Biomedical Engineering Reference
In-Depth Information
However, no immunoassay is 100 per cent 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 42 000 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 relied exclusively upon to prevent pathogen
transmission. Many of the processing techniques used to derive blood products from whole
blood (e.g. precipitation, but especially chromatographic purifi cation) can be effective in sepa-
rating viral or other pathogens from the fi nal product (see also Chapters 6 and 7). Fractionated
products, therefore, are less likely to harbour undetected pathogens.
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 annu-
ally due to errors in blood group typing or the presence of bacteria in the product.
Box 14.6
Product case study: Carticel
Carticel is a preparation of autologous cultured chondrocytes used in the treatment of sympto-
matic cartilage defects of the femoral condyle (the rounded protuberance at the end of the hip
bone), caused by acute or repetitive trauma, in patients for whom surgical repair has proven in-
adequate. It is produced by Genzyme Biosurgery and gained approval for medical use in 1997.
Each single-use product container contains approximately 12 million cells devoid of microbial
contamination, in a fi nal volume of 0.4 ml sterile, buffered cell culture media (Dulbecco's
modifi ed Eagle's medium; Chapter 5).
Chondrocytes are found embedded in the cartilage matrix that they originally produced. Car-
tilage itself is a dense connective tissue consisting mainly of proteoglycans (biomolecules com-
posed of a protein backbone to which extensive carbohydrate side-chains are attached, such that
the molecule is predominantly carbohydrate based). It is capable of withstanding considerable
pressure and effectively acts as a shock absorber in joints. There are three types of cartilage:
hyaline cartilage is found at bone joints, larynx, trachea, bronchi and nose; elastic cartilage is
found in the external ear; and fi brocartilage is found in the intervertebral discs and in tendons.
Chondrocytes are initially harvested from the patient's own body and undergo in vitro ex-
pansion via cell culture (Chapter 5) in a medium containing foetal bovine serum. The cells
are aseptically fi lled into vials and are tested for viability and microbial sterility. They are
implanted by trained surgeons into the affected area via injection, using a catheter. Typically,
between 1 million and 2 million cells are introduced back into the damaged joint area in this
way. Both preclinical studies in rabbits, goats, dogs and horses and clinical studies in humans
have illustrated the ability of the cultured chondrocytes to promote cartilage repair. However,
a signifi cant number of patients treated with Carticel require later surgical intervention, due to
locking, clicking or painful joints that is believed to occur due to overgrowth of grafted tissue.
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