Corpus: Blood sampling

Blood sampling is the collection of blood from a blood vessel. It can be performed by vascular puncture or by removal from a horizontal peripheral or central, venous or arterial access.

There are many known indications for blood sampling. Blood is usually taken for diagnostics and monitoring purposes, but also as part of preventive measures or for blood donation. Bloodletting as a therapeutic blood collection is also still occasionally indicated today.

3.1. Venous blood collection

Blood sampling from a vein (venipuncture) is the standard procedure for blood collection, in which blood is taken by means of a hollow needle and puncture of a venous blood vessel. The puncture site is disinfected with an alcohol swab before the puncture.

As a rule, the median cubital vein or the cephalic vein on the forearm are chosen as puncture sites. Theoretically, however, blood can be obtained from all superficial or deeper veins.

A special case of venous blood collection is staged blood collection.

3.2. Capillary blood collection

Blood collection from a capillary (capillary blood collection) is a widely used procedure for collecting small amounts of blood. It is used, among other things, to determine blood sugar (BG) or the oxygen saturation of the blood.

Capillary blood is best obtained from tissue with a good blood supply, e.g. from the fingertip or earlobe. Lancing devices are used that provide a quick and therefore painless prick and standardise the penetration depth into the tissue.

This procedure is simple and, thanks to modern blood glucose meters, can be carried out by most patients themselves at home. In paediatrics, capillary blood sampling is an elegant way of minimising the number of venous blood samples, which children find more painful.

3.3. Arterial blood sampling

Arterial blood sampling is used to carry out a blood gas analysis (BGA). Blood is obtained by arterial puncture, usually of the radial artery or the iliac artery. Special sealed disposable capillaries can be used for collection, but for cost reasons a normal, heparinised syringe can also be used to puncture and draw arterial blood.

Arterial blood sampling is significantly more invasive than venipuncture. The risk of secondary haemorrhage is higher in the high-pressure system and the consequences of arterial thrombosis are also more serious. With good sampling technique, a capillary ABG is just as informative as an arterial ABG.

3.4. Sampling from peripheral accesses

Blood should only be taken from indwelling catheters if peripheral venipuncture is not possible. Blood taken from accesses creates pre-analytical problems, e.g. dilution with infusion solutions or contamination with heparin used to block the catheter. There is a risk of the indwelling catheter becoming blocked by blood clots. In addition, contamination of the catheter lumen with blood increases the risk of a catheter-associated infection.

3.5. Sampling from central accesses

Blood can also be drawn from an inserted central venous access - such as a central venous catheter or a port system. It is important here that no air gets into the catheter system. Particularly hygienic working practices are also required. No infusion may be running during the blood collection. Before the actual blood sample is taken, a certain amount of blood must be aspirated and discarded to prevent the blood taken for analysis from being diluted with infusion solution. After the blood sample has been taken, the used catheter lumen must be rinsed (e.g. with sterile isotonic saline solution) to ensure that no blood remains there.

The blood collection tubes must be clearly labelled before blood is taken. Prefabricated labels containing the patient data and a sample number are generally used for this purpose. This prevents the "wrong blood in tube" error.

The collected blood is stored in blood collection tubes with special additives until the examination. Depending on the test requested, the blood must be drawn into the correct tubes. The additives used include:

• Coagulation accelerator or no additive - Serum (e.g. CRP, electrolytes)
• EDTA, (e.g. blood count, FISH)
• Citrate, (e.g. PTT, Quick value)
• Heparin (e.g. chromosome analysis)
• Special tubes (e.g. for homocysteine, platelet count in EDTA pseudothrombocytopenia, trace elements)
• Nutrient media - blood culture bottles

For some laboratory parameters, further pre-analytical measures are required, for example immediate centrifugation and deep freezing or a warm blood sample.

Regular blood sampling at short intervals can cause anaemia. This is a particular problem in neonatology, but also in adult medicine. The total amount of blood samples taken can reach 500 ml per week in an intensive care unit. In a publication from 2014, the author estimates the blood loss for diagnostic purposes in the "western world" at 25 million litres per year. The term "hospital acquired anaemia" is used for this.

7.1. Blood collection by non-physicians

Blood samples do not necessarily have to be taken by a doctor, but may be delegated to qualified non-medical staff. This falls under the employer's right to issue instructions. Although the basis for delegation is not precisely regulated by law, it is recognised in case law that personal intervention by the doctor is only required "where the activity in question requires the doctor's own knowledge and skills."

As a rule, blood samples are therefore taken in clinics and surgeries by trainees and PJ students or by appropriately trained nursing staff or medical assistants. However, the presence or short-term personal availability of an authorised doctor is required.

7.2. Compulsory blood sampling

In Germany, compulsory blood sampling is possible in accordance with § 81a of the Code of Criminal Procedure. It may only be carried out by a licensed doctor on the order of a judge. In exceptional cases (e.g. suspected offences under the influence of alcohol or drugs), the public prosecutor's office or investigating authorities (e.g. police officers) may also order a blood sample to be taken. The blood sample can then also be taken against the will of the accused - if necessary using physical force.

In Austria and Switzerland, the rules are stricter.