenyltriazine antiepileptic drug (AED). Lamotrigine is FDA-approved for the management of various seizure disorders, including primary generalized tonic-clonic seizures, partial-onset seizures, and the seizures associated with Lennox-Gastaut syndrome, acting both as monotherapy and adjunctive therapy. Beyond neurology, it is a cornerstone medication in psychiatry for the maintenance treatment of bipolar I disorder, primarily utilized to delay the time to occurrence of mood episodes. Therapeutic drug monitoring (TDM) of lamotrigine is critical in clinical practice due to its complex pharmacokinetic profile and the significant potential for drug-drug interactions. For instance, co-administration with valproic acid can more than double the half-life of lamotrigine by inhibiting its glucuronidation, thereby dramatically increasing the risk of severe concentration-dependent adverse effects, including life-threatening dermatologic reactions such as Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Conversely, enzyme-inducing agents such as carbamazepine, phenytoin, phenobarbital, and estrogen-containing oral contraceptives enhance the clearance of lamotrigine, potentially resulting in subtherapeutic levels and breakthrough seizures or psychiatric destabilization. The procedure for CPT 80175 involves collecting a peripheral venous blood sample from the patient, predominantly timed to obtain a trough concentration—drawn immediately prior to the administration of the next scheduled dose. The blood is typically collected in a standard plain red-top or green-top (heparinized) tube, avoiding gel separator tubes which might interfere with the drug concentration measurements. Upon arrival at the clinical laboratory, the specimen is centrifuged to isolate the serum or plasma. The sample is then subjected to highly precise analytical techniques to quantify the total circulating amount of the drug. Analytical methods frequently employed include homogeneous enzyme immunoassays, high-performance liquid chromatography (HPLC), or liquid chromatography-tandem mass spectrometry (LC-MS/MS). The resulting quantitative value allows the treating physician to make evidence-based pharmacological interventions, individualize dosing regimens, ascertain patient compliance, and manage the delicate balance between maximum therapeutic efficacy and minimal systemic toxicity. Routine monitoring is especially vital during physiological changes such as pregnancy, where accelerated clearance can cause rapid declines in serum lamotrigine concentrations, necessitating vigilant tracking and dynamic dosage titration.

Clinical Indications

  • Establishing a baseline therapeutic concentration for a patient with epilepsy or bipolar disorder.
  • Evaluating suspected lamotrigine toxicity (e.g., ataxia, diplopia, dizziness, rash).
  • Monitoring levels during the addition or discontinuation of interacting medications (e.g., valproic acid, oral contraceptives, carbamazepine).
  • Assessing patient adherence to the prescribed medication regimen.
  • Adjusting dosages during significant physiological changes that alter pharmacokinetics, such as pregnancy, or hepatic/renal impairment.
  • Investigating breakthrough seizures or mood destabilization in patients previously well-controlled on a stable dose.

Procedure Steps

  1. Verify the patient's identity and confirm the time of the patient's last lamotrigine dose to ensure appropriate trough timing.
  2. Perform venipuncture using aseptic technique to draw blood into an appropriate collection tube (typically a plain red-top or green-top tube without gel separator).
  3. Label the specimen accurately with patient identifiers, collection time, and date.
  4. Transport the blood sample to the laboratory in accordance with stability and temperature requirements.
  5. Centrifuge the blood specimen to separate the serum or plasma from the cellular components.
  6. Prepare the analytical instrument, performing any necessary calibrations and running standard quality control samples.
  7. Execute the quantitative assay for lamotrigine utilizing high-performance liquid chromatography (HPLC), liquid chromatography-tandem mass spectrometry (LC-MS/MS), or immunoassay.
  8. Analyze the instrument's output to calculate the total lamotrigine concentration in micrograms per milliliter (mcg/mL).
  9. Review the final quantitative result for technical accuracy and release the report to the ordering clinician.

Coding Guidelines

  • Report CPT 80175 only for a quantitative analysis of lamotrigine. Qualitative screening should be reported with the appropriate presumptive toxicology codes.
  • If multiple specimens are collected at distinct times on the same date of service (e.g., peak and trough levels), report 80175 for each distinct assay and append modifier 91 to the subsequent tests to indicate a repeat clinical diagnostic laboratory test.
  • Do not bill 80175 in conjunction with unlisted or generic drug assay codes if lamotrigine is the specific and sole analyte being quantified.
  • Medical necessity must be clearly documented and supported by appropriate ICD-10-CM diagnosis codes, such as those for epilepsy, bipolar disorder, adverse drug effects, or long-term drug monitoring.
  • Ensure documentation explicitly records the time of the last dose and the time of the blood draw to support the clinical validity of the trough measurement.

Associated ICD-10 Codes