The TPMT (Thiopurine Methyltransferase) Enzyme Activity test measures the activity of the TPMT enzyme, which is involved in metabolizing thiopurine drugs like azathioprine, used in conditions like autoimmune diseases and organ transplants. This test helps identify individuals with low or absent TPMT activity, who may be at risk for severe side effects from thiopurine medications. It aids in dose adjustments to ensure drug safety and efficacy.