Read This Before You Take the Test

A few principles of screening:

• Finding hidden disease can be a huge advantage—but not always. Screenings are useful when early detection—finding something before symptoms appear—leads to effective treatments that in turn lead to better outcomes like a longer life or less illness and suffering. With cervical cancer, early detection makes all the difference. By contrast, doctors could detect lung cancer earlier by screening apparently healthy patients, but that has not been shown to help people survive the disease.
  
  

• Harms, like benefits, often come "downstream." With some screenings, like colonoscopy, there's a very small risk of complications from the test itself; more often the harms associated with screening come once the results are in, in the form of stress from false positives or ambiguous results, further testing and, sometimes, unnecessary treatments that might not have been administered without screening. "I don't think people should stop with the question, 'Well, what's the harm in getting a blood test?' " says Michael L. LeFevre, M.D., a family medicine specialist at University of Missouri Health Care and a member of the prevention task force. "Because the answer is there really is no harm. It isn't just the screening test we need to think about, it's what happens next."
  
  

• Age makes a difference. "As they get older, people need to look critically at any proposal to do something about their health, particularly if it's something where the benefits play out over a number of years," says Harold C. Sox, M.D., a professor emeritus at Dartmouth Medical School and former editor of the Annals of Internal Medicine. "Something else is likely to get them before they're going to enjoy much of that long-term benefit." While doctors, partly for their own protection, might be focused on "big-impact items" like cancer, he says, issues like vision, hearing, balance and loss of muscle mass might be just as important to assess in older people. On the other hand, says Sox, it's life expectancy and not chronological age that matters. He once referred a hearty 92-year-old patient for a mammogram, a screening whose benefits take at least a few years to accrue. "I expected her to live to a hundred," he says.
  
  

• Your values count. For some tests, the balance of benefits and harms weighs clearly in favor of getting screened. For others, it's a judgment call in which people's choices will diverge. In discussing prostate cancer screening with his patients, LeFevre susses out their preferences by describing two kinds of patients and asking them which they most resemble. "One says, 'Look, doc, I feel well. Unless you're sure you can do me more good than harm, you should leave me alone,' " says LeFevre. "The other says, 'I worry about cancer. I'm afraid of cancer. If I have a cancer I would like to find it and do something about it, and I understand that there are risks.' "
  
  

Katharine Greider lives in New York and writes about health and medicine.