What do blood tests look for?

These tests look at samples of blood. They can help find out:

  • whether your child has the right amount of important chemicals in his or her body
  • whether your child has an infection
  • the number of different types of blood cells, the living parts of your child’s blood
  • your child’s blood group – this is important if he or she needs an operation or blood transfusion
  • how well your child’s kidneys are working (kidney function and glomerular filtration rate – these are described in the next section).

Important chemicals

We need the right balance of chemicals to stay healthy. Your child’s blood will be checked to find out if they have too much or too little of any of these chemicals.

Urea and creatinine

The body makes some chemicals after it uses energy. Urea is made from the food we eat, and creatinine is made when we use our muscles. These are waste products and the kidneys remove them from the body into urine.

If there is too much urea or creatinine in the blood, this may be a sign that the kidneys are not working as well as they should.

Electrolytes

Electrolytes are important chemicals in the body. They help make sure that the muscles, including the heart muscle, bones, teeth and blood, are healthy. If we have too much or too little, this can cause problems. Some important electrolytes include sodium, potassium, bicarbonate, phosphate and calcium.

The kidneys control the amounts of some important electrolytes. If we have too much or too little, this can cause problems, and may mean there is a problem with the kidneys or another part of the body.

  • Sodium helps balance the amount of water in the body. Hypernatraemia is too much sodium.
  • Potassium is needed for muscles, including the heart muscle, to work properly. Hyperkalaemia is too much potassium (“hyper” means too much) and hypokalaemia is too little potassium (“hypo means too little) – either of these may mean there is a problem with the kidneys.
  • Bicarbonate balances the amount of acid in our body, or the pH balance. If there is not enough bicarbonate then the blood is acidic – this is called acidosis.
  • Phosphate is important for bones, teeth and muscles. Hyperphosphataemia is too much phosphate.
  • Calcium is important for bones and teeth, helps blood to clot, and also helps the muscles, including the heart muscle, work.

Other substances

Blood tests can be used to look at other substances.

  • Vitamin D helps the body take in calcium, which it needs to grow. We can get vitamin D from food and sunlight. The kidneys change the vitamin D to an active form that our bodies can use.
  • Serum albumin is the main protein in blood. It helps control the amount of water inside blood vessels.  
  • Parathyroid hormone (PTH) is a hormone – a chemical carried in the blood that sends message to other parts of the body. PTH controls the amount of calcium, phosphorus and vitamin D levels in the blood and bone.
  • Glucose is a form of sugar.
  • Cholesterol is a type of fat.

Infection and immune system

Different blood tests can check whether your child has or has had an infection, which is caused by germs, such as bacteria, viruses and yeasts (a type of fungus). Tests can also show whether there are any problems with the immune system, the body’s way of defending itself against infection and disease.

Blood cultures check if the blood sample has germs, such as bacteria and yeasts (a type of fungus). It can take up to 48 hours for this result to be available.

Immunology tests look for antibodies. When germs come into the body, the body makes these chemicals to kill them. The body now has immunity to this type of infection.

These tests help find out if your child has or has had an infection, such as chicken pox. They can also show if there are problems with the immune system. A few people have autoimmune conditions in which antibodies attack the body’s own tissues and organs.

Blood group

Each of us belongs to one of eight blood groups. If your child is going to receive blood from a donor (someone who gives blood), it is important to find out his or her blood group. This is because the blood groups need to match.

Your blood group is based on whether or not you have certain substances on your red blood cells. It is inherited from your mother and father.

Blood groups are defined by two systems:

  • ABO system – identified by the letters, A, B, O and AB
  • Rhesus system – identified as either positive or negative.

This means there are eight blood groups. Each of us has one blood group:

  • A+
  • A–
  • B+
  • B–
  • O+
  • O–
  • AB+
  • AB–

Full blood count

The body has different types of blood cells. It is important we have the right amounts of these cells. A full blood count (FBC) or a complete blood count (CBC) counts the number of blood cells.

An FBC can help check for problems in your child’s body, such as infections or inflammation (swelling). It can also help find out if your child has anaemia, a condition that causes people to feel very tired or low energy.

In a full blood count (FBC), blood is examined under a microscope. The amounts of different blood cells are measured.

This test can give clues about possible problems with your child’s health. It measures the following.

  • Haemoglobin: a protein in the red blood cells which carries oxygen round the body and removes carbon dioxide. If the amount of haemoglobin is below the normal level, or if there are fewer red blood cells than normal, this is called anaemia. There are many types of anaemia, with different causes. Anaemia is a common complication of some kidney conditions, including chronic kidney disease.
  • White blood cells help the body fight infections. If there is a higher than normal number of white blood cells, there may be an infection. If there is a lower than normal number this may mean that the body is not able fight infection normally.
  • Platelets help to prevent excessive bleeding – for example, when we graze our skin.

Occasionally, because of medicines, the bone marrow does not make as blood cells as it should. Measuring the FBC helps check if this is happening.

More information about tests and diagnosis

  • Tests and diagnosis

    Find out more detail about some of the tests used to diagnose or investigate kidney conditions.

  • Getting blood samples

    Preparing your child for a blood test, the kinds of samples that are taken and how you will receive the results.

  • GFR for kidney function

    Measuring glomerular filtration rate (GFR), the amount of fluid (liquid) that the kidneys filter each minute, will help your child's doctor understand how well their kidneys are working. There are two main methods of measuring GFR in children: a blood test and a radioactive tracer.