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Archive for Diagnostic tests & procedures

Screening Tests – Cumulative Incidence of False Positives

It’s easy to think of medical tests as black and white. If the test is positive, you have the disease; if it’s negative, you don’t. Even good clinicians sometimes fall into that trap. Based on the pre-test probability of the disease, a positive test result only increases the probability by a variable amount. An example: if the probability that a patient has a pulmonary embolus (based on symptoms and physical findings) is 10% and you do a D-dimer test, a positive result raises the probability of PE to 17% and a negative result lowers it to 0.2%.

Even something as simple as a throat culture for strep throat can be misleading. It’s possible to have a positive culture because you happen to be an asymptomatic strep carrier, while your current symptoms of fever and sore throat are actually due to a virus. Not to mention all the things that might have gone wrong in the lab: a mix-up of specimens, contamination, inaccurate recording…

Mammography is widely used to screen for breast cancer. Most patients and even some doctors think that if you have a positive mammogram you almost certainly have breast cancer. Not true. A positive result actually means the patient has about a 10% chance of cancer. 9 out of 10 positives are false positives.

But women don’t just get one mammogram. They get them every year or two. After 3 mammograms, 18% of women will have had a false positive. After ten exams, the rate rises to 49.1%. In a study of 2400 women who had an average of 4 mammograms over a 10 year period, the false positive tests led to 870 outpatient appointments, 539 diagnostic mammograms, 186 ultrasound examinations, 188 biopsies, and 1 hospitalization. There are also concerns about changes in behavior and psychological wellbeing following false positives.

Until recently, no one had looked at the cumulative incidence of false positives from other cancer screening tests. A new study in the Annals of Family Medicine has done just that. (more…)

Posted in: Clinical Trials, Diagnostic tests & procedures

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PSA – To Screen or Not to Screen

You’ve all heard the dramatic testimonials in the media: “I had a PSA test and they found my prostate cancer early enough to treat it. The test saved my life. You should get tested too.” The subject of screening tests is one that confuses the public. On the surface, it would seem that if you can screen everyone and find abnormalities before they become symptomatic, only good would result. That’s not true. Screening tests do harm as well as good, and we need to carefully consider the trade-offs.

About half of American men over the age of 50 have had a PSA (prostate-specific antigen) screening test for prostate cancer. Recommendations for screening vary. The US Preventive Services Taskforce (USPSTF) says there is insufficient evidence to recommend screening. The American Urological Association and the American Cancer Society recommend screening. Urologists practice what they preach: 95% of male urologists over the age of 50 have been screened. But other groups like the American Academy of Family Physicians recommend discussing the pros and cons of screening with patients and letting them make an informed choice.

Two recent studies published simultaneously in The New England Journal of Medicine have added to the controversy. One concluded that screening does not reduce deaths from prostate cancer; the other concluded that it reduces deaths by 20%. (more…)

Posted in: Clinical Trials, Diagnostic tests & procedures

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How not to think

Thankfully, I don’t receive all that much blog-related mail.  But this weekend I received several communications about a piece in popular liberal blog.  The piece is (ostensibly) about Lyme disease, which coincidentally happens to be one of the topics of my first post here at SBM.  In fact, I’ve written about Lyme disease a number of times, and Dr. Novella has a very good summary of the controversy at one of his other blogs.  Since we’ve discussed this so many times, I won’t be reviewing the entire controversy, but looking at this particular blog post to examine how our personal experiences and errors in reasoning can distort our view of reality.

The topic of Lyme disease has come up recently in the press, and as the weather improves, cases in the northeastern U.S. should start to increase soon.  Just as a reminder, so-called “chronic” Lyme disease is not Lyme disease at all.  Lyme disease can have early and late manifestations, none of which correspond to the vague, protean symptoms labeled as “chronic” by some.  The disease is often diagnosed without resort to objective evidence, such as reliable, positive lab tests.  But let’s look at the blog post in question and see what’s there.

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Posted in: Diagnostic tests & procedures, Science and Medicine

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Do over one in five breast cancers detected by mammography alone really spontaneously regress?

ResearchBlogging.orgIt figures.

Last Wednesday, right before the four-day Thanksgiving holiday weekend, as I was far more interested in preparing to have family over the next day than in what was going on in the medical news or the blogs, the results of a most fascinating study hit the news. In Medscape, the title of the news report was Mammography Study Suggests Some Breast Cancer May Spontaneously Regress; on WebMD, the story ran under the title Can Breast Cancer Disappear?; on Medical News Today, Mammograms May Identify Cancers That Would Otherwise Just Regress by Drs. Per-Henrik Zahl, Jan Maehlen, and H. Gilbert Welch. Not surprisingly, the study found its way out of the medical news and into mainstream media outlets as well, given how provocative the findings seemingly are. From the Medscape report on this study:

A mammography study from Norway has come up with the controversial proposal that one fifth of breast cancer detected on screening may spontaneously regress. But there is no easy way to verify whether this is the case, say experts.

The study was published in the November 24 issue of the Archives of Internal Medicine. It found that the cumulative incidence of invasive breast cancer in a cohort of women, aged 50 to 64 years, who received 3 mammograms over 6 years was 22% higher than in a control group of age-matched women who received only 1 mammogram at the end of a 6-year period.

In their review of possible explanations for the difference in the breast cancer rates between the 2 groups — which had similar risk factors for breast cancer — the researchers write that the “natural course for some screen-detected breast cancers may be to spontaneously regress.”

“I anticipate that many clinicians will react negatively to the possibility of spontaneous regression, said coauthor Jan Maehlen, MD, PhD, professor of pathology at the Ulleval University Hospital, in Oslo, Norway, in an interview with Medscape Oncology.

Before I discuss the study itself, let me briefly discuss why clinicians may have a bit of a problem with the implications of this study, if they accurately reflect the biology of breast cancer.
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Posted in: Cancer, Clinical Trials, Diagnostic tests & procedures, Science and the Media

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Diagnostic Dilemmas

Sometimes diagnosis is straightforward. If a woman has missed several periods and has a big belly with a fetal heartbeat, it’s pretty easy to diagnose pregnancy. But most of the time diagnosis is much more difficult. Alzheimer’s can’t be diagnosed for sure until the patient dies and you do an autopsy. If only we had one of those Star Trek gadgets to point at our patients and give us a quick and accurate answer! Alas! We are far from perfect. All too often, we really have no idea what’s causing a patient’s symptoms. We do a complete workup and still don’t know. What then?

We all know people who have symptoms that a series of doctors have failed to diagnose, who continue to doctor-shop, hoping to find that one doctor somewhere who will find something the others have missed. Occasionally they do; but far more often these people spend a great deal of time and money chasing a will-o’-the-wisp. Sometimes as they are searching, the illness gradually runs its course and goes away. When this happens, whatever they tried last gets the undeserved credit for the “cure.” Sometimes the symptoms persist and these searches consume their life, encourage unhealthy self-absorption, and permanently ensconce them in the “sick” role.

One of the attractions of alternative medicine is that it offers far more certainty than scientific medicine. If your scientific doctor can’t see anything on x-rays, your chiropractor can. He’ll tell you he knows exactly what’s wrong: a subluxation that he can fix. Sherry Rogers will tell you all illness is due to toxins accumulating in your cells and you must “detoxify or die.” Hulda Clark will tell you it’s all parasites that she can eliminate with her magic zapper. Robert Young says the cause of all disease is acidosis. They all have confident, precise answers. Wrong ones.

The One Cause of All Disease?

It’s really easy to figure out what’s causing a patient’s symptoms if you believe there is one simple cause for all disease. While I was writing this I got sidetracked and searched the Internet for “the one cause of all disease.” I found a lot of them, including: (more…)

Posted in: Diagnostic tests & procedures, Science and Medicine

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