2.4 Venous blood sampling
The most accurate method for measuring blood glucose is to take a sample of blood from a vein (a venous sample) that is then tested in a laboratory. (This will result in a plasma glucose level.) This is always done when diagnosing diabetes – a capillary sample used with a blood glucose meter or a urine test is not accurate enough for a diagnosis. Venous samples are also used for many other blood tests which are important in identifying changes which may lead to long-term complications in people with diabetes.
Veins are large, thin-walled blood vessels which carry blood back to the heart. The most commonly used vein for venous blood sampling is found in the antecubital fossa, that is the area inside the bend of the elbow (Figure 7a). As you can see in this diagram, there are many veins in this area that are close to the surface of the skin. If there is difficulty in getting blood from these veins, other sites such as the back of the hand may be used.
The equipment needed to take a venous blood sample is of two sorts. Either a syringe and needle is used, or a closed vacuum system and needle. The closed vacuum system (also known as a venepuncture vacuum system) has the advantage of enabling several samples to be taken safely at any one time by reducing the risk of blood spillage and needle-stick injury to the person taking the sample (Figure 7b). Venepuncture literally means ‘puncturing the vein’ and is used to describe a needle entering a vein through the skin.
Before blood is taken, it is important that the person with diabetes understands what is going to be done and why, and has given their consent. The person taking the blood should always wash their hands before they start the procedure and wear a fresh pair of gloves.
The equipment used in taking a blood sample includes:
needle and syringe, or venepuncture vacuum system
cotton wool balls and adhesive tape
Can you think what all these items are required for?
You probably thought of at least some of the following:
Disposable gloves: these are used to protect the person taking blood from any accidental blood spillage. Gloves offer some protection from infection risk should a spillage happen. They also help to protect the patient from any bacteria on the health professional's hands, though this person should always wash their hands even when using gloves.
Alcohol swab: this is used to clean the patient's skin before it is punctured to take the blood sample. The alcohol should be left to dry for at least five seconds before the sample is taken; this kills most of the bacteria on the skin so that they cannot enter the blood when the needle enters the vein through the skin.
Tourniquet: this is an elasticated strap used to compress the vein above the proposed site of the injection, allowing the vein to be seen more easily and felt more readily. The tourniquet should be put on tightly enough to raise the vein but not so tightly that arterial blood flow is stopped. Tourniquets should not be left in place for any longer than two minutes, in order to avoid injury. Tourniquets that are left on too long can make the arm quite painful.
Venepuncture vacuum systems: these are closed systems used for collecting blood samples. The vacuum in the specimen tube causes the blood to be drawn directly into it (see Figure 7b). Risk of blood spillage is reduced as each specimen tube can be attached in turn to the system.
Needles and syringes: these are used to obtain the sample of blood when a venepuncture vacuum system is unavailable or not suitable, such as when the patient's veins appear fragile. Needles and syringes come in various sizes and a suitable size should be chosen. The needle should remain sheathed when being applied to the syringe and until the sample is about to be taken. It should not be resheathed after use as this increases the risk of a needle-stick injury. Used needles should be placed in a sharps box.
Specimen tubes: blood samples are placed in these after collection by the needle and syringe method. There are different types of sample tube depending on the laboratory test to be done. Labels on the specimen tubes must be accurately completed, including the patient's name and identification number. (See, for example, Figure 8.)
Sharps box: this is needed for the safe disposal of needles and other used equipment. (See, for example, Figure 6a.)
Cotton wool balls and adhesive tape: these are needed to apply pressure to the vein following the procedure, to avoid haematoma formation (the leakage of blood forming a large bruise) at the site. A small adhesive dressing may be used after bleeding has stopped.
Forms and plastic bags: the forms accompany the specimens to the laboratory in plastic bags to give the patient's details and request the required tests. An appropriate form, accurately completed, accompanies each specimen. In the laboratory, the details on the specimen tube are checked with the details on the form, to make sure the correct tests are performed for the correct patient.