UGT1A1 Testing Before — Irinotecan
A blood test can tell your oncology team whether your body carries a genetic variant that raises the risk of severe diarrhoea or dangerously low white blood cells from irinotecan-based chemotherapy. The test looks at how efficiently your liver can clear the drug's active form. Done before treatment starts, it gives your team the information when it is still useful.
Medically reviewed by Dr. Naresh Gundu, Medical Oncologist, MBBS (Chalmeda Anand Rao Institute of Medical Sciences, Karimnagar) · DNB Internal Medicine (Sir Gangaram Hospital, New Delhi) · DM Medical Oncology (AIIMS) · Last reviewed September 2026
- One blood draw, done once — The result reflects your genetic makeup and stays the same throughout your life — it is not a blood count that changes with each cycle.
- Predicts how you clear irinotecan — Variants in the UGT1A1 gene slow the enzyme that breaks down irinotecan's active compound, allowing it to build up to higher concentrations.
- Most useful before the first dose — Your oncologist needs this information before starting treatment, not after side effects have already appeared.
- Your oncologist decides what comes next — The test result is one input into your treatment plan — the decision about how to use it belongs to your treating team.
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UGT1A1 is a gene that controls how your body breaks down irinotecan's active ingredient. Variants in this gene — most commonly *28 and, in some populations, *6 — slow that breakdown and raise the risk of severe diarrhoea and low white blood counts. A blood test done before treatment can show whether you carry these variants.
What does UGT1A1 testing check for?
UGT1A1 testing checks whether you carry genetic variants that slow the enzyme your liver uses to clear irinotecan's active compound. Irinotecan is converted in the body into a substance called SN-38, which does the anti-cancer work. Your liver must then break SN-38 down and remove it, and the UGT1A1 enzyme does that job.
Some people carry variants in the UGT1A1 gene that make this enzyme less active. The most common is called *28. A variant called *6 is also clinically important in South Asian and East Asian populations. When the enzyme is less active, SN-38 stays in the body for longer and at higher concentrations.
Higher SN-38 concentrations raise the risk of severe diarrhoea and severe neutropenia — a dangerous fall in white blood cells that can leave you vulnerable to serious infection. CPIC and NCCN guidance both identify UGT1A1 variants as clinically important information before irinotecan-based chemotherapy.
The test requires a single blood draw. Because it reads your genetic code rather than measuring something that fluctuates in the blood, the result is the same whenever it is taken. Your oncologist interprets it in the context of your full treatment plan.
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Did you know?
CPIC — the Clinical Pharmacogenomics Implementation Consortium — has published guidance on UGT1A1 and irinotecan since 2014. That guidance identifies patients at substantially higher risk of life-threatening side effects through a pre-treatment blood test.
Knowing your result before the first dose is what allows your team to plan around it.
Source: CPIC Guideline for Irinotecan Therapy and UGT1A1 Genotype
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Frequently asked questions
What exactly does the UGT1A1 test look at?
The test looks at specific positions in your UGT1A1 gene — the part of your DNA that codes for the enzyme responsible for clearing irinotecan's active compound. It identifies whether you carry variants, most commonly *28 or *6, that reduce how efficiently that enzyme works. This is a genetic test, not a routine blood count, and it is processed on a separate sample sent to a genomics or pharmacogenomics laboratory.
Does every patient starting irinotecan need this test?
Your oncologist decides whether to order it based on your specific treatment regimen and clinical picture. CPIC and NCCN guidance considers UGT1A1 status clinically relevant before irinotecan-based treatment, particularly where the result could influence how treatment is planned. If you are being offered irinotecan and have not been told about this test, it is reasonable to ask your oncologist whether it applies to your case.
What happens if my result shows I carry a UGT1A1 variant?
Your oncologist reviews the result alongside your cancer type, stage, treatment goals and overall fitness before deciding how to proceed. The result is one piece of clinical information — it does not make a treatment decision on its own. Ask your oncologist to explain in plain language what your result means for your specific plan, and what, if anything, changes because of it.
Is UGT1A1 testing available in India?
Pharmacogenomic testing for UGT1A1 is available at specialised genetics and genomics laboratories in India, including through some large diagnostic chains. It is not uniformly available at every oncology centre, and your sample may need to be sent to a reference laboratory. Ask your treating team whether they can arrange it and which laboratory they use. Availability and turnaround time vary by location.
How long does a UGT1A1 result take?
Turnaround time depends on whether the sample is processed locally or at a reference laboratory. Results can take anywhere from a few days to two weeks. Your oncology team can give you a clearer timeline once they know which laboratory is handling the sample. If starting treatment soon matters, tell your team so the timing can be planned around the result.
Can I still receive irinotecan if I carry a variant?
Carrying a UGT1A1 variant does not automatically rule out irinotecan-based treatment. Your oncologist weighs the result against your cancer type, the available alternatives and the goals of your treatment. Many patients who carry variants go on to receive irinotecan as part of their plan. The test gives your team information before the first dose rather than a problem to discover after side effects have occurred.
What symptoms should I watch for if I am on irinotecan?
Report immediately any diarrhoea that is clearly worse than your usual pattern — particularly if it is watery, frequent or comes with cramping. Fever alongside any bowel change needs same-day contact with your team, not a wait until the next appointment. Chills, feeling unusually unwell or mouth sores can indicate that your white blood cell count has fallen dangerously low and need urgent assessment. Do not take over-the-counter medicines for diarrhoea without asking your team first.
Does UGT1A1 testing predict whether irinotecan will work against my cancer?
No. UGT1A1 testing predicts how safely your body can handle irinotecan, not whether it will be effective against your cancer. Effectiveness is estimated through different information — your cancer type, stage, molecular profile, and how that cancer is known to respond to this class of drug. UGT1A1 testing is a safety test, not a response test. Your oncologist can explain what separate information guides the judgement that irinotecan is likely to help you.
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