Blood draw

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Drawing blood (drawing blood) is an invasive measure used to collect blood. A distinction is made between venous blood collection, capillary blood collection and arterial blood collection. Read all about the different blood drawing techniques, when they are needed, and the risks.

What is a blood draw?

When a blood sample is taken, a doctor or specialist takes blood from the vascular system for examination. The blood collection is always carried out under sterile (aseptic) conditions so that the risk of infection at the puncture site is kept as low as possible.

Capillary blood collection

Capillary blood sampling is always used when only very small amounts of blood are required. The fingertip, earlobe or heel (in babies) are pricked with a lancet and one or more drops of blood are taken especially for the examination of the composition, but especially often for the determination of the blood sugar or blood gases and the pH and lactate value.

Venous blood collection

Venous blood collection is the standard method for collecting blood. The veins in the crook of the arm or forearm are usually punctured with the help of a hollow needle.

Arterial blood draw

The arterial blood draw is mainly done to carry out a blood gas analysis. It is rarer than taking venous blood, as the oxygen-rich arteries are deeper than the oxygen-poor veins and are therefore more difficult to reach. Also, because of the high blood pressure in the arteries, the risk of rebleeding is much greater.

When to do a blood draw

In addition to donating blood, blood is drawn primarily to gain information. With the help of a blood count, doctors can draw conclusions about diseases or deficiency symptoms based on the composition. The blood consists of red blood cells (erythrocytes), white blood cells (granulocytes), blood platelets (thrombocytes), coagulants and the blood serum with proteins, electrolytes, nutrients and hormones. For example, an increase in the so-called C-reactive protein indicates an infection. In addition, an arterial blood sample can be taken as part of a blood gas analysis.

What do you do if you have a blood sample?

The most common type is venous blood sampling from the crook of the elbow. A cuff, the so-called tourniquet, is placed around the upper arm and pulled so tight that on the one hand the blood can build up in the veins and on the other hand the arterial pulse can still be felt.

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 slight, brief pain. Blood collection tubes are attached to the end of the needle and a negative pressure is carefully created using a stamp, which accelerates the collection of blood.

If the tube is filled enough, 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 that are already in the tubes, rather than clumping. The most common is EDTA blood, in which the ethylene diamine tetraacetate (EDTA) contained in the tube keeps the blood components unbound. This is the only way to examine the cells in the laboratory.

Finally, the doctor opens the tourniquet, pulls the needle and presses on the puncture site with a compress so that there is no bruise. A plaster protects against infections. If the doctor wants to draw arterial blood, he usually chooses an artery in the groin or on the wrist. Because the arteries are deeper than the veins, this procedure is more invasive and is only used when it is necessary to examine the arterial blood, for example to measure oxygen and carbon dioxide levels or pH.

In contrast to this, capillary blood sampling is mainly used when the amount of blood required is only very small. After disinfection, the skin is simply scratched with a sharp lancet and blood is taken from the wound with a measuring strip or a very thin glass tube. If necessary, the capillary blood flow is increased beforehand with a warm water bath, massage or a special ointment.

Do I have to come on an empty stomach to have my blood drawn?

It is important that you come for a blood sample on an empty stomach so that your blood values ​​are influenced as little as possible. What does sober mean to take blood? In principle, this means that you should not eat anything eight to twelve hours beforehand. This makes it easier to compare your blood values, since food mainly influences blood sugar and blood fat values. The enzyme balance also changes as soon as the organs start digestion.

Water and tea without sugar and milk are allowed. However, it is better not to drink coffee before taking a blood sample. Smoking is also not advisable before fasting weight loss, as nicotine, like caffeine, can cause various hormones to rise or fall. If you are taking any medication, discuss how far you should continue to take it with your doctor.

What are the risks of taking a blood sample?

There is no significant risk of infection or injury when taking diagnostic blood samples. Possibly it can lead to psychologically triggered circulatory problems, in very rare cases to shock. However, a bruise develops relatively often at the puncture site.

What do I have to watch out for after a blood test?

Despite the mostly small amounts of blood that are obtained when a blood sample is taken, you should take it easy afterwards. Extra hydration helps the body make up for the loss very quickly.

To prevent a bruise from forming, you should press the patch for a few minutes and hold the puncture site as high as possible. Sustained and strong compression of the puncture site (as soon as the needle has been withdrawn) is important, especially when blood is drawn from the artery or when blood clotting disorders are known. Otherwise very large bruises (hematomas) could form in the tissue or under the skin after the blood sample.

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