TPMT and NUDT15 Testing — Before You Start Thiopurines
Before starting 6-mercaptopurine or azathioprine, a blood test can tell your oncologist whether your genes put you at risk of a dangerous bone marrow reaction. In Indian patients, the gene that matters most is often not the one that is tested most often.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026
- A blood draw, done once — TPMT and NUDT15 testing requires a standard blood sample and does not need to be repeated.
- Identifies a preventable risk — Variants in these genes can cause the drug to reach levels the bone marrow cannot survive.
- Indian patients face a different risk profile — NUDT15 variants are far more common in South and East Asian patients than TPMT variants — and NUDT15 is often not tested.
- Your oncologist decides what follows — A test result informs a clinical decision. It does not automatically change your treatment.
on Panel
Survival Rate*
Treated
(800+ reviews)
TPMT and NUDT15 are genes that control how your body breaks down thiopurine drugs such as 6-mercaptopurine. Variants in either gene can cause the drug to accumulate to dangerous levels, leading to severe bone marrow suppression. Testing before you start identifies that risk so your oncologist can act on it.
What do TPMT and NUDT15 tests actually detect?
Both genes encode enzymes that break down thiopurine drugs — 6-mercaptopurine, azathioprine, and thioguanine — before they accumulate to levels that damage the bone marrow.
If you carry a variant in either gene, the enzyme works less efficiently. The drug then builds up and can suppress the bone marrow severely, causing dangerously low counts of white cells, red cells, and platelets.
The test looks at your DNA from a standard blood draw and classifies your likely enzyme activity as normal, intermediate, or poor. That result tells your oncologist what to watch for — not whether to treat you.
Why is NUDT15 especially important for patients in India?
TPMT testing was developed largely in European patient populations, where TPMT variants are the main driver of thiopurine toxicity.
According to CPIC guidelines, NUDT15 variants that reduce thiopurine tolerance are substantially more common in South and East Asian populations than in European populations. TPMT risk variants show the opposite pattern — they are less common in Indian patients.
A patient of Indian origin who tests normal for TPMT may still carry a NUDT15 variant that creates real risk. Testing both genes gives a more complete picture than TPMT alone, particularly for patients of South or East Asian ancestry.
Ask your oncologist which genes have been tested, and whether both results will be available before treatment starts.
Where to find us
Our centres in and around Hyderabad
Addressed by landmark, because that is how this city navigates. Each centre also names the areas it serves, so you can place it without a map. Consultation and day-care Chemotherapy run at every one of them.
17+ senior cancer specialists. One panel for your case.
Trained at AIIMS, Tata Memorial, and leading international centres. Combined 150+ years of experience. Every complex case is reviewed by 3+ of them — together.
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Dr. Vinay Mamidala
MBBS, MS(General Surgery), M.Ch(Surgical Oncology), FMAS, FARIS(Ongoing)
Dr. Vajja Sandeep Kumar
MBBS, MS (General Surgery), DrNB (Surgical Oncology), FALS Oncology
Want a specific doctor for your case? Mention them when booking.
Book Free ConsultationBook an appointment with our specialist
Share your name and number — we'll call you back within 30 minutes to schedule your consultation.
You do not have to work this out alone
A 45-minute consultation with a specialist who treats this every week.
What should you ask before starting mercaptopurine or azathioprine?
- Has TPMT testing been done on my blood sample before this drug starts?
- Has NUDT15 also been tested — not just TPMT?
- Is this test available here, or does the sample need to go to a specialist laboratory?
- What does my result mean for how I will be monitored during treatment?
- How often will my blood count be checked once I start?
- Which symptoms should I report between counts — and who do I call?
Did you know?
NUDT15 variants that cause reduced thiopurine tolerance are substantially more prevalent in South and East Asian populations than in European populations, while TPMT risk variants show the opposite geographic distribution.
Most early thiopurine safety guidelines were built on data from European cohorts, which is why NUDT15 was identified as clinically important much later — and remains less routinely tested than TPMT in many centres worldwide.
Source: CPIC Guideline for Thiopurines and TPMT and NUDT15 (Clinical Pharmacogenomics Implementation Consortium)
Explore 24 more Before You Start: Tests, Ports & Fertility topics
Pharmacogenomic & Pre-Treatment Safety Testing
- DPYD Testing Before 5-FU and Capecitabine
- Heart, Kidney and Hearing Tests Before Chemotherapy
- Safety Tests Before Your First Chemotherapy Dose
- Should You Ask for Pharmacogenomic Testing?
- TPMT and NUDT15 Testing Before Thiopurines
- UGT1A1 Testing Before Irinotecan
- What If a Safety Test Comes Back Abnormal?
- What Is DPD Deficiency?
Before You Start: Tests & Preparation
HUB — Before You Start Chemotherapy
Ports, PICC Lines & Cannulas
- Cannula Chemotherapy: Managing Difficult Veins
- Chemotherapy Ports and Lines: Which Do You Need?
- Do You Really Need a Port?
- Does a Port Hurt?
- Extravasation: When Chemotherapy Leaks From the Vein
- Looking After Your Port Between Cycles
- Looking After a PICC Line at Home
- Port Infection: Signs and What to Do
- Port Insertion: What the Procedure Involves
- Port vs PICC Line: Which Is Better?
- What Is a Chemotherapy Port?
- What Is a PICC Line?
- What to Report Immediately During an Infusion
- When and How Is a Port Removed?
Still not sure what applies to you?
Send your reports across and a senior medical oncologist will go through what they mean, what is known, and what the options actually are.
Frequently asked questions
What happens if I take mercaptopurine without testing and I carry a variant?
If you carry a poor-function variant in TPMT or NUDT15 and receive a thiopurine without that being known, active drug metabolites can build up to levels the bone marrow cannot tolerate. The result is severe myelosuppression — a dangerous fall in white cells, red cells, and platelets — which can lead to serious infection or bleeding. This is a predictable and potentially preventable event, which is why CPIC and ASCO guidance addresses testing before starting. If you are already on the drug and testing has not been done, tell your oncologist so they can decide whether to test now and how closely to watch your counts.
Which patients are thiopurine testing recommended for?
CPIC recommends TPMT and NUDT15 testing for anyone being considered for a thiopurine drug — 6-mercaptopurine, azathioprine, or thioguanine. In oncology, these drugs are used most commonly in maintenance therapy for acute lymphoblastic leukaemia, but azathioprine also appears in some lymphoma and autoimmune protocols. The recommendation applies regardless of ancestry, though the variant most likely to be found differs by population. Your oncologist will determine whether testing is appropriate in your specific situation and what can be arranged from your treating centre.
Does a normal TPMT result mean I am safe if NUDT15 was not tested?
Not necessarily, particularly for patients of South or East Asian ancestry. TPMT and NUDT15 are separate genes encoding different enzymes, and a result for one says nothing about the other. Because NUDT15 variants are substantially more common in Indian and other Asian patients, a normal TPMT result while NUDT15 is untested leaves a meaningful gap in the safety information your oncologist has before prescribing. Ask specifically whether both genes have been checked, not just one.
Is TPMT and NUDT15 testing available in India?
Testing for both genes is available at some specialist pharmacogenomics laboratories and larger cancer centres in India, though it is not uniformly available at every centre. Some samples need to be sent to a reference laboratory, which adds days to the turnaround. Availability is increasing as pharmacogenomic testing becomes more integrated into oncology practice. Your oncologist is the right person to find out what is accessible from your treating centre, how the sample gets there, and how long the result will take.
Can I still have the drug if my test shows a variant?
A variant result does not automatically mean you cannot receive a thiopurine. It means your oncologist has information they need to plan treatment and monitoring appropriately. What they decide depends on which variant you carry, what it predicts about your enzyme function, which drug is being considered, and the clinical context. The decision is your oncologist's to make, with the test result as one input. Do not delay starting treatment or refuse a prescribed drug because of a result before talking to your team.
What does the test actually involve?
A standard blood draw — the same kind used for routine blood tests. The sample is sent to a laboratory where your DNA is analysed for known variants in the TPMT and NUDT15 genes. The result classifies your predicted enzyme activity based on the variants found. It needs to be done only once, as your genetic makeup does not change. The result is then interpreted by your oncologist alongside your diagnosis, the drug being considered, and your overall clinical picture.
How long does the result take?
Turnaround time varies by laboratory. At centres where testing is done in-house, results may be available within a few days. If the sample must be sent to a reference laboratory, it typically takes longer. Ask your team when the sample was dispatched and when the result is expected, so there is a clear timeline before your planned treatment start date. If your treatment is urgent and a result is not yet available, your oncologist will make a clinical judgement about timing and monitoring in the interim.
What cancers and conditions are treated with thiopurine drugs?
In oncology, 6-mercaptopurine is used most commonly as part of maintenance therapy for acute lymphoblastic leukaemia, particularly in children and young adults. Azathioprine appears in some lymphoma protocols and in autoimmune conditions that can overlap with cancer care. Thioguanine is used in certain leukaemia regimens. These drugs are taken by mouth and are typically part of a prolonged course rather than short-cycle chemotherapy. Your oncologist will explain which drug applies to your diagnosis and the role it plays in your specific treatment plan.
Chemotherapy Topics
Browse our complete chemotherapy guide — 1133 pages on side effects, medicines, cost, daily life and care in Hyderabad. Tap any topic to read more.