Blood Draw

In addition to blood donation, blood is drawn primarily for the purpose of obtaining information. Based on the composition of the blood count, doctors can draw conclusions about diseases or deficiencies. Blood consists of red blood cells (erythrocytes), white blood cells (granulocytes), platelets (thrombocytes), coagulation factors, and blood serum containing proteins, electrolytes, nutrients, and hormones. For example, a rise in so-called C-reactive protein indicates an infection. In addition, an arterial blood sample may be taken as part of a blood gas analysis.

The most common method is venous blood collection from the antecubital fossa (the inside of the elbow). A cuff — known as a tourniquet — is applied to the arm and tightened so that, on the one hand, blood can pool in the veins, and on the other, the arterial pulse can still be felt.

The doctor palpates the best puncture site and disinfects it thoroughly. The vein is then punctured with a needle or using the so-called butterfly system, in which the needle is also guided by two wings. The puncture through the skin may cause mild, brief pain. Blood collection tubes are attached to the needle and a plunger is used to create negative pressure, which speeds up the blood collection process.

Once the tube is sufficiently full, it is removed from the needle, the plunger is kinked, and the tube is inverted several times. This causes the blood to mix with anticoagulants or other reagents already present in the tubes, preventing clotting. The most common is EDTA blood, in which ethylenediaminetetraacetic acid (EDTA) contained in the tube keeps the blood components unbound. Only in this way can the cells be examined in the laboratory.

Finally, the doctor releases the tourniquet, withdraws the needle, and presses a gauze compress against the puncture site to prevent bruising. A plaster protects against infection.

It is important that you come to your blood test fasting, so that your blood values are influenced as little as possible. What does fasting for a blood test mean? In principle, it means that you should not eat anything for eight to twelve hours beforehand. This allows your blood values to be compared more accurately, as food — particularly fatty foods — primarily affects blood glucose and blood lipid levels. Enzyme levels also change once the organs begin digesting.