What does it consist of?
Blood tests are a very useful tool for diagnosis. Blood is composed of different kinds of cells and a liquid part, called plasma, which contains various substances, such as salts and proteins. Blood outside the body coagulates because its cells and proteins become solid, leaving a liquid part called serum, which can be analyzed in chemical tests and analysis of the immune system. In addition, blood samples can be taken to culture and observe if microorganisms that cause infectious diseases are growing, in order to detect them and also see how they behave exactly and the sensitivity they show to different antibiotics.
How do you do a blood test?
Blood samples can be taken from a vein (which carries blood to the heart) or from an artery (which carries blood from the heart). If only a few drops are needed (for example, to control the sugar in a patient with diabetes) it is enough to make a small puncture in the yolk of a finger and to apply a slight pressure so that the necessary blood leaves.
Most blood tests are done with venous blood samples (usually veins close to the elbow). First, the vein is punctured with a rubber band tied around the arm, which, although annoying, facilitates the extraction. The area is then cleaned with alcohol and the needle is inserted into the vein. This needle is connected with a syringe or with low pressure containers, until enough blood has been withdrawn. The needle is withdrawn and pressed with a piece of cotton on top of the wound, to then place a dressing.
If blood is to be drawn from an artery, such as in the case of arterial blood gases, it is usually done on the wrist, since there is an artery very close to the skin. This extraction is more annoying, since there are nerve endings in the arteries. After extraction, a strong pressure is applied on the area for 5 minutes to stop the bleeding.
Some people are very sensitive to the sight of needles and their own blood, and may fade at the time of extraction. For this reason, the extractions must be done sitting or lying down, and at the slightest symptom of dizziness, the person who does the extraction should be advised.
What is analyzed in blood?
Blood has two main elements, the liquid (plasma) and the cells, of which there are three types: red blood cells (red blood cells), white blood cells (leukocytes) and platelets.
In this analysis, we make a count of the different types of blood cells, which are:
Red blood cells (Hematíes)
They are the cells in charge of transporting oxygen to the tissues, and of cleaning them of carbon dioxide thanks to a protein called hemoglobin. With the hemogram, we can know the amount of hemoglobin in blood and the number of red blood cells. Another important fact is the corpuscular volume (VCM), which indicates the size of the red blood cells. A person with anemia will have hemoglobin levels lower than normal, and depending on the size of those red blood cells different types of anemia are differentiated.
Hematocrit is also obtained, which measures the percentage of volume occupied by the red blood cells in relation to the total blood volume. Almost all anemias cause low levels of hematocrit (ie, decrease in red blood cells) just as in major hemorrhages. In contrast, high hematocrit levels are often caused by dehydration from poor fluid intake or water loss from diarrhea, burns and certain types of surgeries, or from excess red blood cells.
The color of the red blood cells is also analyzed so that if they are pale -hypocromic- it usually indicates anemia due to lack of iron, and the form, which may indicate sickle-cell anemia or pernicious anemia. But to see the shape of the red blood cells, it is necessary to spread the blood on a slide and observe them under a microscope.
Dyeings may also be added to blood extensions to detect parasites (in cases of malaria and sleeping sickness) or bacteria.
White Blood Cells (Leukocytes)
They are the cells that defend us from infections. The blood count makes a total count of the number, as well as the determination of the different types in blood (called leukocyte count).
The number of white blood cells usually rises in bacterial infections, burns and bleeding, and less frequently as a result of leukemia, cancer or malaria.
The decrease in the number of white blood cells may be due to autoimmune diseases (ie, antibodies that fight disease agents are turned against the body itself), viral infections, and less frequently, due to some drugs.
The white blood cell count is useful for monitoring the evolution of diseases, and thus modifying treatments if necessary.
The blood count also counts the platelets. Platelets are small cells that cluster together in areas of blood vessels that have suffered damage. They are the basis of blood clotting.
People with low platelet levels are at increased risk of bleeding, even without an injury. Some causes of low levels of platelets are autoimmune diseases (the body produces antibodies against its own platelets), chemotherapy, some drugs, leukemias and some viral infections.
The increase in the number of platelets makes the person more likely to form clots. This occurs in some diseases of the bone marrow, such as essential thrombocytosis or polycythemia vera.
Another elemental analysis is the blood biochemistry, through which the level of ions and proteins in blood can be known. Other tests can also be done such as the study of coagulation, immunological study, etc.
What is a coagulation study?
When clotting disorder is discovered, either because the blood does not coagulate enough or because it does too much, further analysis is needed. Coagulation analysis measures the ability of blood to carry out coagulation. Coagulation tests are also performed before subjecting a patient to a surgical operation.
When a vein is damaged, a small clot forms inside it, made up of platelets and plasma proteins called coagulation factors.
A person will bleed more than normal if he has a low number of platelets or if his clotting factors are absent or not working properly. This requires more analysis and can be measured with a coagulation study. Some disorders in clotting factors are hereditary (eg hemophilia), but they can also be due to liver disorders (since the liver produces many factors of coagulation), or to the effect of some drugs.
Some diseases require treatment with anticoagulant drugs such as warfarin or coumarins, and in those patients, periodic coagulation tests should be done to adjust the doses of the medication.