Pathology/Lab Coding Alert

Nevada Subscriber

Answer: The ordering physician should document that the reason for the test is to evaluate the therapeutic level of cytarabine, which the leukemia patient is taking as a chemotherapy agent. Even if the physician provides an ICD-10-CM code for the leukemia (such as C92.0- Acute myeloblastic leukemia), you should not use that as the primary diagnosis.

Here’s why: The first listed diagnosis code for this test should be Z51.81 (Encounter for therapeutic drug level monitoring), followed by Z79.631 (Long term (current) use of antimetabolite agent) to define the drug. ICD-10-CM specifies that Z79.631 is for 5-fluorouracil, 6-mercaptopurine, and cytarabine.

Although ICD-10-CM lists both Z51.81 and Z79.631 with a “code also” note that doesn’t provide sequencing direction, the proximate reason for the test is for therapeutic drug level monitoring.

Myth: Some coders believe they are not allowed to use Z codes as a primary diagnosis, but that is incorrect. In fact, the Centers for Medicare & Medicaid Services (CMS) in their guideline sheet “ICD-10-CM Official Guidelines for Coding and Reporting FY 2024” states, “Z codes may be used as either a first-listed … or secondary code, depending on the circumstances of the encounter.”

Procedure: The appropriate procedure code for this test is 80299 (Quantitation of therapeutic drug, not elsewhere specified). Using a code from the “Therapeutic Drug Assays” section is appropriate based on the diagnostic documentation and the information you provided that the lab performed a quantitative test.

Caution: CPT^® provides more specific codes for certain therapeutic medications, which you should always use, when available. For instance, another antimetabolite drug has its own code, 80204 (Methotrexate), so you would not report a therapeutic drug test for that medication using the “not elsewhere specified” code, 80299.