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Difficulty: HardMonitoring and Treatment Evaluation

A 28-year-old woman is evaluated in the outpatient gastroenterology clinic for poorly controlled, moderate-to-severe ulcerative colitis. She continues to have bloody diarrhea and abdominal cramping despite optimization of oral mesalamine and repeated courses of oral budesonide. The physician decides to initiate azathioprine as a steroid-sparing immunomodulator. In addition to a comprehensive metabolic panel for assessing baseline hepatic function, which of the following baseline diagnostic tests or ongoing monitoring parameters are essential for this patient's new pharmacotherapy? (Select 2)

  1. Thiopurine methyltransferase enzyme activityAnswer
  2. Complete blood count with differentialAnswer
  3. C
    Interferon-gamma release assay
  4. D
    Glucose-6-phosphate dehydrogenase assay
  5. E
    Routine periodic serum amylase and lipase

Answer

The essential baseline and ongoing parameters are testing for Thiopurine methyltransferase (TPMT) enzyme activity and monitoring with a complete blood count with differential.
Azathioprine is a prodrug that is converted to 6-mercaptopurine, which is subsequently metabolized by the enzyme thiopurine methyltransferase (TPMT). Individuals with homozygous or heterozygous TPMT mutations have low or absent enzyme activity, leading to the accumulation of cytotoxic active metabolites (6-thioguanine nucleotides) and an extraordinarily high risk of profound, potentially fatal myelosuppression. Therefore, baseline assessment of TPMT phenotype or genotype is mandatory before prescribing the drug. Additionally, even in patients with normal TPMT activity, dose-dependent bone marrow suppression can occur. Consequently, regular monitoring of the complete blood count with differential is an essential ongoing requirement to detect early signs of leukopenia, anemia, or thrombocytopenia.

Step-by-Step Solution

1
Identify the proposed new pharmacotherapy and its drug class.
The patient is being started on azathioprine, a thiopurine immunomodulator.
Determining the specific drug is required to identify its unique toxicity profile and specific monitoring guidelines.
2
Determine the baseline testing required prior to drug initiation.
TPMT enzyme activity must be assessed to identify patients at high risk for life-threatening bone marrow suppression.
Azathioprine is primarily metabolized by TPMT; genetic deficiency shifts metabolism toward toxic metabolites.
3
Identify the required ongoing laboratory monitoring during therapy.
Routine complete blood counts are necessary to monitor for dose-dependent myelosuppression.
Even with normal TPMT activity, dose-dependent leukopenia, anemia, or thrombocytopenia can occur during active treatment.

Key Concept

Prior to initiating azathioprine, baseline thiopurine methyltransferase (TPMT) activity must be assessed to prevent severe myelosuppression, and ongoing complete blood counts are required to monitor for dose-dependent bone marrow toxicity.
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Monitoring and Treatment Evaluation

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