Summary & Overview
CPT 84433: Thiopurine S–methyltransferase (TPMT) Enzyme Level Assay
CPT code 84433 represents a laboratory test that measures thiopurine S–methyltransferase (TPMT) levels in blood. TPMT testing is clinically important because reduced enzyme function increases the risk of myelosuppression in patients treated with thiopurine medications such as azathioprine, mercaptopurine, and thioguanine. Nationally, TPMT testing informs medication selection and dosing to reduce serious hematologic adverse events and is increasingly part of precision prescribing workflows.
Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. The publication gives readers a concise overview of how CPT code 84433 is used, typical sites of service, and clinical rationale for testing. It summarizes payer coverage patterns and common billing considerations relevant to laboratory services for pharmacogenetic and enzyme assays.
Readers will learn clinical context for TPMT testing, operational details for ordering and specimen handling, and typical billing and service-line placement for laboratory enzyme assays. Where available, benchmarks and policy highlights outline payer coverage approaches and common administrative issues. Data not available in the input is noted where applicable.
Billing Code Overview
CPT code 84433 describes a laboratory assay to measure thiopurine S–methyltransferase (TPMT) levels in a blood specimen. TPMT is an enzyme that affects metabolism of thiopurine drugs; testing identifies patients at increased risk for myelosuppression when treated with thiopurines.
Service type: Laboratory testing (therapeutic drug-related enzyme assay)
Typical site of service: Clinical laboratory or hospital outpatient laboratory, with specimens collected in ambulatory clinics or physician offices and processed by reference or hospital labs.
Clinical & Coding Specifications
Clinical Context
A typical patient is an adult or pediatric patient being considered for therapy with thiopurine medications (for example, azathioprine, mercaptopurine, or thioguanine) for inflammatory bowel disease, autoimmune hepatitis, organ transplant immunosuppression, or certain hematologic malignancies. Prior to initiating therapy or when unexplained cytopenias occur during treatment, the clinician orders a TPMT activity or genotype test billed with 84433 to assess thiopurine S‑methyltransferase enzyme function. The clinical workflow: the clinician documents indication and informed consent in the medical record, places a laboratory order, phlebotomy collects a whole blood specimen (typically EDTA), the sample is sent to the clinical laboratory for enzymatic activity or genotyping, laboratory performs analysis and issues a report with TPMT activity level (or genotype interpretation), and the clinician documents results and incorporates them into the medication decision (dose adjustment, alternative therapy, or enhanced monitoring). Typical site of service is an outpatient clinic or ambulatory laboratory draw station; testing may also be performed from inpatient hospital blood draws when clinically indicated.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
00 | No modifier reported — standard claim. |