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Blood is normally taken from a vein in the arm, using a fine needle with a syringe and a series of small vacuum tubes that ‘suck’ blood out automatically when they’re popped into the syringe housing. You’ll normally have a pressure cuff wrapped around your upper arm to increase the blood pressure and make collection easier, and you may also be asked to squeeze a small ball, or pump your hand into a fist. Apart from the slight scratch when the needle is put in, blood tests are not a problem for most people, but do tell the doctor or nurse if you are needle phobic, if you have one arm that gives blood more easily than the other, or if you have tricky veins.

The following list includes the various kinds of tests and the more common factors that doctors can study in blood, in order to understand what is going wrong.

Full blood count: This is one of the most common tests. It measures the levels of basic components in the blood, the number of red blood cells, white blood cells and platelets. A reduced number of red blood cells or lower levels of haemoglobin may indicate, for example, anaemia, while an increase in the numbers of the various white blood cells may indicate an infection.

Erythrocyte sedimentation rate (ESR): The ESR measures how quickly red blood cells settle at the bottom of a blood sample. If there is active inflammation, red blood cells tend to clump together, making them sink faster. A raised ESR can therefore indicate active inflammation.

C-reactive protein: A raised level of C-reactive protein indicates there is inflammation. This test is often done in conjunction with the ESR.

Ferritin: Low ferritin levels may indicate the need for iron supplements; the levels also vary when there is inflammation.

Vitamin B12 and folic acid: Because vitamin B12 is only absorbed in the end portion of the ileum, anyone who has had severe damage to (or removal of) the ileum may need regular Vitamin B12 injections. Low folic acid levels can indicate poor nutrition and perhaps poor absorption from the small intestine.

Liver function tests (LFTs): Liver function can be affected by inflammatory bowel diseases (IBDs) and also by certain medication. LFTs also measure the levels of important proteins like albumin, which can be affected by poor nutrition or absorption.

Urea and electrolytes (U&E): The levels of urea and creatinine, sodium, potassium, chloride and bicarbonate give an indication of how the kidneys are functioning. They also give clues about nutritional status – especially useful if you are being given extra fluids and salt because of severe diarrhoea.

Endomysial antibody (EMA) screening test: This test looks for special antibodies in the blood which are always present in people with coeliac disease, but which are never found in people who don’t have it.

Bone chemistry: This test measures blood calcium, phosphate and levels of alkaline phosphatase, which helps doctors see how well nourished we are. It is also used to check for bone problems – though it doesn’t give much information about the possibility of osteoporosis.

Magnesium: Levels of magnesium can become very low with severe diarrhoea and can cause muscle weakness.