What is a blood draw?
In a blood draw, a doctor or specialist draws blood from the blood vessel system for examination. Careful attention is paid to germ-free (aseptic) conditions to minimize the risk of infection of the puncture site.
Capillary blood collection
Venous blood collection
Venous blood collection is the standard procedure for obtaining blood. A hollow needle is used to puncture veins – usually in the crook of the arm or forearm.
Arterial blood collection
When do you do a blood draw?
A blood sample is primarily taken to obtain information. In the course of a blood test, the small blood count can be made. It indicates the number of individual blood cells, i.e. red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). In addition, the concentration of hemoglobin, various erythrocyte parameters (such as MCV) and the hematocrit are measured, among other things.
The small blood count and the differential blood count together make up the large blood count.
A blood sample is also often required to determine substances contained in the blood serum (= liquid portion of the blood without blood cells and clotting factors) more precisely – such as blood sugar, blood fats (such as cholesterol), enzymes (such as CRP) and hormones.
In many cases, a blood sample is also needed for blood gas analysis.
What do you do when you take a blood sample?
Doctors choose either a vein, an artery or a capillary for blood sampling – depending on the medical issue.
Venous blood draw
The most common type is venous blood sampling from the crook of the arm:
First, a cuff, the so-called tourniquet, is placed on the patient’s upper arm and pulled tight so that, on the one hand, the blood can pool in the veins and, on the other hand, the arterial pulse can still be felt.
Blood collection tubes are attached to the end of the needle and a vacuum is carefully created by pulling on the plunger. This speeds up the blood collection process.
Finally, the doctor opens the tourniquet, pulls out the needle and presses on the puncture site with a compress to prevent bruising. A plaster protects against infection.
Arterial blood sampling
For an arterial blood sample, the doctor usually chooses an artery in the groin or at the wrist.
Capillary blood collection
In contrast, capillary blood collection is used primarily when the amount of blood required is very small. For this purpose, the skin is merely 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 increased beforehand with a warm water bath, massaging or a special ointment.
However, water and tea without sugar and milk are permitted during the “fasting phase”. However, it is better to refrain from drinking coffee before the blood sample is taken.
Smoking is also not advisable before a fasting blood sample is taken, as nicotine, like caffeine, can cause a rise or fall in various hormones.
If you are taking medication, discuss the extent to which you should continue taking it with your doctor beforehand.
What are the risks of a blood draw?
What do I need to be aware of after a blood sample is taken?
If the doctor has to take a blood sample, it is usually only a small amount. Nevertheless, you should take it easy afterwards. Additional fluid intake helps the body to quickly compensate for the blood loss.