Sunday Public Service Announcement

Okay, it’s not quite a PSA; it’s more of a PSA and a discussion about our blood supply and the mis-use of statistics.

One of the things going around Facebook is a picture of one of those bags that donated blood goes into, and a thing about how a gay man was turned away from donating (and saving up to three people’s lives) because his blood is ‘too gay’.

Let’s be clear on the policy of which gays are not allowed to donate blood.  Lesbians are allowed to donate blood, as are celibate gay men.  Bisexual men who have not recently had sex with men are not prohibited from donating blood.  The questionnaire doesn’t ask men if they like show tunes, flower arrangements, or Lady GaGa.  Straight men must answer the same questions and are subject to the same standards. Orientation is irrelevant.  The only people who are not allowed to donate (under the relevant provision) are men who have had sex with men within the last year.

According to the Centers for Disease Control, men who have had sex with men (MSM) account for a small percentage of the population (approximately 2-4%), but account for approximately 60% of new HIV cases.  (Source.)   All blood is tested for HIV, or rather, the antibodies that are produced in response to HIV, but ‘false negatives’ are possible, i.e. HIV-positive blood may be incorrectly classified as not having the virus.  The false-negative rate is approximately three in one thousand, i.e. out of any thousand people who have HIV and are given a test for it, three will show a negative test result. (Source.) (The test may also result in false positives, but those aren’t at issue here, because that blood isn’t going into a patient.)  False negatives are most likely in the three weeks immediately after infection.

The government’s rationale for excluding MSM from the blood-donor population is two-fold.  It is expensive, time-consuming, and not without risk to the nurses and lab techs, to take and test blood that cannot be used, so it’s easier sand safer to screen out high-risk people than to take their blood, test it, and then go through the procedures of disposing of it properly.

The bigger issue is not having dozens of Ryan Whites every year.  In the desire to balance getting enough blood donors with reducing risk, it makes sense to eliminate a group that is 3% of the population but 60% of the new HIV cases (and, as per above, new HIV cases are the ones most likely to give a false negative). Forget talk about the 80-20 rule – this is the 97-40 rule.

Now, I’m sure anyone who has read this far says, “But bridget, didn’t you know that senior citizens are the fastest-growing group of people with new HIV infections? that straight women are getting HIV more than ever before?”  For you, dear reader, a thought experiment:

Two groups each contain 100 people.  The prevalence of a highly contagious, sexually-transmitted deadly disease is measured among the groups, last year and this year.  The rates of infection are as follows:

2012: Group A, 0; Group B, 75.

2013: Group A, 1; Group B, 100.

Two questions:

1. Which group has the highest percent increase in infection?

2. If you had to pick one person from either Group A or Group B at random and have intercourse with them, which group would you rather choose from?

Answers: (1) – Group A has an infinite rate of increase, swamping out the 33% rate of change in Group B.  (2) I sure as hell hope you would say “Group A.”

Statistics/public policy alert: Unless you are talking about groups with similar levels of risk, “greatest increase in X” usually means “safest group”.  Rate of change = (final – initial)/initial; a smaller “initial” mean a smaller denominator, which means a higher rate of change.   I used an extreme example to illustrate my point, but going from 2% to 3% is a greater percent increase than going from 20% to 28%, although the latter exhibits a higher prevalence/rate/risk.

Back to AIDS and ethics and public policy:

I don’t think it helps young, gay men to not know that they are approximately forty-four times as likely as their straight peers to get HIV from sleeping with other gay men. Groups like Fenway Health advertise this fact because their primary concern is with ensuring that people don’t die of a preventable disease.  It does not help young gay men to look at their sexually active, healthy-as-horses friends and think, “They aren’t getting AIDS, so I don’t have to worry much, either.”

Yes, it is good for the geriatric crowd to know that they are at risk of various diseases and that they still need to figure out how to protect themselves.  But we can’t pretend that Mabel and Edmund have the same risk of getting HIV as do young gay men – which is of only marginal benefit to Mabel and Edmund, and is detrimental to young gay men.

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Filed under Bioethics, Miscellanea

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