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whose mothers contracted CMV during pregnancy
Treponema pallidum Bacterial causative agent of syphilis
Protozoa of the genus Plasmodium Malaria
Trypanosoma cruzi Protozoal causative agent of Chagas’ disease
Before any blood donation is released for issue/processing, it must be tested for the presence of various pathogens particularly likely to be present in blood. In most countries, these tests include immunoassays capable of detecting:
• hepatitis B surface antigen (HBsAg);
• antibodies to HIV;
• antibodies to hepatitis C virus (HCV);
• syphilis antibodies.
However, no immunoassay is 100% accurate and all will report a low number of false negatives (and false positives). It is believed that in the order of 1 in every 42000 blood units reported to be HIV antibody-negative actually harbours the virus.
In general, processes capable of inactivating viral or other pathogens (e.g. heat or chemical treatment) may not be applied to whole blood or most blood-derived products. Thus, for whole blood at least, effective screening of donations is exclusively relied upon to prevent pathogen transmission. Many of the processing techniques used to derive blood products from whole blood (e.g. precipitation, but especially chromatographic purification) can be effective in
BLOOD PRODUCTS AND THERAPEUTIC ENZYMES 353
separating viral or other pathogens from the final product (see also Chapter 3). Fractionated products, therefore, are less likely to harbour undetected pathogens.
Whole blood is blood that has been aseptically withdrawn from humans. A suitable anticoagulant is added (often heparin or a citrate-dextrose-based substance), although no preservative is present. The blood is usually stored at temperatures in the range 1-8°C, and has a short shelf-life (48 h after collection if heparin is used as the anticoagulant; or up to 35 days if citrate-phosphate-dextrose with adenine is employed). In addition to being screened for likely pathogens, the ABO blood group and the Rh group is also determined. In the USA alone, in the region of 35 transfusion-related deaths occur annually due to errors in blood group typing or the presence of bacteria in the product.
The blood is generally warmed to 37°C immediately prior to transfusion. Whole blood is often used to replace blood lost due to injury or surgery. The number of Units (1 Unit ~ 510 ml) administered depends upon the health and age of the recipient, along with the therapeutic indication. Administration of whole blood may also be undertaken to supply a recipient with a particular blood constituent (e.g. a clotting factor, immunoglobulin, platelets or red blood cells). However, this practice is minimized in favour of direct administration of the specific blood constituent needed.
PLATELETS AND RED BLOOD CELLS
Platelets play a central role in the blood clotting process. They are often administered prophylactically or therapeutically in order to prevent/minimize blood loss due to haemorrhage in persons suffering from thrombocytopenia (low blood platelet levels; see Chapter 6).
Platelet concentrate consists of platelets obtained from whole blood. The platelets obtained from a unit of blood are usually resuspended in 20-50 ml of the original plasma and contain not less than 5.5x1010 platelets/unit (for blood preparations, a ‘unit’ refers to the amount of that product purified from a single Unit of whole blood). Like whole blood, platelets have a short shelf-life and must be used within 72 h of blood collection.
Concentrated red blood cells are prepared from the whole blood of a single donor, from which the anticoagulant and some of the plasma have been removed. According to the British Pharmacopoeia (Chapter 3), the preparation must have a packed cell volume greater than 70% (plasma-reduced blood is similar, except that its packed cell volume must be between 60-65%).
Red blood cell preparations are usually stored at 2-8°C (although the United States Pharmacopeia includes frozen storage below —65°C). If stored unfrozen, its useful shelf-life must not exceed the shelf-life of the whole blood from which it was derived. If stored frozen, the shelf-life is extended to 3 years (although the product must subsequently be used within 24 h after thawing).
Red blood cell fractions are administered to patients suffering from severe anaemia, including patients with sickle cell anaemia and new born babies suffering from haemolytic disease.
Blood substitutes, as defined here, refers to any substance which can:
• replace or maintain blood volume; or
• ensure oxygen delivery to body tissues.
Table 9.3. Some colloidal plasma volume expanders currently in therapeutic use. In addition to these, albumin and plasma protein fraction may also be used