My girlfriend accidentally told the donation center she went to Mexico, and they banned her from donating for four years.

Apparently you'd only go to Mexico to eat brain tacos and share needles with cows. Surely there's a better way to filter out risky blood.

> Surely there's a better way to filter out risky blood

It's simple Bayesian probability. Blood tests have a relatively high error rate. Hep-B tests have a 6-12% false negative rate early in the disease, and Hep-C is 3-6% even later in the course of the disease. That's considered a "very low" false-negative rate for a blood test.

In Bayesian terms, blood tests don’t “screen” for a disease. They reduce the odds ratio of contaminated blood by a factor of 10 or so. But the ultimate odds still depends heavily on the prior odds—the prevalence of the disease in the donor population: https://en.wikipedia.org/wiki/Prior_probability. Even with testing, you can reduce the risk of contaminated blood by drawing blood from a pool of donors that has lower prevalence rate of diseases.

Yes... travel, tattoos, drug use and sexual behavior can and should disqualify a person from donating blood.

All of these things can mostly be tested. When I donated regularly in the UK after being in the southern US, they screen me for west nile virus but still take my blood and use it.

The UK also has a wait time for many countries including Mexico.

https://my.blood.co.uk/eligibility/travel/article?id=47&titl...

Granted it’s shorter, but there are longer wait periods depending on the country. It’s defense in depth because false negatives happen and some viruses take time to show up on tests.

"If you spent over 4 weeks in primitive or rural accommodation you have to wait for 4 months after your return before giving blood. However, if you were travelling and staying in places which are modern and clean, such as typical tourist areas, please wait 4 weeks from your return due to Tropical Virus risk which includes Chik V, Dengue or Zika risk providing you do not have symptoms of these."

Not quite the four years.

The first paragraph is

“ If you visited the rural areas of the states - Chiapas, Chihuahua, Durango, Sinaloa districts, Quintana Roo, you need to wait for 4 months after your return to the UK before giving blood. ”

The US rules also aren’t 4 years. It’s 3 years after undergoing malaria treatment. And 3 months after visiting a malaria prone area.

https://www.redcrossblood.org/faq.html#eligibility-travel

Blood is tested for disease, but the false negative rate for each test is its own risk.

If you got blood from an addict living on the street engaging in prostitution and tested it, would you trust that blood?

I wouldn't.

The bigger worry isn't really false negatives, modern PCR based testing is incredibly good. There is always risk but it's frankly extremely low. The bigger issue is that blood is pooled before testing to make that powerful testing economical. If you increase the rate at which you get (also possibly false) positives you risk having to throw out whole batches of pooled donations.

The false negative rate for HIV testing in newly infected people is actually pretty high, like 50% within the first few months high.

No, it's not though. That's true for antibody tests commonly used for individuals because it's cheap. The window period for nucleic acid testing that is used for blood supply is very short. This test is similar to what's used to confirm a positive antibody test, but isn't used for individual testing because it is too expensive. Hence the pooling.

The UK had mad cow eons ago, and that's still a risk factor for giving blood in some places.

With travel, I understand that there is a higher risk of lots of diseases, and testing against all possible infectious diseases is not feasible. Drug use is also obviously disqualifying. But why would you care about someone's sexual behavior? The blood must be tested for common drugs and common blood borne diseases regardless, and it's perfectly possible to engage in sexually risky behaviors and not have any venereal disease (unlike with drug use, where it implicitly means you will have levels of those drugs in your blood), just like it's possible to be very careful with your sexual behavior and still get a disease.

Note: for tattoos, I have no idea if the problem is also related to venereal diseases, or if there is any problem from contamination with the tattoo ink itself, and I don't care enough about this subject to look it up.

> But why would you care about someone's sexual behavior? The blood must be tested for common drugs and common blood borne diseases regardless, and it's perfectly possible to engage in sexually risky behaviors and not have any venereal disease

Men who have sex with men are something like 50-100x more likely than the general population to acquire HIV. HIV tests do not have a 0% false positive. They will not catch all very recent infections. The rationale for excluding them until recently is that it’s defense in depth and it doesn’t hurt the blood supply much because they only make up about 2-3% of the population.

The current rule is that MSM don’t face a blanket ban, but if you’ve had anal sex in the last 3 months you have to wait because anal sex is inherently more likely to transmit HIV and the tests may not catch a very new infection. Other diseases like Hepatitis have a similar issue.

The answer to all of that is mainly hepatitis C, that can have a window detection of 6 months, even more.

And yes, you can be very careful and get a disease. But they are playing statistics here: over 60% of injected drug users have Hep-C, that means a lot of prostitutes. They won't and shouldn't trust anyone who say "hey, I had unsafe sex against all advice, but was very careful with the tattoo in a dark cellar and the heroin party, pinky promise".

Unfortunately tests are not 100% accurate and there's a window between when the pathogen is present and when it's detectible. Add in that many viruses aren't directly detectable, the tests look for antibodies to the virus.

This is why they usually ask if you've had a new sexual partner in the past 3 or 4 months. This is the window period for detecting some STDs and other diseases.

For tattoos, if the artist isn't using a brand new set of needles for you, you risk bloodborne disease transmission, with hepatitis B being a particular danger.

Well, it's the having of an infectious blood borne thing that disqualifies you.

m-m sex is still disqualifying even if all parties are completely clean and safe. it's discriminatory.

All anal sex in the US requires a 3 month waiting period (as does any kind of sex with a new partner) not just male to male.

Anal sex is inherently much more likely to transit HIV and HIV tests have a higher false negative rate for new infections.

MSM is one of the highest risk activities you can do for bloodborne diseases. If this is discriminatory, life is discriminatory. Life is not fair, and there is nothing any artificially constructed social system will ever do to force mother nature's hands to make life fair. I fully support the rights of people to love and be in relationships with whoever they wish, but that doesn't mean you have the right to contaminate the blood supply due to the inherently high risk of your day-to-day life.

I have never been able to donate blood because of my travels due to their locations and frequencies. I am not going to give up travel to donate blood. MSM is a much higher risk activity than travel, and yet I am also excluded due to my lifestyle risk. It's not fair, but it is reasonable.

0.4% of the US population has HIV.

12-15% of gay men in the US have HIV.

The rules exist for a very real reason.

People have more unprotected, regrettable sex during travel and vacations, so maybe they're on to something?

Sounds like something you should test rather that just rely on heuristics

Tests cost money. They have blood enough to allow them to discriminate. If they were lacking, they will be less picky: in a catastrophic event they call for blood donors and they rely on tests, risking a detection window.

But under normal conditions, letting only the best candidates donate is the most efficient way.

Heuristics work, why would you not rely on heuristics?

Generally, because they're discriminatory. It's like how racial profiling in law enforcement can be a heuristic that can work. Though, in the case of blood donations, it seems that tests aren't accurate enough to be able to rely solely on them. Also, being discriminated against on being able to donate blood is not as big of a deal. Though in some places with public health systems, stuff like getting a surgery may somewhat depend on you and family/friends donating a certain amount of blood.

Reality is discriminatory. Discriminating between blood containing disease vs blood that doesn't is, quite literally, discriminatory.

Sounds like something you should evaluate with a cost/benefit analysis, including the false negative and false positive rates.

should test? As in test the blood? Or test the hypothesis?

if the latter I'm not sure it's enough of a stretch to say unprotected, hi-risk, promiscuity is a risk factor for blood borne illness like HIV? (Such that we need to research it separately)

I don't get the sense we have any standards for actually vetting the blood that's donated, which is deeply concerning

We do test the blood, but they also do coarse grained screenings like this to avoid some level of waste on intake.

We do test the blood, but they also do coarse grained screenings like this to avoid some level of waste on intake. It's like having client and server side validation.

We do, they are just not cheap enough to do on individual donations so you have to throw away a big batch every time they catch something.

I was banned roughly the same time for being in the US. I guess its mostly so they don't need to check for unexpected things.

I get it, just seems like it could be more granular, especially since she could have just said no.

> eat brain tacos

What's wrong with that? Animal brains are a common dish in many countries, including France, Asia, and parts of the United States

Vector for prion disease.

"Accidentally"? Implying she should've lied about it even though they probably have some reason (evaluating risk presumably) for asking?

Let's say a year ago, you walked across a bridge in Mexico for five minutes to see if it's true that Mexico has a yellowish haze like in the movies. Oh, it doesn't. Then you walked back to Texas.

A year later, you go to a blood donation center and they ask you: "Did you go to Mexico in the last N years?"

If you say "Yes", you are banned for four years. If you say "No", you donate liters of blood over the next four years.

If you were in this exact situation, how would you weight which answer is better?

I understand where you're coming from. But on the off chance something unrelated were to go wrong with the blood down the line, it could be shown that a false answer was knowingly given. You have to weigh this risk against keeping "the privilege to give blood". I guess it depends on your values and risk tolerance.

Sounds like a wild party.

There are other ways she can donate her blood.

I used to donate blood regularly but now that I'm in Japan they require me to be decently fluent in Japanese to "understand" the risks, despite having done it a bunch of times in other countries (and other medical procedures not requiring Japanese knowledge).

Bloodletting making a comeback? And having actual benefits this time?

There's kinda a significant difference between bloodletting and blood donation.

For starters, you're not supposed to donate blood when you're sick.

The other being the quantity. A donation is 1-2 pints. Wikipedia lists bloodletting as easily 3 pints [1].

[1]: https://en.wikipedia.org/wiki/Bloodletting#Use_in_the_1600s_...

Although PFAS is helping bloodletting make a comeback https://www.theguardian.com/uk-news/2025/jan/16/bloodletting...

I really love the pedantry, thanks for the laugh

So get routine bloodwork done?

me to the plague doctors: we are SO back

It never went away; now they call it "therapeutic phlebotomy"

Medical leeches are actually a real thing.

I wonder what it would cost in the US to have a pint of blood taken - I can't donate. Guess I could do it myself...

I'm not sure there are any regulations around opting to do that in the US. Do you have a phlebotomist friend? If so, they might do it for you, but it can be risky and they might not want to take the risk, get sued, etc.

It is an interesting question. Are there companies that draw and discard?

> we pass the PFAS to others

Is there no way to filter them out of withdrawn blood?

That's correct.

Aren’t we just donating the PFAS to potentially sicker patients?

I'd assume donated blood matches the average level that people already have in them, so not sure it really matters. But if you donated regularly enough, you could be donating blood that has lower than average levels!

PFAS doesn't immediately harm you like missing a few pints of blood does.

And it's not like you're concentrating higher levels of PFAS into the recipient, they likely have the same average blood concentration levels as the donor does since we're all equally exposed to the same sources.

Do blook banks have a way of filtering out PFAS? Or are we giving each other forever chemicals through blood donations?

A life saving blood transfusion or avoid forever chemicals likely already in my body, hmmm what to choose...

But does it have to be one or the other? Or is there some possibility of somehow removing the PFAS from donated blood?

What makes PFAS so difficult to deal with is the fact that they are particularly inert. Teflon works because it doesn't react with almost anything. I've seen some studies about using UV light to kill off PFAS, but that's not going to be desirable when you want those red blood cells to also survive.

Not without filtering other things we need.

I think people getting blood usually have lost a bunch of their own first. And blood isn't usually given just for fun, it's to save their lives, i.e. PFAS isn't a huge concern at that point.

Reason #136 for why tech-bros need a blood boy to infuse from daily.