Equipment

It is important to assemble a tray or prepare a workspace that has all the requirements for blood collection ( Box 1-2 ). The selection of needle diameter is a compromise between achieving adequate flow with minimal turbulence and minimising patient discomfort. A 19-gauge (19G) or 21G *

* The International Organisation for Standardisation has established a standard (ISO 7864), which relates the following diameters for the different gauges: 19G = 1.1 mm; 21G = 0.8 mm; 23G = 0.6 mm.

Specimen containers

Containers for testing whole blood are available commercially with dipotassium, tripotassium or disodium ethylenediaminetetra-acetic acid (EDTA) anticoagulant, and often have a mark to indicate the correct amount of blood to be added. Containers are also available containing trisodium citrate, heparin or acid–citrate–dextrose, as well as containers with no additive which are used when serum is required. Design requirements and other specifications for specimen collection containers have been described in a number of national and international standards (e.g., that of the International Council for Standardisation in Haematology ) and the European standard (EN 14820). There is no universal agreement regarding the colours used for identifying containers with different additives so phlebotomists should familiarise themselves with the colours used by their local suppliers.

Evacuated tube systems in common use consist of a glass or plastic tube under a defined vacuum, a needle and a needle holder, which secures the needle to the tube. The main advantage is that the cap can be pierced so that it is not necessary to remove it either to fill the tube or subsequently to withdraw samples for analysis, thus minimising the risk of aerosol discharge of the contents. An evacuated system is useful when multiple samples in different anticoagulants are required. The vacuum controls the amount of blood that enters the tube, ensuring an adequate volume for testing with the correct proportion of any anticoagulant.

Phlebotomy procedure

Staff undertaking this procedure should be adequately trained. The phlebotomist must check that the patient’s identity corresponds to the details on the request form and also ensure that the phlebotomy tray contains all the required specimen containers and other equipment necessary for the procedure.

A tourniquet should be applied just above the intended venepuncture site. Blood is best withdrawn from an antecubital vein or other visible veins of the forearm by means of either an evacuated tube or a syringe. It is recommended that the skin be cleaned with 70% alcohol (e.g., isopropanol) and allowed to dry spontaneously before being punctured. The tourniquet should be released as soon as the vein is punctured and blood begins to flow into the syringe or evacuated tube – delay in releasing the tourniquet leads to fluid shift and haemoconcentration as a result of venous blood stagnation. After the vein has been successfully punctured, the piston of the syringe should be withdrawn slowly with no attempt being made to withdraw blood faster than the vein is filling. Anticoagulated specimens must be mixed by inverting the container several times. The risk of unwanted haemolysis of the specimen can be minimised by using minimal tourniquet time, withdrawing blood carefully, using an appropriately sized needle, delivering the blood slowly into the receptacle and avoiding unnecessary agitation when mixing with the anticoagulant. Note that if blood is drawn too slowly or is inadequately mixed with the anticoagulant some coagulation may occur, rendering the sample unsuitable. After collection, containers must be firmly capped to minimise the risk of leakage.

If blood collection fails, it is important to remain calm, communicate with the patient and consider the possible causes. These include poor technique (e.g., passing the needle through the vein, or poor selection of veins), scarring of tissues and haematoma formation.

After obtaining the necessary specimens, remove the needle and press a sterile swab over the puncture site. Gentle pressure should be applied to the swab with the arm slightly elevated for a minute before checking that bleeding has completely ceased. Finally, the puncture site should be covered with a small adhesive dressing.

Obtaining blood from an indwelling line or catheter is an important potential source of error. It is common practice to flush indwelling lines with heparin, so they must be flushed free from heparin and the first 5 ml of blood must be discarded before any blood is collected for laboratory testing. If intravenous fluids are being transfused into an arm, blood should generally not be collected from that arm; however, if this is essential, the specimen should be obtained from below the intravenous infusion with the tourniquet being placed below the site of infusion.

Postphlebotomy procedure

It is essential that every specimen is labelled with adequate patient identification immediately after the samples have been obtained and at the patient’s bedside. The information should include, as a minimum, surname and forename or initials, hospital number or other unique identifying number, date of birth and date and time of specimen collection. Many centres have adopted automated patient identification using a bar code printed on a wrist or ankle band worn by the patient. If this type of system is used both the specimen label and the request form should be bar-coded with identical data, unless the sample is to be used for blood transfusion tests, in which case the label should be handwritten (see Chapter 22 ).

Specimens should be sent in individual plastic bags separated from the request forms to prevent contamination of the forms in the event of leakage. Samples and form should remain together until the request is registered in the laboratory reception area.