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Blood collection: This is how it works

by Josephine Andrews
Published: Last Updated on 277 views

Blood draw (blood drawing ) is an invasive measure to obtain blood. A distinction is made between venous blood sampling, capillary blood sampling and arterial blood sampling. Read everything you need to know about the different blood drawing techniques, when it is necessary and what the risks are.

What is a blood draw?

During a blood draw, a doctor or specialist takes blood from the vascular system for examination. Careful attention is paid to germ-free (aseptic) conditions in order to keep the risk of infection at the bite site as low as possible.

Capillary blood sampling

The capillaries are the finest blood vessels that connect the arterial and venous systems. They can be used to take blood if only very small amounts of blood are required for the examination. For this capillary blood sampling, the fingertip, earlobe or heel (in the case of an infant) is pricked with a lancet and one or more drops of the escaping blood are collected. It is examined for its composition or, particularly frequently, for blood sugar levels , blood gas levels, and pH and lactate levels.

Venous blood sampling

Venous blood collection is the standard blood collection procedure. With the help of a hollow needle, veins – usually in the crook of the arm or on the forearm – are punctured.

Arterial blood draw

The arterial blood sample is mainly used to carry out a blood gas analysis . It is less common than venous blood collection because the oxygen-rich arteries are deeper than the oxygen-poor veins and are therefore more difficult to reach. Due to the high blood pressure in the arteries, the risk of secondary bleeding is also significantly greater.

When do you take a blood test?

A blood test is primarily used to gain information. As part of a blood test, the small blood count can be created. It indicates the number of individual blood cells, i.e. the red blood cells (erythrocytes), white blood cells ( leukocytes ) and blood platelets (thrombocytes). In addition, among other things, the concentration of hemoglobin , various erythrocyte parameters (such as MCV) and the hematocrit are measured.

In the differential blood count, the proportion of the various subgroups of white blood cells (e.g. lymphocytes , monocytes ) is determined.

The small blood count and the differential blood count together make up the complete blood count .

It is often also necessary to take a blood sample in order to more precisely determine the substances contained in the blood serum (= liquid part of the blood without blood cells and coagulation factors) – such as blood sugar, blood fats (such as cholesterol), enzymes (such as CRP) and hormones .

In many cases, a blood sample is also required for blood gas analysis.

Apart from such blood tests, a blood donation can also be the reason for a blood draw.

What do you do when taking a blood sample?

The most common type is venous blood sampling from the crook of the elbow:

First, a cuff, the so-called tourniquet, is placed on the patient’s upper arm and tightened so that the blood can accumulate in the veins and the arterial pulse can still be felt.

Then the doctor feels the best puncture site and carefully disinfects it. Now he punctures the vein with a needle or with the help of a so-called butterfly system, in which the needle is also guided through two wings. The prick through the skin can cause a slight, brief pain. Blood collection tubes are attached to the end of the needle and a negative pressure is carefully created by pulling on the plunger in order to accelerate blood collection.

When the tube is sufficiently filled, it is removed from the needle, the stamp is snapped off and the tube is tilted a few times. This causes the blood to combine with anticoagulants or other reagents already in the tubes rather than clot. The most common is EDTA blood, in which the blood components remain unbound due to the ethylenediaminetetraacetate (EDTA) contained in the tube. This is the only way the blood cells can be examined in the laboratory.

Finally, the doctor opens the tourniquet, pulls out the needle and applies a compress to the puncture site to prevent bruising. A plaster protects against infection.

For an arterial blood draw , the doctor usually chooses an artery in the groin or wrist. Because the arteries lie deeper than the veins, this procedure is more invasive and is used only when arterial blood testing (eg, to measure oxygen and carbon dioxide levels or pH) is warranted.

In contrast, capillary blood collection is mainly used when the required amount of blood is very small. To do this, the skin is simply scratched with a sharp lancet after disinfection. The escaping blood is collected with a measuring strip or a very thin glass tube. If necessary, capillary blood flow is previously increased with a warm water bath, massage or a special ointment.

Do I have to be sober for the blood test?

In many cases it is important that you come to the blood test on an empty stomach so that your blood values ​​are influenced as little as possible. But what does “fasting for blood collection” mean? In principle, this means that you should not eat anything for eight to twelve hours beforehand. This makes it easier to compare your blood values, as food affects various readings, especially blood sugar and blood fat levels. But the enzyme balance also changes as soon as the organs start digesting.

Allowed during the “fasting phase” are water and tea without sugar and milk. However, you should refrain from drinking coffee before taking your blood.

Smoking is also not advisable before you lose weight on an empty stomach, since nicotine, like caffeine , can cause an increase or decrease in various hormones.

If you take medication, discuss with your doctor in advance to what extent you should continue to take it.

What are the risks of taking a blood sample?

There is no significant risk of infection or injury when taking diagnostic blood. Psychologically triggered circulatory problems may occur, and in very rare cases shock. A bruise (bruising = hematoma) develops relatively frequently at the puncture site, but this usually disappears again soon.

What do I have to consider after a blood draw?

If the doctor has to draw blood, it is usually only small amounts. Nevertheless, you should take it easy afterwards. Additional fluid intake helps the body to quickly compensate for the loss of blood.

To avoid bruising, hold the injection site up and press down on the patch for a few minutes. A sustained and strong compression of the puncture site (once the needle has been withdrawn) can prevent very large bruises (hematomas) from forming in the tissue or under the skin after blood collection – especially in blood collection from the artery or known blood clotting disorders .

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