Corpus: Blood sampling

Blood sampling refers to the collection of blood from a blood vessel. It can be performed via direct puncture of a blood vessel or through an existing peripheral or central venous or arterial access.

Blood sampling is commonly performed for diagnostic and monitoring purposes but is also used in preventive healthcare and blood donation. In some cases, therapeutic bloodletting is still indicated as a treatment.

3.1. Venous blood collection

Venous blood sampling (venipuncture) is the standard method for obtaining blood. It involves inserting a hollow needle into a vein after disinfecting the puncture site with an alcohol swab.

The median cubital vein or cephalic vein in the forearm are typically chosen as puncture sites, though blood can theoretically be drawn from any superficial or deeper vein.

A specific variant of venous blood collection is staged blood sampling, which involves taking multiple blood samples over time.

3.2. Capillary blood collection

Capillary blood sampling is a common method for obtaining small blood samples, often used for measuring blood glucose levels or oxygen saturation.

The preferred puncture sites are areas with good blood supply, such as the fingertip or earlobe. Lancing devices create a quick and standardized puncture, minimizing pain.

This method is simple and widely used, especially for self-monitoring at home with blood glucose meters. In pediatric medicine, capillary blood sampling is often preferred to minimize the need for more painful venous blood draws.

3.3. Arterial blood sampling

Arterial blood sampling is primarily performed for blood gas analysis (BGA). Blood is typically drawn from the radial or femoral artery using a sealed capillary tube or a heparinized syringe.

Since arterial puncture is more invasive than venipuncture, it carries a higher risk of complications, including bleeding and arterial thrombosis. However, if performed correctly, capillary blood gas analysis can provide comparable results to arterial sampling.

3.4. Blood sampling from peripheral accesses

Blood should only be drawn from indwelling catheters when peripheral venipuncture is not feasible. Sampling from these devices can lead to pre-analytical issues, such as dilution with IV fluids or contamination with heparin. Additionally, drawing blood from an indwelling catheter increases the risk of clot formation and catheter-associated infections.

3.5. Blood sampling from central accesses

Blood can also be drawn from central venous access devices, such as central venous catheters or port systems. Special precautions must be taken to prevent air from entering the system and to maintain strict hygiene standards.

During sampling, no infusions should be running. A small volume of blood must first be discarded to prevent dilution of the sample. After collection, the catheter lumen must be flushed (e.g., with sterile isotonic saline) to prevent clot formation and contamination.

Blood collection tubes must be properly labeled before sampling to ensure correct patient identification. Preprinted labels containing patient details and a sample number are typically used to prevent errors, such as the misallocation of samples ("wrong blood in tube" errors).

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 tests, additional pre-analytical steps are necessary, such as immediate centrifugation, deep freezing, or maintaining the blood at body temperature.

Frequent blood sampling can lead to anemia, a significant concern in both neonatology and adult intensive care. In critically ill patients, the total blood loss from diagnostic sampling can reach up to 500 ml per week. A 2014 study estimated that in Western countries, blood loss due to diagnostic sampling amounts to approximately 25 million liters per year. This phenomenon is referred to as "hospital-acquired anemia."

7.1. Blood collection by non-physicians

Blood sampling is not exclusively performed by physicians and can be delegated to qualified healthcare professionals, such as nurses, medical assistants, and medical students. The legal basis for delegation is not explicitly defined, but case law generally allows delegation when the procedure does not require a physician’s expertise. However, a physician must be available for supervision or immediate intervention if needed.

7.2. Compulsory blood sampling

In Germany, compulsory blood sampling is permitted under § 81a of the Code of Criminal Procedure. It must be ordered by a judge and performed by a licensed physician. In urgent cases, such as suspected offenses involving alcohol or drug intoxication, prosecutors or law enforcement officers may order a blood sample without prior judicial approval. If necessary, blood samples can be obtained against the individual’s will, including through the use of physical restraint.

In Austria and Switzerland, the legal requirements for compulsory blood sampling are stricter.