Blood tests explained: what are doctors looking for?

If you have Crohn’s disease or ulcerative colitis, you’ll probably have blood tests from time to time. You might have them during a hospital appointment, at a local blood collection centre, or before starting a new medication.

Blood tests help your IBD team understand what’s happening inside your body. They can check for signs of inflammation, see whether you’re getting the nutrients you need, and make sure your medication is safe.

You might wonder why you need blood tests when you’re already feeling better. That’s because IBD can sometimes cause changes inside your body even when you don’t notice any symptoms.

Let’s look at what your doctors are checking for.

🩹 Checking for inflammation

Crohn’s disease and ulcerative colitis cause inflammation in your digestive system. Blood tests can help your doctor look for signs that inflammation might be happening.

Two things doctors often check are:

• CRP (C-reactive protein): This is a substance your liver makes when there is inflammation in your body. A higher level can be a sign that inflammation is present.

• ESR (erythrocyte sedimentation rate): This test looks at how quickly your red blood cells settle in a tube. A higher result can sometimes indicate inflammation.

If these levels are high, your doctor may want to investigate further or check whether your treatment needs adjusting.

However, blood tests don’t always show inflammation, even when your IBD is active. That’s why your team looks at your symptoms and other test results, too.

🩸 Checking for anaemia and low iron

Have you ever felt unusually tired, dizzy, weak, or short of breath? Sometimes, these feelings can be caused by anaemia.

Anaemia happens when your body doesn’t have enough healthy red blood cells or haemoglobin to carry oxygen around your body.

Your IBD team may check:

• Haemoglobin: A protein in red blood cells that carries oxygen around your body.

• Red blood cell count: This measures how many red blood cells you have.

• Ferritin: A protein that stores iron in your body. Low ferritin can mean your iron stores are running low.

IBD can contribute to anaemia because inflammation can affect how your body uses iron, and bleeding from your bowel can cause you to lose it. You might also struggle to get enough iron from food.

If your iron levels are low, your doctor may recommend changes to your diet, iron supplements, or iron given through a drip, depending on what you need.

🥗 Checking your vitamins and nutrients

Your body needs vitamins and minerals to grow, repair itself, and give you energy.

Sometimes, IBD can make it harder for your body to absorb certain nutrients, especially if your small intestine is affected. Eating less when you feel unwell can also make it harder to get everything you need.

Your doctor may check nutrients such as:

• Vitamin B12: Helps keep your nerves and blood cells healthy.

• Folate: Helps your body make new cells and red blood cells.

• Vitamin D: Helps your body absorb calcium and keep your bones strong.

• Albumin: A protein in your blood that can give your doctor information about inflammation, nutrition, and your general health.

Not everyone needs all these tests at every appointment. Your IBD team will decide which ones are useful for you.

If your levels are low, they can help you work out how to get the nutrients your body needs.

💊 Making sure your medication is safe

Some IBD medications need regular blood tests to make sure they’re working safely and aren’t causing problems.

Depending on your medication, your doctor may check:

• Liver function: To see whether your liver is working properly.

• Kidney function: To check how well your kidneys are working.

• White blood cells: These help your body fight infections.

• Platelets: These help your blood clot when you have a cut or injury.

Certain medications can affect your liver, kidneys, or blood cells, so these checks help your team spot possible problems early.

You may need blood tests more often when you first start a medication or change your dose. Once things are stable, you might not need them as frequently.

Even if you feel completely fine, it’s important to attend your blood tests when your team recommends them.

📊 Understanding your blood test results

When you receive your results, you might see lots of numbers and medical words that don’t make much sense.

Most blood tests have a reference range, which shows the range of results usually expected in a particular group of people. Your results may be marked as high or low if they fall outside this range.

But a result outside the reference range doesn’t automatically mean something is wrong. Your age, medication, symptoms, and other results all help your doctor understand what it means.

Your doctor will look at the whole picture rather than relying on just one number.

It’s also helpful to compare results over time. For example, if your inflammation markers are getting lower, this may suggest your treatment is helping.

😟 Does having a blood test hurt?

It's normal to feel nervous about having blood taken, especially if you've had lots of tests already.

A healthcare professional will usually clean a small area of your arm, place a tight band around it, and insert a small needle into a vein to collect your blood.

You might feel a quick scratch or pinch, but the uncomfortable part usually doesn't last long. Afterwards, they'll remove the needle and put a small plaster over the spot.

Here are some things that might help:

• Tell the person taking your blood if you're worried or have felt faint during a blood test before.

• Look away if watching the needle makes you uncomfortable.

• Bring headphones and listen to music or a podcast.

• Take slow breaths and try to relax your arm.

• Ask to lie down if you feel faint or think you might become dizzy.

If you're allowed to drink before your test, having water can sometimes make it easier to find a vein. Some blood tests require fasting, so check your instructions beforehand.

⏰ How often will you need blood tests?

How often you need blood tests depends on your IBD, your symptoms, and the medication you take.

You might need them more frequently if:

• You've recently been diagnosed with IBD.

• You're starting a new medication.

• Your doctor is checking whether a treatment is working.

• You've been having more symptoms than usual.

• Your previous results showed something that needs monitoring.

If your IBD is stable and your medication doesn't require frequent monitoring, you may have fewer blood tests.

Your IBD team will let you know when your next tests are due. If you're unsure, it's always okay to ask.

💙 Remember, blood tests help your team look after you

Blood tests are an important part of managing Crohn’s disease and ulcerative colitis. They help your doctors check for inflammation, monitor your nutrition, spot anaemia, and make sure your medication is safe.

You don't need to understand every number on your results. Your healthcare team can explain what they mean and what, if anything, needs to happen next.

If you have questions about your results, write them down and take them to your next appointment. Understanding a little more about your blood tests can help you feel more confident about looking after your health.