School of Biosciences
University of Westminster
Blood has a vital role in the human body and blood transfusion can be life saving in patients with either massive blood loss like in disseminated intravascular coagulation cases and accidents or in those unable to produce blood due defective haemopoesis (Hoffbrand et al., 2001). Nowadays, with modern separation techniques blood can be fractionated into many useful products like fresh frozen plasma (FFP), platelets concentrate, red cells concentrate and cryoprecipitate. They are used as replacement or therapeutic products (Overfield, Dawson & Hamer, 1999).
However, all these three blood components have the potential to carry pathogens like viruses, bacteria and parasites (Goldman & Blajchman, 1991) and significant number of morbidity and mortality cases were noted in immunocompromised individuals (Lin et al., 1994).
Today, blood components transfusion are safer than in the last few decades due to the introduction of more sensitive screening tests like enzyme linked immunosorbent assay (ELISA). More recently, with the use of nucleic acid testing the `window period' was reduced from 70 days to 13 days in hepatitis C screening test (Regan & Taylor, 2002). The introduction of safeguard systems like detailed donor education, selection and deferral procedures, post donation product quarantine, donor tracing and notification when any infectious agents is detected occurs also helped in making blood a safer product for transfusion (Klein, 2000).
Currently, the risk of transmitting a lethal virus is very low. The table below summarised the risk of viral transmission through transfusion (BCSH guidelines, 2001):
Estimated frequency per unit transfused
Deaths per million units
1 in 100 000 to
1 in 400 000
1 in 300 000
1 in 4 000 000
However, even if the screening techniques are reliable enough to detect many lethal viruses blood is still not a safe product for the following reasons:
1. Most of the screening techniques cannot detect the pathogens in the blood sample during the “window period”. The problem is worsening as pooling of blood fractions from many asymptomatic infected blood donors, in order to raise product yield, can raise the infection rates (Council of Europe expert committee in blood transfusion study, 2001).
2. There is also the possibility of introducing a new emerging pathogen, for which no contemporary test exists, into the blood supply. A very good example to illustrate this is the transmission of the Human Immunodeficiency Virus (HIV) in the late 1970's early 80's to thousands of haemophiliac patients (Corash, 1998).
Bacteria and parasites also, represent infective risks to blood products. Bacteria may be present as silent infections in the donor or more frequently enter blood by way of veni-puncture. In platelets products stored for 5 days at 20- 24Â° c the pathogens can multiply to high levels and can cause morbidity and mortality if transfused (Blajchman et al., 1994). The risk of septic death per million of pooled platelets transfusion is 60 (Ness et al., 2001). The estimated transmission of bacteria in red cell concentrate per unit transfused is 1 in 500 000 and death can result in nearly 20% of these cases (Perez et al., 2001). UK SHOT 2001 annual surveillance report cites 15 incidents between 1995 and 2000 due to bacterial contamination and four of them were fatal (McDonald et al., 2002). In Britain, there is also, the theoretical possibility of transmission of variant Creutzfeldt-Jakob disease (CJD) by blood and currently no tests are available to detect the prion in blood or blood products (Regan & Taylor, 2002).
Labile blood products can also, transmit various parasitic diseases like malaria, chagas disease (Moor et al., 1999) and they are not currently tested in the routine serological screening tests in blood bank services.
To further reduce the risks of pathogen transmission via blood transfusion pathogen inactivation steps have been routinely applied to labile blood product like fresh frozen plasma (FFP) and possibly could be applied to blood cellular components (Council of Europe expert committee in blood transfusion study, 2001).
Several pathogen inactivation methods are available (Council of Europe expert committee in blood transfusion study, 2001):
1. Solvent/ detergent treatment of FFP
2. Methylene treatment of FFP
3. Leukocytes depletion
4. Ultraviolet B irradiation
5. Gamma Irradiation
Some procedures in clinical trials are photoinactivation using Psoralens, cyanines and riboflavin. Two processes are currently available for pathogen inactivation in the UK: Methylene blue and solvent-detergent treatment for FFP (Gibson et al., 2002).
Pathogen inactivation of labile blood products has multiple advantages. It can inactivate many viruses in the “window period” and pathogen inactivation methods like Psoralen-303 could inactivate all potential pathogens except prions. This treatment would make it unnecessary to irradiate blood components to prevent transfusion associated graft v/s host disease (Regan & Taylor, 2002).
However, all Pathogen inactivation methods only inactivate labile plasma components and cannot be applied to blood cellular components. Whereas, viruses like HIV can exist both freely in the plasma and also, in the nucleus of leucocytes (Council of Europe expert committee in blood transfusion study, 2001).
The other disadvantages are:
1. Solvent-detergent (SD) and methylene blue(MB) treatments kill enveloped viruses such as hepatitis B, C and HIV but some non-enveloped viruses like hepatitis A, parvovirus and prions are unaffected. The inactivating reagents can also, be toxic to the cellular components of blood (Koenigbauer et al., 2000; Pamphilon, 2000).
2. Pathogen inactivation (Psoralens & UV method) does not inactivate spore-forming bacteria (eg Bacillus Species) (Knitson et al., 2000)
3. There is decreased platelets recovery and in-vivo survival (Corash et al., 1997)
4. An increased in the incidence of hyperfibrinolysis has been noted in SD treated plasma (de Jonge et al., 2002)
5. Virus inactivation significantly alter red blood cells properties during storage (Wagner SJ, 1998)
6. Factor VIII, protein S and Î±2-antiplasmin are partially removed or damaged and high molecular multimers of von Willebrand factor are absent and several plasma proteins are affected in MB treated FFP (Council of Europe expert committee in blood transfusion study, 2001).
7. Pathogen inactivation of granulocytes, haematopoietic progenitor cells and donor lymphocytes are not available (Council of Europe expert committee in blood transfusion study, 2001).
Caspri 2002, reviewed the different pathogen inactivation methods and made the following observations:
1. single step inactivation cannot destroy all potential contamination unless the virus load is reduced beyond detection limit by previous screening
2. It is very difficult to investigate and evaluate the clinical effect of inactivated cellular components in either healthy, in thrombocytopenic or anaemic patients.
3. All methods inactivate pathogens by modifying their nucleic acids and these inactivated products when transfused in a patient do have a residual mutagenicity and teragenicity. As there is no threshold for mutagenicity and teragenicity therefore we cannot assess the long term effect of these mutagens in the transfused patients and the risk of them developing a cancer later.
4. The safety of the products used in inactivating pathogens has been demonstrated only in model viruses and in vivo studies are yet to be done.
There are also factors other than pathogen inactivation that need to be taken into consideration if we want to ensure total safety in transfusion. Blood products are a very labile commodity; FFP life span is 12 months at -30Â°c, Red blood cells is 35 days at 5Â°c and platelets is only 5 days at room temperature. Therefore blood availability is increasingly becoming a safety issue (Klein, 2000). The quest to eliminate the last potentially risky blood donor has caused the loss of thousands of healthy donors. The possible exclusion of blood donors who received blood transfusion between 1980-1996, to reduce the possible risk of CJD prions transmission via blood, will result in a further loss of 10 % donors in the UK (Regan & Taylor, 2002). In the United States, 8% of elective surgery was postponed and 25 % of transfusions were delayed by 1 or more days because of shortage in blood availability (Klein, 2000).
Blood leucocyte depletion:
Blood leucocyte depletion should be considered as it improves the quality and safety of labile blood products (Murphy, 1999). Brown (1997) & Wilson et al.(2000) suggested the possibility of leukocytes being involved in transmission of spongiform encephalopathies. So, in order to minimise a potential risk for the public leuco-depleted blood is issued by all transfusion centres in the UK (Council of Europe expert committee in blood transfusion study, 2001).
Pathogen inactivation of labile blood will reduce the risk of transfusion-transmitted infections especially in the “window period” but several drawbacks were noted. There are concerns about the cost effectiveness of the methods used and the inactivating products seemed to have a detrimental effect on the blood products biological activity. There is also the possibility of exposing the patients to additional risks as the inactivating substance might affect the nucleic acid of the transfusion recipient resulting in possible mutagenicity or carcinogenicity in the long term. So, as the risk of dyeing from an infectious disease through blood transfusion is very small pathogen inactivation should be done only if it does not give rise to malignancies and other side effects that could lead to a major catastrophe in the long term.
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