Why is it possible to get a disease from a blood transfusion?

Bloodborne infections
Blood banks screen donors for risk factors and test donated blood to reduce the risk of transfusion-related infections, but they occasionally still occur. It can take weeks or months after a blood transfusion to determine that you’ve been infected with a virus, bacterium or parasite.

Estimated risks of contracting these diseases after you receive a blood transfusion:

HIV — 1 in 2.3 million transfusions.
Human T-lymphotropic virus — 1 of every 2 million transfusions.
Hepatitis B — 1 in 350,000 transfusions.
Hepatitis C affects — 1 in 2 million transfusions.
West Nile virus — 1 of every 350,000 units of blood in the United States.
Sepsis (bacterial infection of the blood) — 1 in 1 million transfusions.
http://www.mayoclinic.com/health/blood-transfusion/MY01054/DSECTION=risks

They DO test every donation. It’s just that sometimes there are false negatives (blood that is diseased even though the test said it wasn’t). This is very rare, and I think they test it multiple times to make it even less likely.

The risk is extremely low. Every unit of blood is tested. @KNOWITALL has the data correct. You’re far more risk of getting a transfusion reaction than you are contracting a disease.

That said…we can only test for known diseases. Before Hepatitis C was known about, it was widely transmitted in blood transfusions. Now that we know about it, you can’t catch it. But it’s certainly possible to contract a disease that we do not yet know about.

Anytime that you have humans doing something, there is a possibility for mistakes to happen. A tired overworked blood bank technologist can make a mistake in testing donor blood for pathogenic microbes (viruses, bacteria, etc). Also the microbial level at the time of testing may not reach the labs detectable level. Also the blood bank does not test for every type of human pathogen or possible pathogen.

Off the top of your head, do you think there’s more risk selling your plasma or donating blood?

I have a few friends who sell their plasma like three times a week here, it’s a college town, just curious.

@KNOWITALL There should be no risk at all in donating plasma or blood. Just receiving it.

@Rarebear is right, blood and plasma donations are free of infection risk if they follow normal procedures.

I think all the donation centers affiliated with the Red Cross use a whole new (sterile) kit of needle, tubes, bags for each donation. And they swab the puncture are thoroughly with disinfecting solution before they put the needle in. They certainly do every time I have donated. (anyone else a ‘Gallon Club’ card holder?)

And good point too that they can’t test for diseases we don’t know about. My mom found out she had hepatitis-C in the early ‘80s and it probably came from transfusions needed during one of three C-section births in the 50’s.

Under what circumstances would a test yield a false negative?

@Bruan: Under what circumstances would a test yield a false negative? For instance if exposure is recent so the donor hasn’t yet had time to mount an immune response with production of antibodies, even though virus is present.

This article also admits a certain low percentage of actual human errors in the lab.

The safety of the blood supply improved in the late 1980s with hep C screening. Before that the virus was mysteriously known as “non-A, non-B.”

Are there any preferred blood suppliers who have stringent records and can show this never happened with their blood supply especially in the instance of known disease detection prior to transfusion?

@Brian1946 no test is 100%. It’ll take a lesson in Bayesian statistics to explain it properly, but basically every test, no matter what, has a rate of failure, both false positive and false negative.