Tests are done for a number of reasons:

• So that the cancer or leukaemia can be diagnosed accurately. Sometimes it is hard to tell the difference between specific types of cancer. Your child’s doctor may speak to other doctors to ask their opinion and advice about the diagnosis. They will have the support of other cancer experts around the UK and overseas.

• To see where the cancer is in the body and also whether it has spread.

• To assess your child’s general health, as this may affect the treatment that is given.

The tests may mean that treatment can’t be started for a few days. However, it’s important to have the information from the tests, so that the right treatment can be chosen. Generally, cancer develops slowly and waiting to start treatment for a short time will not make the treatment less effective.

With some types of cancer it is very important to start treatment straight away. If this is the case, your child’s doctor will discuss this with you.

It is important to get all the necessary information together from the tests to make sure that the best treatment is given.

Some common tests are described here. Several of these tests are carried out again during the course of treatment, to see how the treatment is affecting the cancer. Some tests can also check for any side effects of treatment.

If your doctor thinks that a tumour may be a cancer, the surgeon may remove part of it to look at under a microscope.

• Needle biopsy A special needle is put into the tumour through the skin to remove a small part of the tumour.

• Open biopsy A tiny piece of the tumour is taken during a small operation.

The sample of cells is sent to a laboratory to be examined by the histopathologist (a person who studies body tissues). The histopathologist can tell if it is a cancer and exactly what type of tumour it is. It usually takes several days to get the results of biopsies. With an open biopsy, the surgeon may sometimes be able to remove the whole tumour rather than just taking a sample.

There are lots of different types of blood tests. Blood samples will be sent to different laboratories depending on the type of test. Blood tests may be done when your child is diagnosed, during treatment and afterwards at follow-up appointments.

• Full blood count The numbers of different types of blood cells in the blood are counted in the haematology laboratory. This is called a full blood count. This is one of the main ways to monitor the side effects of treatment.

• Blood chemistry The blood may also be looked at in the laboratory to measure the levels of certain chemicals. This gives information about how well organs of the body, such as the liver and kidneys, are working.

• Blood cross-match Blood samples can be used to find blood from a donor that matches your child’s blood. Cross-matching is done in case a blood transfusion is needed.

• Blood culture This is when microbiologists (people who study types of infection) check a sample of blood to see whether there is any infection.

Blood may also be taken to see if your child has had any viruses in the past, such as measles, hepatitis or German measles (rubella).

In several types of cancer, the bone marrow may be affected. Bone marrow is the area of soft spongy cells in the middle of some of the bones. It produces cells (called stem cells) which develop into the three different types of blood cells:

• red blood cells, which carry oxygen to all cells in the body
• white blood cells, which are part of the immune system and essential for fighting infection
• platelets, which help the blood to clot and control bleeding.

To take a sample of bone marrow, a needle is put into the bone of the hip and some of the marrow is drawn out using a syringe. This is sent to be looked at in a laboratory. This test (also known as a bone marrow aspirate) is usually done under a general anaesthetic, or sedation, to make sure your child feels as little  discomfort as possible. Older children may prefer to have a local anaesthetic. The bone where the sample is taken from can feel very sore for a few days after the test.

In some conditions, such as leukaemia or lymphoma, cancer cells can pass into the fluid that surrounds the brain and the spinal cord - the cerebrospinal fluid or CSF. To see whether this has happened, a few drops of the CSF are removed. This is done by inserting a fine needle into the CSF between two spinal (vertebral) bones in the lower part of the spine. This is called a lumbar puncture and is usually done under a general anaesthetic. The CSF is examined to see if any cancer cells are there.

Sometimes, as part of the treatment, drugs are injected into the CSF at the same time. This is known as intrathecal chemotherapy.

We have a section with detailed information about lumber puncture and intrathecal chemotherapy.

On an x-ray a tumour often looks different from healthy tissue. An x-ray can show if there is a tumour in the chest, tummy (abdomen) or bones. Sometimes special dyes are injected into a vein to make particular parts of the body show up better on the x-ray.

This uses sound waves to build up a picture of the inside of the body. A gel will be spread onto the part of your child’s body being scanned. A small device, like a microphone, which produces sound waves, is passed over the area. The sound waves are then converted into a picture by a computer.

Ultrasound scans are completely safe and are used particularly to look at the tummy (abdomen) and heart.

A CT (computerised tomography) scan shows up areas of the inside of the body. The CT scanner takes a series of x-rays from different angles. A computer uses the x-rays to build up a 3-D picture of the inside of the body. CT scans are painless but your child may be given a sedative or general anaesthetic to make sure that they lie still.

An MRI (magnetic resonance imaging) scan is similar to a CT scan but uses magnetism instead of x-rays to build up the pictures. The scan is painless, but the machine is very noisy. It can look frightening, as your child has to lie in a narrow tunnel. They may be given a sedative or general anaesthetic to make sure that they lie still. They will also be given earplugs or headphones to wear. Some scanners can show videos as the scan is being done. It may be possible for you to stay in the room with your child while they have the scan.

If your child has a bone tumour or a tumour which has spread to bone, this scan will show how much of the bone is affected. Small doses of a radioactive substance are injected into a vein, usually in the arm. The radioactive substance is taken up by the bone. Abnormal areas of bone absorb more of the substance than healthy bone and are highlighted on the scanner as hotspots.

A PET (positron emission tomography) scan uses low-dose radioactive glucose (a type of sugar) to measure the activity of cells in different parts of the body. A very small amount of a mildly radioactive substance is injected into a vein, usually in the arm. A scan is then taken a couple of hours later. Areas of cancer are usually more active than surrounding tissue and show up on the scan.