Children's blood routine test: How to determine whether it is a bacterial infection or a viral infection?

Author: Song Wenqi, National Children's Medical Center (Beijing), Chief Technician, Beijing Children's Hospital, Capital Medical University

Reviewer: He Yanling, Chief Physician, Beijing Chaoyang Hospital, Capital Medical University

In daily life, we can easily find that if children have a cold, fever, or go to the hospital for other diseases, the doctor will always ask them to do a routine blood test first.

You may wonder, why do children with colds and fever often have their blood tested first?

Cold is usually the common name for acute upper respiratory tract infection. Children may have symptoms such as fever, runny nose, sneezing, coughing, sputum, and red and swollen throat.

More than 90% of acute upper respiratory tract infections are caused by viruses. Common respiratory tract infection viruses include influenza virus, parainfluenza virus, respiratory syncytial virus, adenovirus, etc.

Some acute upper respiratory tract infections are caused by bacteria, such as beta-hemolytic streptococci, Staphylococcus aureus, Streptococcus pneumoniae, etc., and some are caused by mycoplasmas, such as Mycoplasma pneumoniae.

It is particularly noteworthy that catarrhal symptoms such as runny nose, sneezing, and watery eyes may be common acute upper respiratory tract infections, or they may be early symptoms of certain diseases, such as measles, epidemic cerebrospinal meningitis, scarlet fever and other acute infectious diseases, the first symptoms of which are often similar to those of a cold. At this time, clinicians need to make a distinction, because acute infectious diseases require appropriate isolation and symptomatic treatment. If they are not distinguished in time, cross-infection may occur.

There are also some diseases whose only early symptom is fever, which are easily mistaken for colds, such as roseola infantum, Kawasaki disease, rheumatic fever, etc. We must also make timely identification.

When children have a cold or fever, doctors often first conduct a preliminary assessment of the disease through routine blood test results to determine whether it is a bacterial or viral infection. Then, based on the child's symptoms, signs and other auxiliary examinations, they give appropriate treatment so that the child can recover as soon as possible.

Generally speaking, if the total white blood cell count increases, especially the percentage (or absolute value) of neutrophils in the white blood cell classification, the possibility of bacterial infection is high.

Figure 1 Original copyright image, no permission to reprint

Neutrophils in white blood cells are of great significance in bacterial infections. White blood cells are generally in blood vessels. Some white blood cells are in the bloodstream and flow with the blood circulation, which is called the circulating pool; the other part of white blood cells is attached to the blood vessel wall, which is called the marginal pool. Under normal circumstances, the two are in a dynamic balance.

The white blood cells and neutrophils we usually count are those in the blood circulation pool. When certain tissues of the body are infected with bacteria, white blood cells quickly move to the infected area to play a defensive role. White blood cells first engulf these bacteria. There are a large number of lysosomes in the cytoplasm of white blood cells, which can digest the engulfed bacteria. But as time goes on, after engulfing dozens of bacteria, neutrophils will lyse and release a large amount of lysosomal enzymes to digest the necrotic tissues and cells. These digested necrotic tissues and cells finally form pus, which is where the symptoms of runny nose in children come from.

Therefore, when there is a bacterial infection, the number of neutrophils usually increases, and the total number of white blood cells will also increase accordingly.

When a child shows symptoms of infection, if the total white blood cell count is normal or decreased, and the proportion of lymphocytes in the white blood cells increases, it indicates a high possibility of viral infection.

Figure 2 Original copyright image, no permission to reprint

When fighting against viral infections, lymphocytes play a major role. Lymphocytes are the most important cells in the human immune response system, including B lymphocytes, T lymphocytes and natural killer cells.

When infected by a virus, B lymphocytes will synthesize corresponding antibodies, which will bind to the virus and destroy it. At the same time, T lymphocytes will transmit the information that the human body is infected by the virus to other cells, such as natural killer cells and killer T cells. With the help of these cells, the virus-infected cells can be further and thoroughly eliminated, eliminating the problem forever.

Viral infection may also inhibit the production of neutrophils in the bone marrow. Therefore, during viral infection, the percentage (or absolute value) of lymphocytes will increase slightly, while the percentage (or absolute value) of neutrophils will decrease, and the total white blood cell count will be normal or decrease.

Of course, in actual work, the situation is more complicated. For example, if a child initially has influenza, the total white blood cell count will decrease and the proportion of neutrophils will also decrease. However, as the disease progresses, bacterial infection may occur and cause pneumonia, and the total white blood cell count will increase. However, there are also individual cases in which the total white blood cell count does not increase but decreases when there is a serious infection.

Therefore, clinicians will use their professional knowledge to accurately interpret the results of routine blood tests, conduct a comprehensive analysis based on the patient's symptoms and other examination results, and provide the patient with the most appropriate treatment plan.