Dacie and Lewis Practical Haematology E-Book

Barbara J. Bain, Imelda Bates, Mike A Laffan

• Editore: Elsevier
• Anno: 2016
• ISBN:
  9780702069253
• Edizione: 12
• Argomento: Médecine
• Categoria: Hématologie

For more than 65 years, this best-selling text by Drs. Barbara J. Bain, Imelda Bates, and Mike A. Laffan has been the worldwide standard in laboratory haematology. The 12th Edition of Dacie and Lewis Practical Haematology continues the tradition of excellence with thorough coverage of all of the techniques used in the investigation of patients with blood disorders, including the latest technologies as well as traditional manual methods of measurement. You'll find expert discussions of the principles of each test, possible causes of error, and the interpretation and clinical significance of the findings.

• A unique section on haematology in under-resourced laboratories.
• Ideal as a laboratory reference or as a comprehensive exam study tool.

• Each templated, easy-to-follow chapter has been completely updated, featuring new information on haematological diagnosis, molecular testing, blood transfusion- and much more.
• Complete coverage of the latest advances in the field.
• An expanded section on coagulation now covers testing for new anticoagulants and includes clinical applications of the tests.

1

Collection and Handling of Blood

Christopher McNamara

Chapter Outline

Biohazard precautions

Procurement of venous blood

Equipment

Specimen containers

Phlebotomy procedure

Postphlebotomy procedure

Capillary blood

Collection of capillary blood

Blood film preparation

Differences between capillary and venous blood

Sample homogeneity

Serum

Cold agglutinins

Anticoagulants

Ethylenediaminetetra-acetic acid (EDTA)

Trisodium citrate

Heparin

Effects of storage on the blood count

Effects of storage on blood cell morphology

Acknowledgement

The author wishes to acknowledge the contribution of previous authors of this chapter – the late Corrine Jury, Yutaka Nagai and the late Noriyuki Tatsumi – and of Gareth Ellis, who reviewed the content of this chapter.

Following an informed decision to analyse a blood sample, a specimen must be safely and correctly procured. It is essential to be aware that variation in this pre-analytical phase of the testing process can lead to errors in the analytical phase (see Box 1-1).

Box 1-1

Causes of misleading results related to specimen collection

Pre-collection

• Urination within 30 min; food or water intake within 2 h

• Smoking

• Physical activity (including fast walking) within 20 min

• Stress

• Drugs or dietary supplement administration within 8 h

During collection

• Different times (diurnal variance)

• Posture: lying, standing or sitting

• Haemoconcentration from prolonged tourniquet pressure

• Excessive negative pressure when drawing blood into syringe

• Incorrect type of tube

• Capillary versus venous blood

Handling of specimen

• Insufficient or excess anticoagulant

• Inadequate mixing of blood with anticoagulant

• Error in patient and/or specimen identification

• Inadequate specimen storage conditions

• Delay in transit to laboratory

Venous blood is used for most examinations. Capillary blood samples may be satisfactory for some purposes but in general the use of capillary blood should be restricted to children and to some point-of-care screening tests.

Biohazard precautions

Laboratory policies must be in place to ensure that staff who collect blood samples and transfer them to the laboratory minimise the risk of infection from various pathogens during all aspects of specimen handling (see Chapter 24). Additional precautions should be taken when handling high-risk specimens (e.g. those from patients suspected of having a viral haemorrhagic fever).¹ In this circumstance, the collection policy should stipulate the use of personal protective equipment, such as disposable gloves, body apron and protective eyewear. Care must be taken to prevent injuries, especially when handling and disposing of needles and lancets. Recommendations for standardising blood collection have been published.^2,3

Procurement of venous blood

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* needle is suitable for most adults. A 23G needle is often selected for children. The shaft of the needle should be short (about 15 mm). It may be helpful to collect the blood by means of a winged needle (often referred to as a ‘butterfly’) connected to a length of plastic tubing that can be attached to the nozzle of the syringe or to a needle for entering the cap of an evacuated container (see Specimen Containers).

Box 1-2

Items to be included in a phlebotomy tray

• Syringes and needles

• Tourniquet

• Specimen containers (tubes or evacuated tube system) – plain and with various anticoagulants

• Request form

• 70% isopropanol swabs or 0.5% chlorhexidine

• Sterile gauze swabs

• Adhesive dressings

• Self-sealing plastic bags with a separate compartment for the request form

• Rack to hold specimens upright during process of filling (except when an evacuated tube system is used)

• Puncture-resistant disposal container

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.⁴ 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 Haematology⁵) 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.⁶ 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 unnecessa...