It's a very poor choice to write long blog posts and then to post them against poison-green background. Few people will read them and then they will sue me for the headache. That's the nice thing about a blog of my own: I can agree to all this and just tractor on.
This is the second post on matters medical. The first one discussed the confusion between correlation and causality. This one will talk about screening for disease. Its hook is this piece of news:
Federal health officials Thursday recommended regular, routine testing for the AIDS virus for all Americans ages 13 to 64, saying an HIV test should be as common as a cholesterol check.
The U.S. Centers for Disease Control and Prevention guidelines are aimed at preventing the further spread of the disease and getting needed care for an estimated 250,000 Americans who don't yet know they have it.
"We simply must improve early diagnosis," said CDC Director Dr. Julie Gerberding.
Nearly half of new HIV infections are discovered when doctors are trying to diagnose a patient who has already grown sick with an HIV-related illness, CDC officials said.
"By identifying people earlier through a screening program, we'll allow them to access life-extending therapy, and also through prevention services, learn how to avoid transmitting HIV infection to others," said Dr. Timothy Mastro, acting director of the CDC's division of HIV/AIDS prevention.
Although some groups raised concerns, the announcement was mostly embraced by health policy experts, doctors and patient advocates.
Screening is the odd guy out in medical care, because it's not really prevention and it's not really treatment. It is usually justified as allowing diseases to be spotted earlier. The hope is that earlier diagnosis makes for better cure or at least longer survival rates. In many cases we know this to be true, in some cases we just hope this to be true.
The benefits of screening can be found in what I stated in the prior paragraph, with one addition: When the disease we screen for is an infectious one, such as is the case with AIDS, the benefits of screening may also include the ability to reduce the spread of the disease by making an infected person aware of the infection, assuming that this awareness changes behaviors (such as engaging in unprotected sex or donating blood) which are dangerous for others.
Any other benefits to screening? It can help in producing useful information for medical research and the relief people get when they are told that they don't have a particular disease also might count as a benefit. All this is very good, and most likely the reason why Americans are fervent supporters of various types of mass screening programs.
Now let's dive into criticisms of screening. You knew I would go there, I always do.
Medical programs don't have just benefits, they also have costs. In the widest sense these costs include not only the financial costs of the program but also the nonfinancial negative or harmful consequences of the program. As an example of the latter, think about someone who gets told that she might be HIV-positive, based on one these tests, even though she doesn't actually have the infection (a false positive finding). She will then undergo further tests, both costing money and a lot of mental suffering. It is this suffering which also counts as costs of the program.
Or think about someone who is told that she doesn't have the infection even though she does (a false negative finding). She'd be relieved and perhaps less likely to notice subtle symptoms or to take proper precautions in stopping the spread of the disease. The consequences of this are also costs of the program.
And so are the costs accruing to all those tested. We often look at the costs of a medical program in a narrow sense, by counting the expenditures of the institutions running it, but we ignore the costs to the patients participating in the program.
In an ideal world we would be able to count all the benefits and all the costs of a program in the same units, say, money, and we would then be able to look at the impact of the program on various groups of people from the fairness and human rights angle. This would allow us to make pretty good recommendations about which program to finance or to require someone else to finance.
But we don't live in that ideal world. In reality many choices take place almost totally on political grounds and emotional arguments tend to trump most other arguments. The reason why this is not such a great idea is simple: money and other resources spent in one type of medical program (for instance, mass screening) will not be available for some other type of medical program (for instance, more intensive screening of people at high risk and more help for them to cover the costs of treatment when needed).
Suppose, for the sake of an experiment, that you are told the existence of one person in the United States who has a fatal illness, but one which can be cured if we only could find this one person. Suppose that the only way to find the individual is to screen every single one of us at the cost of ten dollars per person (plus the costs of time and travel and so on for each subject tested). Should we pay for this program? What if there are two people with this horrible illness out there? A thousand? Ten thousand? What if it costs a hundred dollars per person to do the screening?
To continue the experiment, suppose that researchers have narrowed down the type of person most likely to have this fatal illness. They believe that there is a 0.9 probability that the person is under five feet tall. This means that the probability of finding the person in the group of people over that height is only 0.1. Should we still screen all people to find the one case?
This thought experiment can be enhanced in many ways. We could introduce a second disease to the story and we could ask which of the two diseases we should screen for. Or we could expand the setup in a slightly different way: Consider a disease such as breast cancer. That you don't have it today doesn't mean you won't have it tomorrow. If getting a mammograph once a year is good, why not suggest one every six months? Every three months? Every day?
I may have overhammered my nail home, but one more time may not hurt: Screening has both benefits and costs, and the costs matter, because money and effort spent one way is not available to be spent in other perhaps equally good ways.