Blood drawing

Peripheral venous blood of adults is most often collected using closed collection systems. The person collecting blood is protected from contamination with the patient's blood and at the same time external contamination of the collected biological material is prevented. Mainly vacuum collection tubes are used. The disposable collection tube already contains preparation agents (e.g. anticoagulants or, conversely, coagulation accelerators). It is hermetically sealed and a vacuum is created in it, which ensures that the right amount of blood is sucked in during sampling. This also ensures the correct ratio of blood and preparation reagents.

Serum
Blood serum is the basic material for most routine clinical-biochemical determinations. It is prepared from collected whole venous blood by its precipitation and centrifugation. This will get rid of blood elements, fibrinogen and most other coagulation factors from the material. Precipitation of fibrin reduces the total protein concentration in serum by about 4 g/l compared to plasma. The concentration of calcium, which is consumed during coagulation, also decreases slightly.

The advantage of serum is the good stability of most analytes (usually hours to days when stored in the cold), it is often even possible to freeze the sample and store it for a long time. The disadvantage is the longer preparation of the material, coagulation and centrifugation takes around 45 minutes.

Blood for obtaining serum can be collected in a test tube without any reagents, coagulation occurs after contact with glass. Precipitation, however, takes quite a long time, for reliable results you need to wait at least 60 minutes before centrifugation. This increases the risk of hemolysis and release of intracellular components from blood elements. At the same time, metabolism in blood cells can continue for some time, which also affects the concentration of some substances. That is why tubes are mainly used, in which coagulation activators have been added – e.g. different forms of silicon dioxide. The addition of the activator reduces the time required for reliable coagulum precipitation to 30 minutes.

For easier separation of the serum, there may be a separation gel in the collection tube. During centrifugation, blood elements and coagulum sediment under the gel, the serum remains above it. The gel facilitates the separation of the serum and at the same time it prevents contamination of the serum with the intracellular content of blood elements, which is gradually released. The separation gel is usually made of acrylate polymers with the addition of silicon dioxide, which further accelerates coagulation.

Collection tubes for the preparation of blood serum usually have a golden yellow (with separation gel) or a red (without gel) stopper.

Plasma and heparinized whole blood
The basic material for most statistical biochemical tests is blood plasma. Lithium heparin is used as an anticoagulant for its preparation. The preparation of plasma is faster than the preparation of serum, because the time required for blood clotting is eliminated. The tubes have walls coated with dried heparin. After collection, the blood is mixed with it by repeatedly turning the test tube, and centrifugation can be started immediately afterwards. Tubes for plasma preparation may also contain a gel to facilitate the separation of plasma from blood elements.

The addition of heparin has relatively little effect on common biochemical determinations. The lithium salt of heparin is most often used because, unlike sodium or potassium heparin, it does not change the concentration of sodium or potassium, i.e. basic and commonly determined ions.

As an anticoagulant, heparin can also be used for some examinations of "whole blood", e.g. examination of acid-base balance and blood gases. In that case, heparinized capillaries or special sampling sets with a syringe for anaerobic sampling are used. However, heparinized blood is not suitable for hematological examinations (heparin interferes with the staining of blood smears) or for molecular biological methods (it inhibits the polymerases used in PCR).

Vacuum tubes with heparin are usually marked green – tubes with separation gel have a light green stopper, and tubes without the separation gel have a dark green stopper.