# A client with Crohn disease is starting azathioprine. Which baseline test is essential to determine starting dose and avoid severe myelosuppression?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=391821&lang=en  
> language: en  
> subject: Medical Emergencies  
> category: PA

## Question

A client with Crohn disease is starting azathioprine. Which baseline test is essential to determine starting dose and avoid severe myelosuppression?

## Option

1. Serum vitamin B12, folate, and homocysteine levels, plus CBC, LFT, and comprehensive iron panel
2. TPMT (thiopurine S-methyltransferase) activity or genotype, plus CBC, LFT, and viral hepatitis serology **✔ Correct answer**
3. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), plus CBC, LFT, and stool studies
4. UDP-glucuronosyltransferase (UGT1A1) genotype, plus CBC, LFT, and viral hepatitis serology

**Correct answer: 2**

## Explanation

Correct (2): TPMT deficiency (homozygous low activity ~0.3% of population) leads to azathioprine accumulation and severe, potentially fatal myelosuppression. Genotype/phenotype guides dosing or avoidance. Also obtain baseline CBC, LFT, viral hepatitis serology. NUDT15 testing for Asian populations. (1,3,4) Unrelated to thiopurine safety.

## In-depth explanation

Memory Tip Azathioprine/6-MP = "TPMT test before start." NUDT15 in Asian descent — additional pharmacogenomic check.

## Clinical scenario

Scenario 42-year-old female with Crohn disease starting azathioprine. Pre-treatment workup.

## Key concepts

- **Azathioprine** — Purine analog immunosuppressant; metabolized to 6-MP active metabolite.
- **TPMT enzyme** — Metabolizes thiopurines; deficient genotype → toxicity.
- **NUDT15** — Pharmacogenomic gene with higher impact in Asian populations than TPMT.

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