blood test

Blood transports oxygen from the lungs to the organs and on the way back takes the waste product carbon dioxide with it for exhalation. It is also the main artery for the many other substances that need to move from one place to another in the body. All the substances circulating in the blood can be measured. Blood tests are an important part of most medical examinations.

Blood – body fluid with numerous functions

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 bloodstream every day is huge.

But blood also has other tasks: it distributes heat and is part of the regulatory system that ensures that the optimal internal environment is maintained in the body (homeostasis). The coagulation system seals injuries. Another very important function is the  immune system, in which defense cells and antibodies are made available in the blood and are used to fight pathogens, foreign proteins or diseased cells in the body.

components of blood

In adults, blood makes up about 8 percent of body weight, which corresponds to about 5 to 6 liters.

  • 42 to 44 percent of the blood volume is caused by the blood cells – experts call this proportion the  hematocrit .
  • The remaining 56 to 58 percent is the  blood plasma  or plasma volume. This consists of 90 percent water, 8 percent proteins and 2 percent small molecular substances such as vitamins, sugar or hormones.

Blood plasma without fibrinogen, a coagulation protein, is called  blood serum  .

What is examined and what for?

A large number of different tests can be carried out with a single blood sample – depending on which components of the blood are examined and in what way. In particular, whole blood, blood cells and blood serum are examined. However, each examination is only a snapshot and may have to be repeated.

There are also a number of factors that influence the result and must therefore be taken into account in the evaluation. This includes:

  • Alter
  • gender
  • nutrition
  • time of day
  • medication

The standard values ​​that are used for comparison can vary from laboratory to laboratory and depending on the examination method.

What blood tests are there?

In principle, the following tests can be distinguished:

  1. blood count
  2. blood clotting
  3. blood cell settling rate
  4. Blood serum tests (serological tests)
  5. Blutgasanalyse (BGA)
  6. Blood culture
  7. 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 function tests and imaging procedures such as ultrasound or X-rays.

Blood tests are also good for  monitoring the progress  of diseases and treatments. For example, deterioration in organ function, the discontinuation and side effects of medication or the resurgence of a tumor can be detected at an early stage. 

The individual blood tests are presented below.

1. Small and large blood count

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 observed microscopically and photometrically. Depending on which blood cells are examined, a distinction is made between small and large blood counts.

This examination is used in particular if there is a suspicion of infections, blood diseases such as  anemia  or blood formation disorders and deficiency diseases (e.g.  ironfolic acidvitamin B12 ).

2. Blood clotting

The coagulation system protects the body from bleeding and blood loss. There is a complex balance between hemostasis by  clots  on the one hand and keeping the blood fluid so that the vessels do not become blocked on the other. There are a number of different factors involved in this; the most important are the platelets (thrombocytes), fibrinogen, calcium and vitamin K.

The blood coagulation test is mainly carried out if congenital or acquired coagulation disorders and certain organ diseases (e.g. liver) are suspected.

3. Blood cell settling rate

This is a screening test in which non-coagulable blood is drawn into special tubes and the distance the solids drop over a period of time is determined. If this is larger than normal, this can indicate infections, inflammation and tumours, if it is smaller it means liver inflammation. Further investigations must follow.

4. Blood serum studies

Serological tests are primarily used to assess the function of internal organs such as the liver and gallbladder, kidneys, heart, lungs, stomach and intestines, thyroid,  pancreas , spleen and prostate. Proteins , fats,  minerals , vitamins, hormones, enzymes and cancer markers can be determined – important tests for identifying a wide variety of disorders and deficiency symptoms and for monitoring the course of the disease and therapy.

Für die Funktionsdiagnostik der verschiedenen Organe sind bestimmte Enzyme typisch und werden auch entsprechend benannt (zum Beispiel Herz-, Leber-, Muskelenzyme). Das sind Gruppen von Substanzen, deren Konzentration und prozentuale Verteilung dem Arzt meist wichtige Hinweise auf die Art der Funktionsstörung des speziellen Organs gibt. Sie werden in der Regel in Kombination mit anderen Stoffen wie Eiweiß oder Fett beurteilt.

5. Blutgasanalyse (BGA)

Zu den Blutgaswerten gehören die Konzentration von Sauerstoff und Kohlendioxid sowie der pH-Wert und das Bicarbonat. Die Blutprobe wird meist aus der Arterie im Handgelenk oder den Kapillaren im Ohr entnommen. Damit lässt sich der Gasaustausch in der Lunge beurteilen, beispielsweise bei Krankheiten wie Asthma.

6. Blutkultur

Bei diesem mikrobiologischen Verfahren wird Blut in einem Brutschrank bebrütet, um Bakterien nachzuweisen und dann das passende Antibiotika zur Therapie zu bestimmen. Es wird zum Beispiel bei hohem Fieber ungeklärter Ursache eingesetzt.

7. Blutausstrich

Hier wird frisches Kapillarblut auf einem Glasträger ausgestrichen und unter dem Mikroskop beurteilt. Es kann eingefärbt werden und dient zur Untersuchung auf Parasiten (zum Beispiel Malariaerreger) und Betrachtung und Zählung von Blutzellen.

Gewinnung einer Blutprobe

Je nach beabsichtigter Untersuchung werden ein oder mehrere Röhrchen Blut abgenommen, die Menge liegt in der Regel bei 2 bis 50 Milliliter. Meist wird das Blut aus der Vene (zum Beispiel in der Armbeuge), bei speziellen Fragestellungen auch aus Arterie oder Kapillaren entnommen. Bei manchen Tests bedarf es bestimmter Vorbereitung des Patienten – nicht selten muss er nüchtern sein, beispielsweise bei Bestimmung von Blutfetten oder Blutzucker. Der Arzt wird den Betroffenen im Einzelfall über notwendige Maßnahmen und Vorbereitungen aufklären.

A large number of different tubes are available for storing and transporting blood samples. For example, the blood for determining coagulation must be prepared differently than that for blood sedimentation. Today, the tubes are already supplied by the manufacturer with the appropriate additives, usually recognizable by the different colored stoppers. The collection systems are designed in such a way that you only have to pierce once and still be able to fill several tubes.

In some cases, the doctor only removes one large syringe and then fills it into different containers. When taking blood for a culture, the doctor must pay particular attention to sterile work. If the sample is contaminated, for example with normal skin germs, it may otherwise not be able to be assessed correctly.

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