Azathioprine monitoring blood tests: what is checked and how often

Azathioprine is an immune-suppressing medicine used for conditions such as rheumatoid arthritis, Crohn's disease, ulcerative colitis, lupus and after an organ transplant, according to the NHS. Because it acts on the immune system, it can occasionally affect the liver, kidneys and bone marrow. The bone marrow is where blood cells are made, so the usual monitoring centres on a blood count, liver function and kidney function.

The NHS describes a pattern of testing that is front-loaded. You have tests before treatment begins, then at least weekly for the first 8 weeks, then less often. Your prescriber advises on exactly how often, and the schedule can be tighter if your dose is high or you already have kidney or liver problems.

This page explains the general approach described by the NHS and Arthritis UK. It cannot replace the plan agreed by your own specialist team, and it does not set out a personal schedule or interpret individual results.

The NHS says that you will have blood tests to check your liver function, kidney function and blood count before you start taking azathioprine, and that a blood test is done beforehand to make sure the medicine is safe for you. These baseline results give your clinician something to compare later results against.

Arthritis UK explains that, before prescribing azathioprine, a doctor will order a blood test for an enzyme called TPMT (thiopurine methyltransferase). TPMT helps break down and remove azathioprine from the body. Low levels may mean a slightly increased risk of side effects, so a doctor may suggest a lower dose, and if the result is very low azathioprine may not be the right medicine. The NHS also mentions NUDT15, a gene linked to a rare inherited condition, and says to tell your doctor if you have ever been told your body produces too little TPMT.

Arthritis UK adds that, before starting, you might also be checked for previous hepatitis B or C infection, because the drug can increase the risk of these infections starting up again, and that you will usually need a chest X-ray. These are additions to the routine monitoring bloods rather than part of the weekly schedule.

The pattern the NHS describes is simple. For the first 8 weeks you have blood tests at least once a week. From week 9 onwards the tests become less frequent, and your doctor will advise how often. The NHS says you may only need them every few months, and that ongoing monitoring continues for as long as you take azathioprine.

Each part of the monitoring panel answers a different safety question.

• Full blood count: looks at the blood cells made in the bone marrow, which azathioprine can affect. This is why infection and bruising symptoms matter.

• Liver function tests: look for signs that the liver is being affected. The NHS also says that itching while taking azathioprine in pregnancy should prompt a conversation, as tests may be needed.

• Kidney function (urea and electrolytes): looks at how well the kidneys are working. The NHS lists a change in pee colour or amount as something to report.

• TPMT (and, where relevant, NUDT15): usually a one-off pre-treatment test that guides dosing rather than ongoing monitoring.

Scheduled blood tests are a safety net, but they do not replace paying attention to how you feel. The NHS says to stop taking azathioprine and speak to a doctor or call 111 now if you feel tired all the time, dizzy or sick, or you are vomiting or have diarrhoea; you have a high temperature with shivering or chills, cough or a sore throat; your joints or muscles are hurting; your pee changes colour or you pee more or less than usual; you feel confused, light-headed or weak; you are bleeding or bruising more easily than usual; you notice lumps anywhere on your body; or you have severe stomach ache and back pain.

Arthritis UK similarly advises telling your doctor or nurse specialist straight away about a sore throat, fever or other signs of infection, unexplained bruising or bleeding, and yellowing of the skin or eyes. It also says to contact your rheumatology team if you get chickenpox or shingles, or have contact with someone who has them and have never had chickenpox.

A serious allergic reaction is rare but is an emergency. The NHS says to call 999 if lips, mouth, throat or tongue suddenly swell or if you are struggling to breathe.

Arthritis UK says you may be asked to keep a record of your blood test results in a booklet and to take it with you to the GP or hospital. Keeping the record up to date helps avoid gaps, especially when a GP practice, hospital clinic and community phlebotomy service are all involved.

Other medicines matter too. The NHS says to tell your doctor or pharmacist if you take allopurinol, ciclosporin or tacrolimus, warfarin, or are having chemotherapy, and to mention azathioprine before any surgery or vaccination, especially a live vaccine. Arthritis UK also says alcohol and azathioprine can both affect the liver, so it is important not to exceed the recommended limits.

The NHS notes that taking azathioprine for a long time can increase the chance of certain cancers, including skin cancer, and advises avoiding strong sunlight and using sunscreen of SPF 30 or more. If you are planning a pregnancy, are pregnant or are breastfeeding, the NHS says to talk to your doctor rather than stopping the medicine yourself, since it is particularly important to keep taking it if you have an autoimmune condition or have had an organ transplant.

There is no guidance in the NHS pages about making up a missed blood test, so the practical step is to contact the team that prescribes the medicine and rebook promptly. For missed doses, the NHS says that if you forget one dose you should take it as soon as you remember unless it is nearly time for the next one, in which case skip the missed dose; do not take two doses to make up. Check with your doctor or pharmacist if you miss two doses or more.

If you take more than your prescribed dose, the NHS says to contact 111 for advice even if you feel well, and to call 999 or go to A&E if you feel unwell.