Blood test in children, preparation and parameters

Pediatricians sometimes need more than a medical history and physical examination to be able to make a more accurate diagnosis and offer appropriate treatment. One of the important tests that are done is blood tests.

Blood, stool and urine tests help us to refine more to be able to determine a disease in the child. In addition, in children, routine tests are not necessary if there is no very specific cause that justifies them. In this article we will focus on blood tests in children.

What is a blood test and what is it for?

It is a laboratory test where the different components of the blood are analyzed. Its removal is not pleasant but it is not very painful either. What happens is that needles are always scary.
Blood is made up of 45% cells(white, red blood cells, and platelets) and 55% plasma (the liquid part of blood, made up of 90% water, proteins (albumin, immunoglobulins, and clotting factors). enzymes, hormones, glucose, fats (cholesterol and triglycerides), mineral salts, gases in dissolution and vitamins.
When we carry out an analysis we do not ask for everything we can find, but only those substances that will allow us to carry out the correct orientation of the child.
With the analysis of a small sample of venous blood we can know, roughly, if our body works well.
In addition, in serious infections, we can also culture the blood(this is called a blood culture) to observe the growth of some bacteria and to be able to give the best antibiotic treatment.

Are blood tests in children fasting?

In most cases, we must obtain the sample with a minimum of eight to 12 hours of fasting, that is, without eating any food (water is allowed). Glucose (blood sugar), cholesterol, or some hormones, such as thyroid, are best assessed if the child has not eaten at all.
Only in newborns, infants or some children with metabolic diseases is not necessary to perform the analysis in the fasting state.

Strategies to Prepare Your Child for a Blood Test

The needles used for tests in children are thinner than in adults. The staff is highly qualified to perform the technique quickly and painlessly. Still, fear is the great enemy.
In children who can understand it, it is important to explain the procedure and that the parents are present during the extraction, so the child will feel more calm and accompanied.
In some cases, analgesics(such as sucrose in newborns) or topical anesthetics cream can be used to decrease pain.
The puncture of the vein is performed with a needle that is finer than the one used in adults after disinfecting the skin. In addition, a rubber strip-shaped compressor is placed to facilitate the rapid filling of the veins and facilitate the extraction of blood.

Are preventive tests necessary in the follow-up of the healthy child?

No. There is no evidence or recommendation for routine testing in children who do not need it or within the well child health program. Therefore, children without notable health problems do not have to undergo “preventive” tests.

When are blood tests necessary in children?

There are conditions that do justify periodic follow-up tests in children:

  • Familial hypercholesterolemia(genetically caused high cholesterol levels).
  • Metabolic diseases.
  • Diabetes.
  • Chronic and hormonal diseases.
  • Chronic taking of drugs, such as anticonvulsants(medicines for epilepsy): to control drug levels in the blood or evaluate their side effects.
  • Immuno-deficiencies.
  • Child with cancer.

In children without chronic problems, we may need laboratory tests to study potentially serious illnesses that present acutely or unexpectedly, such as severe infection, anemia, or poisoning. Or for the study of diseases not so serious, but with an impact on the habits of the child, such as allergies.

What parameters are most frequently evaluated in a blood test in children?

The analytics are always requested under medical criteria and it is the doctor who selects the parameters to be evaluated:

  • Hemogram: the three main cells of the blood(red blood cells, leukocytes and platelets) are analyzed. It is very important to rule out anemia or the presence of an infection.
  • Coagulation: in cases of surgery, it is vitally important to know how the patient coagulates, that is, their ability to plug possible wounds. It is very important for safe surgery.
  • Liver function (liver): enzymes such as transaminases and bilirubin are determined. They are markers that show us if the liver is inflamed or if it is not working well.
  • Renal(kidney) function: ions in the blood, urea and creatinine give us an idea of ​​how the kidneys work.
  • Lipid profile: indicates the level of triglycerides and cholesterol.
  • Proteins that indicate infection or inflammation: such as ESR or CRP.
  • Blood glucose: important for assessing glucose(sugar) metabolism.


  • There is no evidence or recommendation to perform routine tests in children who do not need it or within the well-child health program.
  • There are conditions that do justify the performance of periodic follow-up tests in children(familial hypercholesterolemia, diabetes, taking drugs, cancer…)
  • The needles used for tests in children are thinner than in adults. The staff is highly qualified to perform the technique quickly and painlessly.

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