Criteria For Screening

Photo: zilupe

I had a patient once who wanted an exercise stress test even though he had neither symptoms nor risk factors to suggest the presence of coronary artery disease (such as chest pain with exertion).  I argued vociferously against it.  However, extenuating circumstances (not relevant here) prompted our mutual decision to go ahead with it anyway.  To our surprise and dismay, it came back positive.

WHEN TO SCREEN

Given the amazing advances in medical technology in the last four decades or so, the American public has come to expect and believe that if a test exists for a disease, it should always be done.  Nothing, however, could be further from the truth.  The decision about which tests to perform on which types of patients and when to perform them actually requires a complex calculation.

First, we must distinguish between a screening test and a diagnostic test.  Often a single test can be used as either, the difference arising from the circumstances under which it’s performed.  So, in my patient who had no symptoms or risk factors to suggest the presence of coronary artery disease, putting him on an exercise treadmill by definition represented a screening test:  that is, an attempt to identify the presence of a disease before it produced symptoms.  Were he to have complained of chest pressure while climbing stairs, in contrast, putting him on a treadmill would have represented a diagnostic test:  that is, an attempt to confirm or exclude a disease that had already manifested symptoms.

Why don’t we screen everyone for every disease we can?  First:

  1. The disease must represent an important health problem.  We could, for example, screen everyone for mononucleosis.  But given that 90% of the population has already been infected without ever knowing it by the time they leave adolescence (and having had it are now immune to having it again), no reason exists to screen for it.
  2. An effective treatment for the disease must exist.  Scientists are working on tests to detect Alzheimer’s dementia in its early stages.  But because we still have no effective treatment for it, screening for Alzheimer’s at this point makes little sense.  In fact, doing so may cause real harm—the harm that comes from knowing you’re highly likely to develop a fatal disease for which there exists no cure.
  3. A latent stage of the disease must exist.  If no such stage exists (e.g., asthma), no opportunity to identify the disease before it becomes symptomatic exists.  If the first time it becomes possible to identify the presence of a disease is only once it becomes symptomatic, why bother attempting to screen for it?

Second, putting aside the issue of affordability, specific criteria exist that must be met before any test can be considered appropriate for use in screening effectively and safely.

  1. The screening test must be able to detect the disease in the latent stage early enough to affect outcomes.  The key isn’t just catching the disease in the latent stage but catching it before what’s known as the critical point—the point beyond which, even though the disease hasn’t yet produced any symptoms, it’s no longer curable.  A case in point:  studies have been done showing that obtaining screening chest x-rays in patients lacking signs or symptoms of lung cancer does indeed identify asymptomatic cancers earlier than if the x-rays weren’t performed—but still too late to affect the rate of cure.  Interestingly, a recent study suggested that performing CT scans as a screening test does catch at least some lung cancers early enough to increase the likelihood of cure (by about 20%).  This is probably because CT scans can pick up much smaller tumors than chest x-rays can, presumably increasing the likelihood of identifying the presence of lung cancer prior to the critical point.
  2. The risk of false positives must be acceptably low.  A false positive result occurs when the test says you have a disease that you really don’t.  The likelihood that a test is right when it says a patient has lung cancer (what’s called the positive predictive value of a test) turns out to be greater in smokers because smokers have a higher risk than non-smokers of getting lung cancer.  (This is because the more prevalent a disease is in a given population, the more times the screening test will actually find the disease it’s designed to detect.)  This matters a great deal because once a screening test says a patient has a disease, to confirm the presence of the disease progressively more invasive tests and procedures are often required, which exposes patients to progressively greater risks of complications.  For example, a single chest CT poses little risk to anyone (the rate of contrast reactions is quite low in the general population), but the lung biopsy or wedge resection that will likely follow a positive scan are invasive enough to pose significant risk.  And though uncommon, the following scenario can occur:  a patient has a positive CT, gets a biopsy or wedge resection of her lung, and develops a complication that ultimately leads to her death—only to have the pathology on the tissue taken from her lung turn out to be benign.  The risk of a similar scenario prevents us from recommending exercise treadmill tests, like the one my patient had, for people without symptoms or risk factors for coronary disease.  False positive treadmill tests happen frequently in those patients, often leading to cardiac catheterizations, which carry a small but definite risk of death—a risk that’s only justified when the suspicion of a false positive result is sufficiently low.
  3. The risk of the test itself must be acceptably low.  Blood tests represent almost the lowest-risk tests we have (the needle stings and some people get woozy or faint, but that’s about it) other than physical exam maneuvers.  A colonoscopy is slightly more invasive and carries a slightly higher risk (colonic perforation is a catastrophic event but happens rarely).  A cardiac catheterization, on the other hand, is distinctly more invasive (the puncture is arterial rather than venous) and therefore carries a distinctly higher risk, one high enough to disqualify it as a screening test altogether.

The number of tests that meet these criteria are far fewer than most people realize.  Further, the utility of many that do satisfy them remains controversial.  Consider the PSA test, a blood test used to identify asymptomatic prostate cancer.  While it satisfies the three criteria listed above, the more studies that are done on it the less clear we become about what number actually represents a positive result.  It used to be a PSA below 4.0 was thought to effectively rule out the presence of prostate cancer.  Then we found a small but significant minority of patients with PSAs between 2.0 and 4.0 actually had it.  Now we’re thinking this may also be true for patients with PSAs between 1.0 and 2.0.  (To explain how researchers figure out what “normal” is for any given test would take an entire post in itself.)

In the end, convinced my patient’s treadmill test represented a false positive result, rather than take him to cardiac catheterization, I put him on a treadmill test again, this time using nuclear imaging (an addition that conveys a negligibly increased risk but far greater sensitivity), which, thankfully, was negative.  We both heaved sighs of relief, and he became convinced that just because we can test for something doesn’t mean we always should.

Next WeekWhy We Need To Know Why

10 comments to Criteria For Screening

  • I couldn’t agree more with this post! My specific personal objection is to the proposed “screening” test for Alzheimer’s—when there are no cures, not even agreed-upon useful treatments—leaving bewildered, perhaps even despairing, human beings in its wake. A horrible death sentence, with no possibility of pardon: for what? Because we can?

    That’s how the human condition becomes intolerable, from global warming to nuclear catastrophe. It is sufficient for us to be able to do something, to do it, and damn the consequences.

  • I think really the main concern is one of cost, no? It’s certainly a valid concern for us here in the U.S., although sadly, no matter how many millions we could save by eliminating unnecessary tests, those saving would more likely go into the pockets of managers and stockholders well before they were passed on to citizens.

    But if I could find out if there was a very high likelihood of me getting Alzheimer’s, I would want to know. I would lead a very different life, and the opportunity to decide to do so would be very much appreciated.

    Allen: Cost to society is certainly an issue, but I’d hoped to emphasize that screening certain people for certain diseases actually poses more risk than it does provide benefit. And though I certainly understand your comment about leading a different life if you knew you were going to develop Alzheimer’s—you already know you’re going to die from something. Why aren’t you already leading the life you want?

    Alex

  • I agree with almost all of the post. However, I would like to plead for specific testing for people who are at a higher risk for certain diseases. For example, if several close family members have heart disease, then it makes sense to me to screen early and periodically. That way minimally invasive treatment at an early stage could prevent worse. Please, let’s not wait until people turn gray, clutch at their chests and fall down dead in the street, if their higher risk has been known all along.

    Catrien: Yes, in general, we’re more interested in screening people for diseases they’re more likely to get. But in the case of heart disease, finding it before it becomes symptomatic doesn’t give us any advantage in treatment. The risks of intervention with an angioplasty or bypass operation coupled with their likely small degree of benefit in people with asymptomatic coronary calcification doesn’t justify their early use. So even if we did screen all high-risk people, we’d tell those in whom we found asymptomatic disease the same thing we tell everyone else: reduce your risk factors (e.g., stop smoking, lower your cholesterol, lose weight, control your diabetes and hypertension, and exercise).

    Alex

  • John

    I guess I have to say that I’m surprised the article focuses on why screening based on a patient’s own feeling that something is wrong, without medical evidence, is bad policy.

    We learn our bodies…we feel when something is not right…patients have valuable insights that are hard to vocalize in medical terms. Those feelings/intuits should be respected. Supporting/encouraging patient ignorance is just crazy.

    Should health insurance pick up the tab when such tests, undertaken with only a patient’s feeling, come back negative? I personally think not. However, if like in this case it comes back positive, yes.

    And medical technicians should praise such proactive efforts by patients rather than come up with excuses as to why proactive testing is bad.

    John: The post was focused specifically on situations in which patient don’t have any symptoms or feelings that something is wrong and attempted to demonstrate that what seems like a straightforward issue—the decision to screen or not—is, in fact, more complex than most people realize. I’m a great supporter of health care providers listening carefully to their patients and taking what they say very seriously, as I wrote in a previous post, When Doctors Don’t Know What’s Wrong.

    Alex

  • Leslie

    Thank you for this post. I got a severe form of trigeminal neuralgia during the course of a common dental procedure. I had never heard of TN before then, and it has been hard to live with.

    Since experiencing one of those “rare” events, I evaluate screening, tests, and procedures the same way you recommend here. I also factor in how much pain/discomfort is involved. There have to be worthwhile or necessary reasons for me to accept anything painful on top of the TN.

  • Tara C

    Thanks for discussing this topic. After reading several studies about the rather negligible results of having yearly breast cancer screenings from 40-50 years old and the high rate of false positives, I’ve decided to stop screening until I turn 50 and only be screened every other year, according to the European recommendations. I feel comfortable with this.

  • Dr. Stephen Parker

    Re: Diagnosing heart problems—

    Ten years ago (age 50) I had dream of an airplane with four-cylinder red engine that was smoking and leaking oil. The plane took off, and then crashed and burned.

    I sat bolt upright in bed at 5:00 am in the morning, and the first thought in my head (that came out of nowhere) was, “I have heart problems.” [Later, much later, I came to realize that a red four cylinder engine is symbolically equivalent to the heart.]

    My physician and I discussed the situation, and I had no symptoms and no risk factors. We decided not to do a treadmill test, for many of the valid reasons you mentioned.

    Three months later I had emergency surgery and a long stent for severe blockage of the LAD, a classic “widow-maker” situation The dream presaged five more years of heart problems, culminating with a severe heart attack.

    Since recovery, I have wondered why the doctor and I (and the culture) have not paid more attention to such dreams that would provide an early warning system. (I learned not to tell cardiologists about the dream; they would roll their eyes and dismiss anything else I had to say.)

    There is actually substantial anecdotal information (and some legitimate research) about these “prodromal” dreams, yet I know of almost no physicians who would ask about dreams and nightmares as a type of screening device.

    In the 1960′s, Vasily Kasatkin, a Russian physician, collected 10,000 dreams over a forty year period, and correlated numerous dreams with medical conditions. This research is almost unheard of in the United States.
    (http://dreamcurrents.com/kasatkin-characteristics-of-prodomal-dreams/)

    Most readers will immediately dismiss the possibility that dreams might contain important medical information; I certainly would have before my experience. (http://www.heartak.com)

    Thanks for your thorough article on screening and diagnostic tests: consider inquiring about dreams and nightmares as an inexpensive addition to your screening and diagnostic tools.

    Dr. Parker: What a fascinating notion. It doesn’t strike me as improbable at all that some kind of communication about the state of an organ could be communicated to the brain and mind through dreams, which are, after all, stories we tell ourselves.

    Alex

  • Very relevant post. The false positive issue is a serious one, especially because it’s hard to tell whether a patient would have been fine without treatment (and in some cases, the treatment causes some harm). Of course, there are also a lot of medical device folks who’d love to see screenings become more routine for business reasons. I think your guidelines mark a sensible approach.

  • Jonathan

    Your post nicely illustrates a principal danger of asymptomatic screening: false positives leading to further unnecessary tests (in your example leading to the use of an imaging technique often identified in the US with over-utilization), adding further costs to an over-burdened healthcare system.

  • Great post.

    The requirement that “The screening test must be able to detect the disease in the latent stage early enough to affect outcomes” is important. We can screen for Huntington’s disease …BUT if there is no treatment or prevention for the disorder, then implementing a screening program is irrelevant.

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