|Year : 2001 | Volume
| Issue : 3 | Page : 119-126
Microbes and blood transfusion
Lister laboratory and research centre, Chennai - 600 034, India
Lister laboratory and research centre, Chennai - 600 034, India
Transfusion medicine has been constantly evolving through the years with improved technologies that enhance the capability of identifying existing and newer emerging transfusion transmissible infections (TTI). In spite of the efforts made by blood banks the risk of TTI remains. This article deals with the various steps involved in ensuring blood safety, i.e. donor selection, role of screening donated blood for known and emerging infections, issues and assessment of threat posed by the risk, methodologies employed for testing and possible suggestions to improve transfusion services. While the threat of TTI remains, with a concerted effort of private and government organisations, and co-operation from the diagnostic companies, it is possible to raise the levels of blood safety. A surveillance system is also essential to identify any new agents that might pose a threat in a geographic area and to include them too in the screening process.
|How to cite this article:|
Narayan S. Microbes and blood transfusion. Indian J Med Microbiol 2001;19:119-26
Ever since 1628, when the English physician William Harvey discovered the circulation of blood, the field of transfusion medicine has made very rapid progress. Soon afterwards the first dog to dog and subsequently, lamb to human blood transfusions were attempted. In 1932, the first blood bank was established at Leningrad and within a few years blood banks were started all over the United States.1 In the 1940s Syphilis screening of donated blood was introduced in blood banks. By the mid 1950s the demand for blood had increased tremendously in response to open heart surgeries that became the miracle cure. It was however only in 1971 that screening donated blood for viral infections was begun. Since then several new tests have been added on to the screening process to make blood transfusions safer. Donated blood all over the world is now routinely screened for several infections and depending on the endemicity and / or prevalence, these tests vary from continent to continent.
This article deals with the various steps involved in ensuring blood safety, role of screening blood for transfusion transmitted infections, the need for constant review of our blood banking practices and the future of blood banking.
Various steps involved
Most often we believe that screening donated blood is sufficient to ensure blood safety. However, even before screening, careful donor selection is the most vital step. Voluntary and preferably non-replacement donors are ideal, followed by the actual donation process which again needs sophisticated equipment. Using a closed system for collecting blood that cannot be tampered with or re-used is also one more step in ensuring the safety of the blood collected. If the blood is going to be separated into its components, care needs to be taken during this step too. After the screened, safe units have been correctly labeled, checked and stored at the proper temperatures, compatibility testing at the time of issue is the last link in the chain of ensuring safety of blood from the blood banks point of view. Once the blood reaches the hospital, the onus for ensuring that the correct unit reaches the intended patient lies with the hospital.
Role of screening donated blood
At no point in time is blood transfusion really safe. While there are some agents that are known to be capable of transmission through blood transfusion, there are still several more that are identified over a period of time and sometimes through retrospective screening of stored blood samples as in the case of Hepatitis C.
The known transfusion transmitted infections can be classified into:
Some of these infections pose a threat only in certain geographic areas and universal screening for these infections everywhere is neither required nor recommended.
Screening for Hepatitis B infection started in 1971 by screening for Hepatitis B Surface antigen (HBsAg) or Australia Antigen as it was earlier called. This is a Hepadna virus and currently there are 300 million carriers of this virus worldwide. Hepatitis B is transmitted through infected blood, other body fluids (seminal fluid, vaginal secretions, breast milk, tears, and saliva) and open sores. After an incubation period that can range between 50 -180 days, this infection has an insidious onset. The virus is detectable in the blood for several months or even years and about 5 - 10% of individuals develop chronic infection or become chronic carriers. Chronic infection could lead to the development of cirrhosis and hepatic malignancy several decades later. Chronic carriers are those who have HBsAg in their blood for more than six months but who do not exhibit any signs or symptoms of infection. They remain infected and infective and are capable of transmitting infection to others.
Antibody to the core protein of Hepatitis B (anti HBc total) was introduced in the screening process in 1992 as a surrogate marker for post-transfusion Non A Non B Hepatitis. It has outlived its usefulness after the introduction of direct screening tests for Hepatitis C but due to the overlapping epidemiology of Hepatitis B and HIV, it still has a role in identifying recently infected donors who are in the window period of HIV infection, prior to the detection of HIV antibodies.
Anti HBc (total) screening has substantially reduced the number of Hepatitis B viral infectious units from entering the donor pool.3 These units can come in from two sources - chronically infected individuals in whom Hepatitis B virus is not detectable and donors in the stage of acute Hepatitis B in whom HBsAg has disappeared but in whom antibody to surface antigen has not yet appeared. Despite the benefits of additional screening for anti HBc, there may be a loss of several thousands of donor units all over the world. An anti HBs testing in those donors who are HBsAg negative but anti HBc total positive will indicate a prior HBV infection. Those donors positive for Anti HBs are unlikely to be infectious while those that are anti HBs negative are likely to have circulating HBV-DNA and will therefore be the ones that really need to be eliminated.
Screening for any of these markers is by ELISA and the sensitivity of these tests for HBsAg detection is around 0.05 U/mL or 0.5ng/mL. Latex agglutination and other rapid testing devices may be convenient to use but they compromise on both sensitivity and specificity.
Referred to as “the silent epidemic”, the Hepatitis C virus that belongs to the family of Flaviviridae causes this infection. This, like Hepatitis B, is spread through contact with infected blood. The most common route is through injection of drugs or by sharing of razors and needles. Activities like tattooing and body piercing can also be potential routes of infection.
After an incubation period of 40 - 120 days this infection is insidious in onset. It affects adults predominantly and again like Hepatitis B, virus is present in the blood for months to years. However, unlike Hepatitis B, chronicity of infection is the major complication of Hepatitis C infection. Almost 50% - 90% of individuals develop a chronic infection and it has been estimated that for every 100 of infected individuals, 85 develop long-term infection, 70 develop chronic liver disease, 15 develop cirrhosis over 20 - 30 years and five die. About 15-25 appears to resolve the infection. There is no vaccine currently available as the virus undergoes frequent mutations. However, unlike Hepatitis B, Hepatitis C infection progresses slowly, life expectancies are not reduced and with the treatment options available patients may recover completely.
Prevalence of Hepatitis C is higher in under developed countries being 1.5% in India and 14.5% in Egypt. 200 million people are infected worldwide and with no large-scale efforts to contain the spread and treat people, the death rate for Hepatitis C is all set to surpass that of AIDS by the start of the next century. In India, screening of donated blood for Hepatitis C had not been made mandatory until June 1st 2001 as policy makers felt it was too expensive to be implemented.
Diagnosis of Hepatitis C infection is by detection of antibody to Hepatitis C and confirmation by a recombinant immuno-blot assay (RIBA). Presence of antibody however does not indicate an acute infection and the only way to confirm this is by detection of the presence or absence of the HCV RNA in the blood. Earlier Anti HBc (total) and the liver enzyme ALT were used as surrogate markers but with the advent of specific screening tests, these are no longer required for this purpose.
In 1982, the HIV-I virus was discovered. In 1985 the first blood-screening test to detect HIV antibody (Ab) was licensed and screening was quickly implemented by blood banks. In 1986, the first HIV-1 infected patient in India was identified in Madras. In 1992, HIV-2 was identified and Ab screening for this was also introduced. In 1996, P24 Antigen screening was introduced. The rationale behind this was to reduce the window period before antibody appearance by a week. By the end of 1998, according to estimates of the joint UN programme on HIV/AIDS (UNAIDS) and the WHO, the number of people living with HIV had grown to 33.4 million, 10% more than the previous year. Eleven men, women and children all over the world were infected in each minute of 1998. In 1998 alone there were 2.8 million deaths from AIDS. Perhaps the most frightening is that one tenth of all newly infected people in 1998 were less than fifteen years old.
The virus is transmitted sexually, exposure to infected blood, from mother to child across the placenta or through breast milk. In India available statistics show us that infection is mainly spread through the heterosexual route.
The typical course of untreated HIV infection passes through the stages of primary infection, dissemination of virus to lymphoid organs, clinical latency, elevated viral expression, clinical disease and death. The duration between primary infection and progression to clinical disease is usually around ten years while death occurs about two years after onset of clinical symptoms.
Following primary infection there is viremia for about 8-12 weeks. Virus is widely disseminated to all organs and an acute mononucleosis like syndrome develops 3-6 weeks after primary infection. An immune response occurs any time between one week and three months, plasma viremia drops and levels of CD4 rebound. This immune response is unable to eliminate the virus completely and the virus gets firmly lodged in the lymph nodes. It is estimated that ten billion HIV particles are produced and an equal number are destroyed every day. Because of the rapid viral proliferation and the inherent error rate of HIV reverse transcriptase, it is believed that every possible nucleotide of the HIV genome mutates daily.6 The delicate balance between replication and destruction is lost and constitutional symptoms and apparent disease develops. HIV found in later stage disease is generally more virulent than that found earlier. There is also a shift in the cell series that are affected, from macrophage to lymphocyte accompanied by the onset of AIDS.
The screening methodology used is the ELISA. The commercially available microtitre kits have a sensitivity of 100% and are able to identify almost all the infections. There are several rapid testing devices available that have a sensitivity of 99%. However on an average they are able to identify an antibody response a week later than the microtitre ELISAs. This is particularly dangerous in a fully component blood bank where each unit of donated blood reaches three recipients. The reason for using anti HBc screening as a surrogate marker for HIV infection is that, the presence of anti HBc total in the absence of Hepatitis B and C could indicate the presence of HIV in the window period before seroconversion. After the introduction of immune-complex dissociated (ICD) P24 antigen screening, transfusion transmitted HIV is almost negligible. Nucleic acid testing and a stringent donor selection process can address the minimal risks that still remain.
Since the HIV mutates at a frightening pace, no vaccines are available as yet, though several candidate vaccines are on the cards. Treatment with anti viral agents and other systems of medicine have yielded tremendous relief to patients and have been able to successfully delay the onset of AIDS. Treatment appears to be lifelong, initially aggressive and later less so. As to whether treatment if initiated soon after diagnosis can eradicate the infection, time alone can tell. Prevention by health education again is possibly the most effective way of controlling this global epidemic.
This is a ubiquitous herpesvirus that is transmitted in the following ways: 1) transplacental 2) perinatal through infected secretions 3) blood transfusion or organ transplantation and 4) close personal contact with infected secretions.
Following an incubation period of 4 - 8 weeks in normal hosts, a mononucleosis type of infection develops after which a life long infection with periodic reactivation develops. Almost every organ in the body can be infected and the symptoms are more severe in the immuno-suppressed in whom the virus is reactivated. Almost 80% of adults in some populations are seropositive for CMV antibodies.
Superinfection in CMV infection occurs when a person with CMV infection of one serotype in the past receives blood or an organ infected with another serotype. This reactivates the original latent infection and causes disease in the recipient. Diagnosis is by detection of IgM and G antibodies to CMV. Screening of donated blood is not mandatory as most of the donor population is exposed to the infection at some time or other.
CMV negative blood is obtained by using 3 log white cell filters. These filters have been found to render CMV positive units negative by PCR8. Situations where CMV negative blood is required are the following:
1) Seronegative low birth weight infants (<1.5kg)
2) Seronegative recipients of seronegative organ transplants or bone marrow transplants.
3) Intrauterine transfusions in a seronegative woman.
4) Transfusion in a seronegative pregnant woman.
| ~ The treatment and control of CMV infections include the following:|| |
a) Antiviral agents-ganciclovir, acyclovir, vidarabine and interferon
b) Isolation of infected newborns
c) Screening of transplant donors and recipients for CMV antibody.
d) Use of seronegative or CMV negative blood by filtration for infants requiring multiple transfusions.
Vaccines of two kinds - “live” and purified polypeptide vaccines are in the midst of trials to determine their efficacy.
HTLV I & II
This was the first retrovirus to be identified. Found in Japan, Caribbean islands, Southern USA, S. America and Africa less than 1% of people worldwide have antibody to HTLV while 10% of people in endemic regions are seropositive. HTLV-I was present in the US donor population in the 1980s and screening was initiated in 1988. Because of the close similarity between the two viruses, the HTLV-I screening test detects most of HTLV-II infections.
| ~ Parasitic Infections|| |
Transfusion induced malaria has been reported after transfusion of red cells, white cells and platelets. The usual incubation period is about twelve days for P. Falciparum and P.Vivax and about one month for P.Malariae. Screening is by a routine thick and thin smear and is now being replaced by the more sensitive fluorescence method. Transfusion induced filariasis needs to be borne in mind particularly in endemic regions. Studies done in cats have shown that microfilariae were seen in the recipient, anywhere between 2-136 days following injection.
Babesiosis More Details caused by the parasite Babesia More Details microti is endemic in restricted areas of the North Eastern part of the US and is transmitted by ticks. Twelve cases of transfusion-transmitted Babesiosis More Details are on record in the US. Human Babesiosis More Details is more severe in the adult than in the young. The American Red Cross has permanently deferred those individuals with a history of Babesiosis More Details.
Trypanosomiasis (Chagas disease) was discovered 86 years ago by a Brazilian doctor called Carlos Chaga. Caused by T Cruzi, eighteen million people in South East and Central America are infected. Every year several thousands die of the disease. There have been four reported cases in literature. About 10% of infected individuals never exhibit symptoms. The American Red Cross defers donors with a history of Chagas.
Transfusion transmitted Leishmaniasis caused by L.donovani has been reported in literature. In 1991 on the assumption that L.Tropica could also be transmitted through transfusion, the Red Cross deferred potential blood donors who had been infected. However, as there has been no substantiating evidence for transmission in US blood donors the ban has since been lifted.
Toxoplasmosis is caused by the parasite Toxoplasma gondii. Approximately 50% of the US population may harbour the organism. Most infections are asymptomatic. Since 1970 there have been no reported cases of transfusion transmitted toxoplamosis
Syphilis is one of the earliest post transfusion hazards to be recognised. Screening for antibodies to Treponema pallidum, the causative organism, has been done for more than fifty years. Post transfusion syphilis has now become very rare possibly because of a combination of factors such as improved donor selection processes, uniform antibody screening of donors worldwide, and shift from transfusion of warm whole blood to refrigerated components. Refrigeration is a vital factor, as the T.pallidum does not survive for longer than 96 hours at 2-80 C. However for those units transfused within 96 hours, the risk of transmission still remains. Furthermore, platelets are stored between 20-24 degrees C and there is no information regarding the viability of T.Pallidum in these conditions. Until molecular techniques to detect the DNA of the organism in serologically positive units become available for use as a routine, antibody screening still remains the safest way of preventing the spread of this infection by transfusion.
Lyme Disease, caused by the spirochete Borrelia burgdorferi, is yet one more transfusion transmitted infection that is associated with the bite of the eastern deer tick. While no cases of transfusion transmitted infection have been reported in the US, patients are deferred and allowed to donate only after a cure.
Leptospirosis is a spirochetal infection caused by the pathogenic leptospire L. Interrogans. It is endemic in some parts of the world particularly the southern states of Tamilnadu and Kerala, Gujarat, Maharashtra and the Andaman Islands in India. The infection is spread through contact with urine of rats, dogs, cats and cattle. Recently transfusion transmitted Leptospirosis has been reported from the city of Madras.10 After an incubation period of two to twenty one days the infection presents in the form of an influenza like illness and is therefore most often not recognised or misdiagnosed. 80% of affected individuals clear the infection spontaneously. 10% have complications while in 10% the infection is fatal.
Leptospira have been isolated from frozen meat and remain viable for almost a week at 2-8 degrees C. Screening of donor blood by Dark Field Microscopy, and arbitary primed PCR when available together with temporary deferral would be an ideal policy particularly in endemic areas until DNA based techniques are available for routine use.
| ~ Bacterial|| |
Brucellosis More Details is a blood borne pathogen, but there have been no reported cases of transfusion-transmitted Brucellosis More Details recorded. The BaCon study deals extensively with the risk of bacterial contamination of blood products and the purpose of the study is to determine bacterial contamination rates of blood components associated with recipient transfusion reaction according to defined criteria for transfusion reaction, to identify pathogens associated with contamination, to identify risk factors for bacterial contamination, and to identify factors associated with transfusion recipient morbidity and mortality.
Signs and symptoms of transfusion-associated bacterial sepsis often include fever, rigors (i.e., shaking chills), tachycardia (i.e., rapid heart rate), change in systolic blood pressure (either a significant increase or decrease), nausea and /or vomiting, shortness of breath, and lower back pain. Case reports and investigation of numerous episodes by CDC have indicated that these are the signs and symptoms that most often occur in close temporal proximity to transfusion with a bacterially contaminated blood component.
Increased awareness of signs and symptoms of transfusion reaction, and of bacterial contamination as the possible cause of transfusion reaction, is obviously important for improved detection, and past studies have confirmed that increased awareness of the possibility of bacterial contamination by clinical personnel results in dramatic increases in the number of cases detected.
BaCon Study data collection has ended as of December 31, 2000. It is recommended in the USA that adverse transfusion reactions suspected to be due to bacterial contamination of blood or blood products should continue to be reported through standard operating procedures.
| ~ Surrogate Markers|| |
ALT level estimation was introduced as a mandatory test in 1986 -1987 in the US as a surrogate marker for Hepatitis C. An elevated ALT level indicates the presence of hepatitis. In HCV infection, the ALT levels rise three weeks prior to the presence of antibody. However, additional screening has not added to the benefit in any way. ALT has also been used for its ability in identifying unknown hepatotropic viruses and in India ALT estimation still has a role to play in the detection of hepatitis of any aetiology. Anti HBc(total) was also introduced as a surrogate marker for post transfusion Non A Non B Hepatitis in 1992. It has, to an extent, outlived its usefulness after the introduction of direct screening for Hepatitis C, but still has a role to play due to the overlapping epidemiology of Hepatitis B and HIV infections.
| ~ Emerging infections|| |
Cruetzfeldt-Jakob disease (CJD)
This is a fatal untreatable disease occurring in humans following transmission of bovine spongiform encephalopathy (BSE) through food. The majority of these cases are inherited and others have been attributed to the transfer of infectious material. This has followed administration of pituitary derived hormones, implantation of duramater and following corneal transplant. Pruisner postulated prions as the cause of spongiform encephalopathies. Prions are resistant to viral inactivation procedures and are capable of converting normal proteins into abnormal ones. In familial CJD there is an increased susceptibility to form these abnormal proteins. The possibility that new variant CJD (nvCJD) may represent the human form of BSE has led to enormous concern in the UK and acting on the suggestions of the Committee on Safety of Medicines (CSM) blood banks in the UK have already taken several precautionary measures. However, so far no conclusive proof exists that the nvCJD can be spread through blood.8,9 Until that time however, people residing in the United Kingdom for the previous ten years and those who have traveled to the United Kingdom in the preceding six months are debarred from donating blood.
Hepatitis G Virus.
Earlier referred to as Non A-E hepatitis, this virus is responsible for about 10-20% of community acquired hepatitis and transfusion associated hepatitis.13 It was until recently referred to as Hepatitis G/GB virus. This virus has about a 25% homology with HCV. Recent studies indicate that the virus seems to be capable of causing a mild acute hepatitis but not clinically significant chronic disease. The prevalence of HGV is more common in patients with Hepatitis B than with Hepatitis C or other hepatotropic viruses. Diagnosis requires the presence of HGV-RNA and anti envelope (E2) antibodies. Until simpler and less expensive testing strategies become available, the disadvantages of screening for this infection far outweigh the probable benefits.
The latest addition to the list is the Transfusion Transmitted Virus (TTV) , the Sen-V variants, and ParvoB19 viruses. Further assessment of the risks that they pose to a transfusion service is required.
| ~ Issues in blood banking|| |
Screening for infectious agents and either deferring or rejecting a prospective donor does not mark the end of the transfusionist's job. Donor counselling is equally important. The implications of a “positive” test must be discussed with the donor. Confirmatory tests and treatment options available may also be discussed.
While newer and more exotic infectious diseases continue to emerge, one needs to evaluate the disease in its proper perspective:
i) Is the microbe a real threat?
ii) How serious are the consequences of the infection?
iii) What are the financial implications of transmission of an infection and is prevention of transmission by proper screening a more cost-effective method of handling a situation?
iv) If it is endemic in an area and has the potential of being transmitted through blood, should not the panel of screening tests be constantly reviewed and screening for such infections implemented too?
Enzyme Linked Immuno Sorbent Assay (ELISA) currently the microtitre method is the recommended method of testing. However, the higher sensitivity and procedure related issues resulting in false positives must be borne in mind, which can cause untold anxiety both to the donor and transfusionist. This is certainly safer than having to deal with a false negative result and the unwary recipient, who has to face the consequences. A qualified microbiologist is essential in a blood bank, to handle these issues, to troubleshoot and to ensure the proper interpretation of the obtained result. Based on population studies, microbiologists must evolve their own cut off criteria that are far more stringent than those recommended for diagnostic use, in the best interest of the recipients.
Rapid Testing Devices (RTD)
Sensitivities of 99% are claimed for HBV and HIV but are not recommended for use in blood banks. For HIV, on an average, these RTDs detect HIV one week later than the routine ELISA assays. In addition, any positive result that is obtained has to be confirmed by a routine test.
Nucleic Acid Testing (NAT)
The false negative result and the problems associated with the window-period still need to be addressed and this is where nucleic acid testing plays a vital role. Nucleic acid testing is probably the only real answer we have for direct identification of the microbe. This has already been implemented routinely in the US since 1999.
Currently in the US, 90% of donor blood is screened for HIV and HCV RNA using the mini pool NAT methods. Currently, Polymerase Chain Reaction (PCR) and Transcription Mediated Amplification (TMA) are the two methodologies employed. Minipool testing (24 samples) for PCR and 16 or 24 for TMA are used.
Important issues relating to NAT testing are as follows :
a) Lack of automation
b) Pool size 16/24 - single unit testing may be ideal but not cost effective. Pool size will have to vary depending on the concentration of virus in a sample and mini pool may be efficient if the load is high.
c) Retesting - Lead to delay in release of units as re-testing of every single sample in a positive pool exists. This is particularly important if a long time donor or a donor with a valuable type.
d) Other viruses - Variant strain - 'HIV 'O' sub group, HIV 2, newer variants. Constant evolution of NAT testing is required. Constant surveillance is required to identify newer emerging viral infections that can be transmitted.
e) Cost effectiveness - Risk / benefit ratio has to be assessed and testing centres with this facility have to be set up. With several diagnostic companies developing automated system and assays for numerous viruses, the costs are bound to reduce and safety standards will improve.
| ~ A few suggestions towards improving the standards of transfusion medicine would be:|| |
i) Regional blood testing centres, which could undertake such specialised testing followed by distribution of screened components to the other peripheral, or satellite centres, for storage, compatibility testing and release. A pilot project has already been in operation since November 2000, between Jeevan Blood Bank and Research Centre and Sundaram Medical Foundation a 130-bedded community hospital based in Chennai. This has been a very successful exercise and will ensure standardisation of the quality of blood components.
ii) Strict enforcement of legislation in all blood centres together with uniform donor selection guidelines and screening methodology.
iii) An external quality assurance programme to monitor the screening techniques in blood banks must be introduced.
iv) Implementation of a follow up programme for the recipients of blood / components for a specified period of time. This will help to assess the impact of technologies used in screening blood.
v) Implementation of a “recipient notification” programme in which the recipient of blood / component coming from a donor who has subsequently sero-converted is informed. Storing donor samples for pre-fixed periods of time will help in retrospective diagnosis of newer emerging transfusion transmissible agents. This was exactly how the existence of the Hepatitis C virus came to be known and several more transfusion-transmitted infections will be identified this way in the future too.
Since 1665, when the first dog to dog transfusion took place, blood transfusion has indeed come a long way but can still never be a hundred percent safe. Donated blood is subjected to heat treatment, solvent detergents, leuco-depletion and irradiation in some centres, to improve the safety. All blood banks must strive to provide the safest blood / components available given the existing state of knowledge at any given point in time. With increasing awareness about the use of autologous blood and synthetic blood substitutes, the day is not far off when we will be able to handle the onslaught from the microbial world and the risk of transfusion transmitted infections reaches abysmally low levels.
| ~ References|| |
|1.||History of Transfusion Medicine-Fact sheet on blood and blood banking. Official site of the American Association of Blood Banks http://www.aabb.org |
|2.||Alter MJ. Public Health Service Inter Agency guidelines for screening donors of Blood, Plasma, Organs, Tissues and Semen for evidence of Hepatitis B and Hepatitis C. Miriam J Alter, Centre for Disease Control. |
|3.||3. Infectious Disease Testing for blood transfusion-NIH Consensus Development Conference Statement Jan 9-12; 1995 |
|4.||Division of Viral Hepatitis, CDC, Viral Hepatitis C - FAQ. National Centre for Infectious Disease http://www.cdc.org |
|5.||HIV/AIDS: The Global Epidemic (1998). UNAIDS Joint United Nations Programme on HIV/AIDS. http://www.unaids.org |
|6.||AIDS and Lentiviruses- Chapter 44, Jawetz, Melnick&Adelberg's Medical Microbiology 21st Edn 1998. |
|7.||BirchM P,Lee L L J, Sallen G A, et al. Time course of detection of viral and serologic markers preceding HIV type I serconversion-implications for screening of blood and blood donors. -Transfusion 1995;35:91-7 |
|8.||The Blue Book- Guidelines for the control of Infectious Diseases-CMV infection |
|9.||Transfusion Transmitted Diseases. http://www.biomed.redcross.org/home/virus.htm |
|10.||Nedunchelliyan S and Venugopalan AT - Blood Transfusion and Leptospirosis, Indian Veterinary Journal 1997; 74:790-791. |
|11.||Assessment of the frequency of blood component bacterial contamination associated with Transfusion Reaction (BaCon Study) - American Association of Blood Banks (AABB), American Red Cross (ARC), Centre for disease control and prevention (CDC), and Department of Defence (DOD). http://www.cdc.gov/ncidod/hip/bacon. |
|12.||Dealler S F. UK adult's risk from eating beef, Lancet, 1996, 347,195-196 |
|13.||Howard J, Worman MD. The incidence of transfusion associated Hepatitis G Virus infection and its relation to liver disease. New Eng J Medicine 1997; 336:747-754. |
|14.||Gaining informed consent for screening Editorial. BMJ 1999;319:722-723. |
|15.||Nucleic Acid Amplification Technology (NAT)- Testing Blood Donors. Vol1, No 4 Feb 1999. |
|16.||Gallarda JL, Dragon E, Molecular Diagnosis. Blood Screening by Nucleic Acid Amplification Technology: Current Issues, Future Challenges. Vol. 5 No. 1; 2000. |