Blood transports oxygen from the lungs to the organs and, on the way back, carries along the waste product carbon dioxide to exhale. It is also a major artery for the many other substances that need to get from one place to another in the body. All the substances that are in the blood can be measured. Blood tests are an important part of most medical examinations.
Blood - body fluid with numerous tasks
Most substances in the body have to move from one place to another. Whether the nutrients in the gastrointestinal tract to other organs, the oxygen in the lungs to the body cells, hormones from the endocrine glands to their target cells - the number of substances that are transported back and forth in the blood stream every day, is huge.
But blood also has other tasks: it distributes the heat and is part of the control system, which ensures that the optimal internal environment is maintained in the body (homeostasis). The coagulation system seals injuries. A very important function is also the immune defense, in which defense cells and antibodies are provided in the blood and to fight pathogens, foreign proteins or diseased cells in the body.
Ingredients of blood
In adults, the blood accounts for about 8 percent of body weight, which is about 5 to 6 liters.
- 42 to 44 percent of the blood volume is due to the blood cells - this proportion experts call hematocrit.
- The remaining 56 to 58 percent are the blood plasma or plasma volume. This consists of 90 percent water, 8 percent protein and 2 percent small molecule substances such as vitamins, sugars or hormones.
Blood plasma without fibrinogen, a protein body of blood clotting, is called blood serum.
What is being investigated and for what?
With a single blood sample, a variety of different studies can be performed - depending on which component of the blood are considered in what way. For example, whole blood, blood cells and blood serum are examined. However, each exam is always a snapshot and may need to be repeated.
There are also some factors that influence the outcome and therefore have to be taken into account in the evaluation. This includes:
The standard values used for comparison may vary from laboratory to laboratory and depending on the examination method.
Which blood tests are there?
In principle, the following tests can be distinguished:
- blood count
- blood clotting
- Erythrocyte sedimentation rate
- Examinations of the blood serum (serological investigations)
- Blood gas analysis (BGA)
- blood culture
- blood smear
Only the synopsis of the various parameters gives the doctor an indication of the possible disorder. Depending on the suspected disease and cause, the blood tests are often supplemented by further diagnostics such as functional testing and imaging techniques such as ultrasound or X-ray.
Blood tests are also good for monitoring disease and treatment. Thus, for example, early deterioration of organ function, recruitment and side effects of drugs or the resurgence of a tumor can be determined.
The individual blood tests are presented below.
1. Small and large blood picture
Microscopically and photometrically, blood cells (red and white blood cells, platelets, immature red blood cells) and blood pigment, as well as their appearance, number, size and percentage distribution are considered. Depending on which blood cells are examined, a distinction is made between small and large blood counts.
This study is particularly useful in suspected infections, blood disorders such as anemia or disorder of blood formation and deficiency diseases (for example, iron, folic acid, vitamin B12) are used.
2. Blood clotting
The coagulation system protects the body from bleeding and blood loss. There is a complex balance between hemostasis due to clots on one side and fluid retention of the blood, so that the vessels do not clog, on the other side. There are a number of factors involved in this; the most important are the platelets (platelets), fibrinogen, calcium and vitamin K.
The blood coagulation examination is carried out especially in suspected congenital or acquired coagulation disorders and certain organ diseases (for example, liver).
3. Blood cell lowering rate
This is an overview test where unrinnable blood is drawn into special tubes and the distance that the solid components descend within a given time is determined. If this is larger than normal, this may indicate infections, inflammation and tumors, it is less on liver inflammation. Further investigations must follow.
4. Studies of the blood serum
Serological examinations are primarily used to assess the function of internal organs such as the liver and gall bladder, kidneys, heart, lungs, stomach and intestines, thyroid, pancreas, spleen and prostate. It can be proteins, fats, minerals, vitamins, hormones, enzymes and cancer markers determine - important investigations in the identification of various disorders and deficiencies, and to control the course of disease and therapy.
For the functional diagnosis of the various organs certain enzymes are typical and are also named accordingly (for example, heart, liver, muscle enzymes). These are groups of substances whose concentration and percentage distribution usually give the doctor important information about the type of dysfunction of the particular organ. They are usually assessed in combination with other substances such as protein or fat.
5. Blood gas analysis (BGA)
Blood gas levels include the concentration of oxygen and carbon dioxide, as well as the pH and bicarbonate. The blood sample is usually taken from the artery in the wrist or the capillaries in the ear. This can be used to assess the gas exchange in the lungs, for example in diseases such as asthma.
6. Blood culture
In this microbiological process, blood is incubated in an incubator to detect bacteria and then to determine the appropriate antibiotic for therapy. It is used, for example, when there is a high fever of unexplained cause.
7. Blood smear
Here, fresh capillary blood is streaked on a glass slide and assessed under the microscope. It can be stained and used to examine for parasites (for example, malaria parasites) and to look at and count blood cells.
Obtaining a blood sample
Depending on the intended examination, one or more tubes of blood are taken, the amount is usually from 2 to 50 milliliters. Most of the blood is taken from the vein (for example, in the crook of the arm), in special cases also from the artery or capillaries. Some tests require specific preparation of the patient - not infrequently, he must be sober, for example, in the determination of blood lipids or blood sugar. The doctor will inform the affected person in individual cases about necessary measures and preparations.
There are a variety of different tubes available for storing and transporting blood samples. For example, the blood for the determination of the coagulation must be prepared differently than that for blood sedimentation. Today, the tubes are already supplied with the appropriate additives finished by the manufacturer, usually recognizable by different colored plugs. The sampling systems are designed so that they only need to be punctured once and several tubes can be filled.
In some cases, the doctor will take only one large syringe and then refill it into different containers. When collecting blood for a culture, the doctor must pay particular attention to sterile work. Otherwise, contamination of the sample, such as with normal skin germs, may not be properly assessed.