If you’re unlucky enough to end up in hospital in need of a blood transfusion, you trust that your transfusion will help you recover. You certainly hope that it won’t infect you with a serious disease with long lasting effects. To prevent that from happening, blood undergoes a bunch of tests so doctors can make sure it’s safe to transfuse into patients.

How do we choose what to test for?

Blood centers need to choose carefully what they test for, taking into account the diseases that their donors may have, and what may be dangerous to the recipients.

Some of the things they consider are:

  • If the donor has a disease, can it be transmitted by blood transfusion?
  • Can the donor’s blood cause a serious illness in the recipient?
  • Is there a period of time where the donor may be infected with something but still feels healthy (called an “asymptomatic period”)?
  • Can the infection be detected by a reliable test that is approved by regulatory bodies (such as the FDA)?

There are some problem diseases that we can’t test for because suitable tests aren’t available. One example is vCJD, or “mad cow” disease, which scientists have had difficulties developing a test for, because it doesn’t have DNA or RNA that can be amplified and detected like most other infectious agents do. In cases like this, blood centers screen donors for known risk factors (for example, if they visited a region that was experiencing a disease outbreak) to keep the disease out of the blood supply. But for many diseases, scientists have developed effective tests for detecting them.

Testing through time

Today, it seems almost obvious that we should test blood for disease before we transfuse it into somebody, but this wasn’t always the case. Here are some examples of diseases that have shaped blood supply safety.


From the 1940s through the 1970s, U.S. blood donors were screened only for syphilis, a disease that was first documented in Europe 1494, and has been said by some to have been introduced to Europe by Christopher Columbus.

Wellcome collection, creative commons

Prosthetic noses like this one carved of ivory were used by people who had been disfigured by syphilis.

Wellcome collection blood screening blog

An illustration of the disfiguring effects of late-stage syphilis.

Syphilis is caused by a spiral shaped-bacterium known as Treponema pallidum, which has the dubious honor of catalyzing advances in cosmetic surgery in Europe in the late 16th century. During this time, surgeons attempted to fix the disfigured noses of syphilis victims using skin grafts from the patient’s arms. Other victims of syphilis used custom-made ivory or metal noses to cover the gaping holes in their faces left by late-stage disease. (Fortunately, the disease can now be cured using antibiotics before it gets this far.)

In the last 50 years, there has been only one case of someone catching syphilis from a blood transfusion in the US. This may be related to advances in testing, but is probably helped along by the fact that Treponema pallidum doesn’t like the cold refrigerators in which blood is stored today.



During the 1950’s, as many as one in three recipients of blood transfusions became infected with viral hepatitis. In the subsequent decades, though, advances in disease research and testing led to the discovery, and introduction of routine testing for, Hepatitis B and Hepatitis C. Disturbingly, some of the pivotal experiments that led to our understanding of hepatitis included studies on prisoners and institutionalized children that would be considered unethical today.

After scientists introduced a test for Hepatitis B in the 1970’s, the number of people who came down with hepatitis after a blood transfusion dropped dramatically to around one in ten.  Hepatitis C wasn’t isolated until 1989, but then scientists rapidly introduced a test for it in 1990. Today, the risk of contracting Hepatitis B or C from a blood transfusion in the USA is around 1 in a million.

HIV testing and the 90’s

Credit: David S. Goodsell, The Scripps Research Institute, Wellcome Images

Watercolor and ink on paper illustration of a cross section through an HIV virus particle (bottom right of image). Blood plasma surrounds the virus (top and left side of image). Width of image is approximately 100 nm.

The AIDs epidemic of the 1980’s presented an entirely new challenge to blood safety, requiring the coordinated efforts of public health systems and scientists to contain a new and devastating disease. By 1983, there was strong evidence that AIDs could be spread through blood transfusions and intimate sexual contact, particularly among gay men, but the cause of the disease was still unknown.

Research identified the HIV virus in 1984, and scientists introduced a test for antibodies to HIV in blood, which the FDA approved in March 1985.

The HIV tests of the 80’s couldn’t detect HIV until six to eight weeks after someone got infected. Since then, scientists have developed more accurate and sensitive tests that detect the genetic material of the virus, which make it possible to detect the disease after less than a week.

Zika: emerging threat

Zika virus is one of the latest threats to blood safety, yet if you were infected with Zika virus, you may not even realize it. Four out of five people who have been infected show no symptoms at all, and some only feel mild flu-like symptoms. Prior to 2007, only 14 cases had been reported globally. Nothing to worry about, right?

Signs of Zika’s more sinister side appeared in 2013, with a huge outbreak in French Polynesia. Carried to Brazil in 2014, Zika found perfect conditions to spread: a warm, damp environment ideal for mosquitoes that carry the disease, combined with large numbers of humans.

As the virus spread, so did the rate of a birth defect known as “microcephaly,” in which babies are born with abnormally small heads. By February 2016, Zika virus was declared a public health emergency of international concern by the World Health Organization.

Although we still don’t know all the effects of Zika virus, or all the ways it can be passed to people, health authorities across the globe have already introduced guidelines to limit the spread of Zika through blood transfusions.

In the US, blood centers have already started testing for Zika viral genetic material in donated blood. It wouldn’t be helpful to screen donors for antibodies to the virus, like we do for other viruses, because many people who have recovered from Zika virus still carry antibodies to the virus their whole lives, and screening out anyone with these antibodies would waste a lot of useful blood.

Today, all blood centers in the US screen blood donors for at least eight transfusion-transmitted diseases using a total of at least twelve different tests, shown in the Table below:

Disease Test Year introduced
Syphilis Antibodies to Treponema pallidum 1940s
Hepatitis B Antibodies to Hepatitis B virus surface antigen 1970s
Antibodies to hepatitis B virus core antigen 1987
Hepatitis B virus DNA 2009
Human Immunodeficiency virus


Test for antibodies to HIV 1, 2 1985
HIV DNA and RNA 1999
Hepatitis C virus (HCV) Antibodies to Hepatitis C 1990
Hepatitis C virus RNA 1999
Human T-cell Lymphotropic virus (HTLV)


Antibodies to HTLV-1/2


West Nile Virus (WNV) WNV RNA 2003
Chaga’s disease Antibodies to Trypanosoma cruzi 2007
Zika virus (ZIKV) ZIKV RNA 2016

Who decides how safe is safe enough?

Blood centers do not make the decisions on donor selection, testing and processing alone.

In the US, the FDA regulates the blood supply because blood and products made from it are used to treat disease. Because regulated treatments, like medicines, need to be made from “safe and pure” ingredients, this extends to ensuring that the donors who provide the “raw material” for blood products are free of infections, as well.

Blood centers all over the world make different decisions about how to ensure their blood supplies are big enough, yet safe enough for their circumstances. Different parts of the world face different dangers to the blood supply, yet at the center of all blood centers’ thinking is the well-being of the patients who receive their products.


  • Alison Gould

    Alison Gould (BSc (Hons) PhD MRACI CChem) is a Scientific Communications Specialist in the Research and Development Team at the Australian Red Cross Blood Service. You can follow her on Twitter @A2ali.

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