Collection and handling of blood

Published on 04/03/2015 by admin

Filed under Hematology, Oncology and Palliative Medicine

Last modified 04/03/2015

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Chapter 1 Collection and handling of blood

In investigating physiological function and malfunction of blood, accurate and precise methodology is essential to ensure, as far as possible, that tests do not give misleading information because of technical errors. Obtaining the specimen is the first step towards analytic procedures. It is important to use appropriate blood containers and to avoid faults in specimen collection, storage and transport to the laboratory. Venous blood is generally used for most haematological examinations and for chemistry tests; capillary skin puncture samples can be almost as satisfactory for some purposes if a free flow of blood is obtained (see p. 4), but in general this procedure should be restricted to children and to some ‘point-of-care’ screening tests which require only a drop or two of blood. Bone marrow aspirates are described in Chapter 7.

Biohazard precautions

Special care must be taken to avoid risk of infection from various pathogens during all aspects of laboratory practice, and the safety procedures described in Chapter 24 must be followed as far as possible when collecting blood. The operator should wear disposable plastic or thin rubber gloves. It is also desirable to wear a protective apron or gown, as well as glasses or goggles, if necessary. Care must be taken to prevent injuries, especially when handling syringes, needles and lancets.

Disposable sterilized syringes, needles and lancets should be used if at all possible, and they should never be re-used. Re-usable items must always be sterilized after use (see Chapter 24).

Venous blood

It is now common practice for specimen collection to be undertaken by specially trained phlebotomists, and there are published guidelines which set out an appropriate training programme.1,4

Phlebotomy Tray

It is convenient to have a tray which contains all the requirements for blood collection (Box 1.2).

Disposable Plastic Syringes and Disposable Needles

The needles should not be too fine, too large, or too long; those of 19 or 21G* are suitable for most adults. 23G are suitable for children and ideally should have a short shaft (about 15 mm). It may be helpful to collect the blood by means of a winged (‘butterfly’) needle connected to a length of plastic tubing which can be attached to the nozzle of the syringe or to a needle for entering the cap of an evacuated container (see below).

Specimen Containers

The common containers for haematology tests are available commercially with dipotassium, tripotassium, or disodium ethylenediaminetetra-acetic acid (EDTA) as an anticoagulant, and they are marked at a level 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 Standardization in Haematology,5 and there is also a European standard (EN 14820). Unfortunately, there is not yet universal agreement regarding the colours for identifying containers with different additives; phlebotomists should familiarize themselves with the colours used by their own suppliers.

Evacuated tube systems which are now in common use consist of a glass or plastic tube/container (with or without anticoagulant) under 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 minimizing 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 which enters the tube, ensuring an adequate specimen for the subsequent tests and the correct proportion of anticoagulant, when this is present. Silicone-coated evacuated tubes can be used for routine coagulation screening tests.

Phlebotomy Procedure

The phlebotomist should first check the patient’s identity, making sure that it corresponds to the details on the request form, and also ensure that the phlebotomy tray contains all the required specimen containers.

Blood is best withdrawn from an antecubital vein or other visible veins in the forearm by means of either an evacuated tube or a syringe. It is usually recommended that the skin should be cleaned with 70% alcohol (e.g. isopropanol) or 0.5% chlorhexidine, and allowed to dry spontaneously before being punctured; however, some doubts have been expressed on the utility of this practice for preventing infection at the venepuncture site.6 Care must also be taken when using a tourniquet to avoid contaminating it with blood because infection risks have been reported during blood collection.7 The tourniquet should be applied just above the venepuncture site and released as soon as the blood begins to flow into the syringe or evacuated tube – delay in releasing it leads to fluid shift and haemoconcentration as a result of venous blood stagnation.4 Except for very young children, it should be possible with practice to obtain venous blood even from patients with difficult veins. A butterfly needle is especially useful when a series of samples is required.

Successful venepuncture may be facilitated by keeping the subject’s arm warm, applying to the upper arm a sphygmomanometer cuff kept at approximately diastolic pressure and tapping the skin over the site of the vein a few times. After cleaning and drying the site and applying a tourniquet, ask the patient to make a fist a few times. Veins suitable for puncture will usually become apparent. If the veins are very small, a butterfly needle or 23G needle should enable at least 2 ml of blood to be obtained satisfactorily. In obese patients, it may be easier to use a vein on the dorsum of the hand, after warming it by immersion in warm water; however, this site is not generally recommended as vein punctures tend to bleed into surrounding tissues more readily than at other sites. Venepuncture should not be attempted over a site of scarring or haematoma.

If a syringe is used for blood collection, the piston of the syringe should be withdrawn slowly and no attempt made to withdraw blood faster than the vein is filling. Anticoagulated specimens must be mixed by inverting the containers several times. Haemolysis can be avoided or minimized by using clean apparatus, withdrawing the blood slowly, not using too fine a needle, delivering the blood gently into the receiver and avoiding frothing during the withdrawal of the blood and subsequent mixing with the anticoagulant. If the blood is drawn too slowly or inadequately mixed with the anticoagulant some coagulation may occur. After collection, the containers must be firmly capped to minimize the risk of leakage.

If blood collection fails, it is important to remain calm and consider the possible cause of the failure. This includes poor technique, especially stabbing, rather than holding the needle parallel to the surface of the skin as it enters, as this may result in the needle passing through the vein. After two or three unsuccessful attempts, it may be wise to refer the patient to another operator after a short rest.

After obtaining the blood and releasing the tourniquet, remove the needle and then press a sterile swab over the puncture site. The arm should be elevated after withdrawal of the needle and pressure should continue to be applied to the swab with the arm elevated for a minute or two before checking that bleeding has completely ceased. Then cover the puncture site with a small adhesive dressing.

Obtaining blood from an indwelling line or catheter is a potential source of error. As it is common practice to flush lines with heparin, they must be flushed free from heparin and the first 5 ml of blood discarded before any blood is collected for laboratory tests. If intravenous fluids are being transfused into an arm, the blood sample should not be collected from that arm.