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Monday, February 19, 2007

Gay men, blood donation and the limits of science.

I gave blood recently, and the barrage of questions asked of all donors reminded me that I'd been meaning to write this article since June of last year. At that time, Art Caplan wrote that it's time to let gay men donate blood, but he did not present much solid evidence for that position. As it happens, we can be pretty sure he is in command of considerable evidence, having served as Chair of the HHS Advisory Committee on Blood Safety and Availability for four years, but the article was presumably written as an exercise in persuasion and contained just enough information to make me curious.

In trying to decide for myself whether Caplan is right, I found myself in something approaching the position in which I imagine legislators must regularly find themselves: I was trying to find answers to policy questions ("should gay men be accepted as blood donors?"), without being able to rely on a personal background in the relevant science. Policy decisions have to be made, but science will rarely give you a hard-and-fast answer even to those questions on which it has something to offer -- and it becomes important to identify which questions those are, and which are not scientific questions at all.

The FDA bans blood donation by any man who has had sex with another man, even once since 1977 (the probable date of the first clinical AIDS case in the US), and by anyone who has been paid for sex during the same time period; donors are also deferred who have had sex in the last 12 months with anyone who qualifies for the since-1977 ban. This policy was most recently formally revised in 2000, at which time the decision was made to stick with the 1998 recommendation (the even-once-since-1977 ban), which in turn was based on policy implemented in the late 1980s at the height of the "AIDS scare".

One way of looking at the question is this: what change in risk could we expect to see if we changed the policy from a lifetime ban for men who have sex with men ("MSM") to the same 12-month deferral that applies to women who have had sex with such men, or men who have had sex with prostitutes (both high-risk behaviours)? Here's where it starts to get complicated, because the risks are already so low that there is essentially no direct way to measure them, and current estimates of risk are derived from sophisticated mathematical models. It's tempting to do something like this:

1. MSM = approximately 2-3% of the general population

2. Of persons living with HIV/AIDS, approx 40-50% are MSM

3. expected increase in donations from the policy change = approx 1%

4. relative risk increase = (45/2.5) x 1 = 18%

Unfortunately, though, (1)-(3) are fairly well accepted estimates taken from the references listed below, but (4) is something I just made up, and it's nonsense. Intuitively appealing, maybe, but nonsense. Inter alia, what makes it nonsense is the relationship between transfusion risk and screening methods. The primary issue here is what's known as the "window period", the time that can elapse between infection and detectable levels of virus. The FDA says:

Studies have shown that up to 2 months may elapse between the time of infection and the time the HIV antibody test is reactive. This period of time is often referred to as the "window period." Accepting men who have had sex with other men since 1977 as blood donors increases the likelihood for the collection of HIV-positive window period blood, because epidemiologic studies have documented higher incidence and prevalence rates in these populations. On March 14, 1996, FDA recommended donor screening with a licensed test for HIV-1 antigen, which has succeeded in further reducing the window period.

In fact, blood collection agencies now test every donation for HIV and Hepatitic C virus (HCV) by nucleic acid testing (NAT). This is an exquisitely sensitive test for viral genomic material; it is more sensitive than the antibody-based test for viral antigen, and unlike the antibody test, does not rely on the host response and is not subject to the resulting delays. According to the Red Cross:

Since 1987, the [HIV] window period has been reduced from 42 days to approximately 12-16 days following the implementation of the HIV antigen test in March 1996.

A variety of expert presentations at a March, 2006 FDA workshop on behaviour based donor deferrals indicated that, with the advent of NAT, the window period for HIV infection is less than 12 days. In the US, the residual risk of transmission of HIV or HCV by blood transfusion is estimated, by a variety of models, to be around 1 in 2,000,000 donations. This is clearly a very conservative estimate, since there are around 15 million donations every year and I could only find mention of four authenticated transfusion-related transmissions of HIV since NAT was implemented in 1999 (none of which involved MSM). At the same FDA workshop, Celso Bianco re-ran an earlier prediction using risk and other estimates that were getting general agreement at the workshop and came up with a figure, which he called conservative, of one infected unit per 32 years.

So, while it seems intuitively likely that including a high-risk group (as judged by increased prevalence) in the donor pool would increase overall risk, calculating -- or rather, estimating -- that increase is far from straightforward. The only numbers I could find were presented by Andrew Dayton to the same FDA workshop:

The 5-year [deferral, instead of a lifetime ban] would result in possibly a 25 percent increase in the current residual risk, and the 1-year would be 40 percent.

So, worst case scenario: 1.4 transmissions per 2 million donations, instead of 1.0 -- or about three extra cases per year (and remember that, to date, the observed level of transmission is much lower than the estimate). I'm not familiar with what sorts of risks are considered acceptable in public policy formation, but I can say outright that I would be prepared to accept that risk to my own person as the cost of allowing MSM to participate on a more equal footing in a profound act of community altruism. (To say nothing of a 1% increase in a critical health resource that is often in short supply.)

Furthermore, given that the window period is less than two weeks and you can only donate every eight weeks, there is an obvious method for reducing the risk even further. According to the AABB, red blood cells can be stored cold for 42 days or frozen for ten years, and plasmaand cryoprecipitated antihemophilic factor can be frozen for at least a year; of the fractions into which whole blood is routinely divided, only platelets have a shorter effective storage life, about five days. It is clearly possible to hold (at least most of the fractions of) any first-time donation until the donor returns and can be re-tested; two clear tests eight weeks apart are definitive proof of HIV-negative status.

This brings us, though, to the reason I said "more equal footing", not "equal footing", above. While MSM (as defined by the current even-once-since-1977 FDA exclusion) includes a great many men who have not had sex with another man for more than eight weeks, it also includes gay men for whom a 12-month deferral, or even an eight-week deferral, represents a discriminatory barrier. Somewhat ironically, when the Public Health Service issued its first formal recommendations in 1983, they offered a partial solution: sexually active homosexual and bisexual men with multiple partners should refrain from blood donation. While this leaves room for improvement in behaviour based deferral, since sexually active heterosexuals with multiple partners are also at increased risk, it would at least allow gay men in stable monogamous relationships to enter the blood donor pool.

Did you notice that we are no longer debating the science? Now we are talking about public policy, social justice and what sorts of risks we are willing to endure.

In that light, here's another way to look at the question: according to the CDC, African Americans make up approximately 13% of the US population but accounted for 49% of new HIV/AIDS diagnoses in 2005 and 61% of people under the age of 25 whose diagnosis of HIV/AIDS was made during 2001–2004; over the same period, black women were more than twenty times as likely to be diagnosed with HIV/AIDS as white women. I think it goes without saying that we are not going to ban African Americans from donating blood on the basis of their ethnicity.

The FDA "believes that there is scientific justification for screening out all potential donors who are men who have had sex, even once, with another man since 1977". After all that, I agree with Art Caplan: the FDA is wrong.

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Some references:

FDA Workshop on Behavior-Based Donor Deferrals in the NAT Era

Epidemiology of HIV/AIDS --- United States, 1981--2005

Twenty-Five Years of HIV/AIDS --- United States, 1981--2006

CDC Basic Statistics on HIV

CDC>HIV/AIDS>Morbidity and Mortality Weekly Reports from 2006

CDC>HIV/AIDS>Fact Sheets

AABB Blood FAQ

AABB: Whole Blood and Blood Components

ARC: 50 quick facts about blood

ARC: Blood information

P.S. World Blood Donor Day is June 14, and you can find your nearest blood donation center here or here.

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This work is licensed under a Creative Commons Attribution 3.0 License.



Posted by Bill Hooker at 06:28 AM | Permalink