Collecting a venous blood sample is a fundamental skill in clinical practice, requiring meticulous attention to detail to ensure patient safety and the sample’s integrity. This section delves into the practical aspects of venepuncture, including the selection of appropriate veins in the arm and hand, the importance of sterile techniques, and the critical role of hand washing. It also covers alternative blood collection methods. We will also explore labelling principles and the proper completion of request forms, vital for accurate laboratory investigations and patient identification.
Vein selection for needle insertion
Preferably, the sample should be taken from the arm, with the antecubital fossa being the optimal site. This area, located at the elbow joint, is chosen because the veins here are usually prominent and easy to locate. Commonly used veins in this area include the basilic vein, the cephalic vein, and the medial cubital vein.
It is also possible to use the median cubital veins located on the dorsal aspect of the hand. This is often where intravenous (IV) cannulas are placed to administer medication and fluids. These veins tend to be smaller and less prominent than those in the antecubital fossa. Although they are a viable option, they are generally avoided in favour of more easily accessible veins, depending on the patient’s specific circumstances.
Figure 1.13 - Veins of the hand
Sterile measures
The significance of sterile measures in blood collection cannot be overstressed. A key component is the use of an alcohol swab to clean the skin before needle insertion. An alcohol-based agent like isopropyl alcohol is generally preferred. Other disinfectants, like iodine, can be used, but excessive iodine can contaminate the sample, potentially affecting test results, including falsely elevating platelet, potassium, and uric acid levels. Therefore, an alcohol swab is usually the preferred choice to cleanse the area before needle insertion.
Hand washing is also a crucial step in this process. It is recommended that hands be washed with water and soap or other detergents containing an antiseptic agent, especially when carrying out any aseptic technique. This significantly reduces the risk of transmitting infections and ensures the sterility of the procedure. Use disposable gloves and tourniquets to maintain hygiene and prevent cross-contamination during venepuncture. All equipment that makes contact with the patient, especially needles, should be single-use and sterile to ensure the highest level of safety and infection control. An exception to this is non-sterile gloves, which are appropriate for blood collection that does not involve central catheters like Hickman lines.
Blood collection systems
Blood collection can be performed using either the needle and syringe method or the vacutainer system. The needle and syringe method involves using a sterile syringe and needle to manually draw blood from a patient’s vein. It carries a higher risk of coagulation, haemolysis and sample contamination as the blood must be transferred from the syringe to collection tubes.
On the other hand, the vacutainer system employs a double-ended needle, a plastic holder, and vacuum-sealed collection tubes. When the needle is inserted into the vein, and a vacuum-sealed tube is attached, the vacuum automatically draws the blood. This method reduces the risk of haemolysis and contamination, minimises exposure to pathogens, and allows for the easy collection of multiple samples from a single venipuncture. It is hence the preferred method of blood collection.
A-needle and syringe, B-vacuntainer with butterfly needle
The venepuncture procedure
Multiple methods exist for venepuncture, each requiring practice and supervision to develop confidence and competency. To ensure safety and accuracy, mastering venepuncture involves understanding the correct technique, equipment, and patient interactions. Figure 1.15 provides a step-by-step visual guide to the practical procedure¹, outlining crucial stages such as patient preparation, vein selection, site preparation, needle insertion, and the final steps for a successful blood draw.
Alternative Blood Collection Methods
Capillary Blood Collection
Capillary blood collection involves using a lancet needle to puncture the skin and milking the blood to obtain a sample. This technique is often utilised in point-of-care devices, such as those used by diabetics for blood sugar monitoring. Some full-blood count analysers also employ a similar approach. Capillary blood can also be directly applied to a glass slide for blood film preparation. This method is particularly valuable in global health settings for rapidly analysing conditions like malaria. Capillary blood collection is also utilised in dry-blotted sample techniques, commonly used in newborn screenings to detect inherited conditions. This method is significant when collecting blood from a patient is challenging, though the quantity of blood obtained is limited.
Figure 1.16 - Capillary blood collection
Cord Blood Collection
Regarding newborns, another method involves drawing blood directly from the umbilical cord. Post-delivery, the umbilical cord is clamped at two locations, and blood is extracted using a needle and syringe. It is crucial to have an anticoagulant, typically heparin, in the syringe to prevent clotting.
Feotal Blood Collection
In prenatal settings, foetal blood can be drawn from the umbilical cord while the baby is still in utero, typically under ultrasound guidance. This procedure carries significant risks for both the mother and the foetus, including haemorrhage, infection, and miscarriage. Therefore, it is performed only for clear medical reasons when other diagnostic procedures, such as amniocentesis or chorionic villus sampling, are unsuitable.
Indwelling Catheters
Blood can also be drawn from indwelling catheter devices, usually for venous but sometimes via the arterial route. Examples include temporary central venous catheters, such as central lines inserted into the patient’s jugular vein, tunnelled lines like Hickman lines placed over the upper chest, and peripherally inserted central catheters (PICC) typically placed in the arm. Another device used in this context is the Port-a-Cath, a small implantable device situated under the skin. A needle can be inserted into the device to either draw blood or administer medications.
These devices, especially Hickman and PICC lines, are frequently used for patients undergoing chemotherapy or requiring prolonged IV medication, such as antibiotics and parenteral nutrition. The tips of these lines are usually positioned just above the heart, most commonly in the superior vena cava, facilitating blood withdrawal and administration of medications or fluids. When extracting blood from an indwelling catheter, maintaining sterile techniques is crucial due to the heightened risk of infection.
Sterile gloves and alcohol swabs are standard tools for ensuring the cleanliness of the extraction site. The initial 10-50 millilitres of blood drawn from these catheters is usually discarded to avoid contamination with residual fluids or medications before collecting the sample intended for analysis.
A-tunneled (Hickman) line, B-PICC line, C-temporary central venous catheter, D-port-a-cath
Blood can be drawn from peripheral venous lines, typically known as IV cannulas, immediately after the insertion needle is removed. However, once these lines are established and in use, drawing blood from them is generally discouraged to prevent the risk of haemolysis and contamination.
Figure 1.13 - Intravenous cannula
Labelling and Request Form Procedures
After collecting the blood sample, it should be labelled correctly. Labels can be created electronically with preprinted information or handwritten. If handwritten, ensure legibility and include all necessary information for processing the sample. Avoid scenarios where labels obscure important details, such as covering the cap or being wrapped around the tube in a way that makes information unreadable. Adequate labelling is pivotal for accurate sample identification and processing.
When completing a request form for identified patients, essential details include the patient’s full name, date of birth, gender, and medical record number, which could be a hospital-specific or a universal number like the National Health Service (NHS) number used in the United Kingdom. Additional important information includes the patient’s location, the date and time of sample collection, the collector’s details, the type of specimen (e.g., blood, urine), and any relevant clinical information. This information is crucial, especially if the laboratory needs to contact the healthcare provider urgently regarding abnormal results.
The three key pieces of information for unidentified patients are a unique number, approximate age, and gender. Once the patient’s identity is ascertained, it is essential to immediately update the laboratory with this information for accurate record-keeping and processing. This ensures that results previously analysed under unidentified information can be accurately transferred to the patient’s record.