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The Downside Of Screening Tests: The Costs Of False Positive Results

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.

They took advantage of the ongoing Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial to gather their data. In this large controlled trial (over 150,000 subjects), men randomized to screening were offered chest x-rays, flexible sigmoidoscopies, digital rectal examinations and PSA blood tests. Women were offered CA-125 blood tests for cancer antigen, transvaginal sonograms, chest x-rays, and flexible sigmoidoscopies. During the 3-year study period, a total of 14 screening tests were possible for each sex. The subjects didn’t all get every test.

By the 4th screening test, the risk of false positives was 37% for men and 26% for women. By the 14th screening test, 60% of men and 49% of women had had false positives. This led to invasive diagnostic procedures in 29% of men and 22% of women. 3% were minimally invasive (like endoscopy), 15.8% were moderately invasive (like biopsy) and 1.6% involved major surgical procedures (like hysterectomy). The rate of invasive procedures varied by screening test: 3% of screened women underwent a major surgical procedure for false-positive findings on a transvaginal sonogram.

These numbers do not include non-invasive diagnostic procedures, imaging studies, office visits. They do not address the psychological impact of false positives. And they do not address the cost of further testing.

These data should not be over-interpreted. They don’t represent the average patient undergoing typical cancer screening in the typical clinic. But they do serve as a wake-up call. Screening tests should be chosen to maximize benefit and minimize harm. Organizations like the U.S. Preventive Services Task Force try to do just that; they frequently re-evaluate any new evidence and offer new recommendations. Data like these on cumulative false positive risks will help them make better decisions than they could make based on previously available single-test false positive rates.

“In a post earlier this year, I discussed the pros and cons of PSA screening. Last year, I discussed screening ultrasound exams offered direct to the public to bypass medical judgment). If you do 20 lab tests on a normal person, statistically one will come back false positive just because of the way normal lab results are determined. Figuring out which tests to do on a given patient, either for screening or for diagnosis, is far from straightforward.

This new information doesn’t mean we should abandon cancer screening tests. It does mean we should use them judiciously and be careful not to mislead our patients into thinking they offer more certainty and less risk than they really do.

*This blog post was originally published at Science-Based Medicine*


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Richmond, VA – In an effort to simplify inpatient medical billing, one area hospitalist group has determined that “altered mental status” (ICD-9 780.97) is the most efficient code for use in any patient work up.

“When you enter a hospital, you’re bound to have some kind of mental status change,” said Dr. Fishbinder, co-partner of Area Hospitalists, PLLC. “Whether it’s confusion about where your room is located in relationship to the visitor’s parking structure, frustration with being woken up every hour or two to check your vital signs, or just plain old fatigue from being sick, you are not thinking as clearly as before you were admitted. And that’s all the justification we need to order anything from drug and toxin screens, to blood cultures, brain MRIs, tagged red blood cell nuclear scans, or cardiac Holter monitoring. There really is no limit to what we can pursue with our tests.”

Common causes of mental status changes in the elderly include medicine-induced cognitive side effects, disorientation due to disruption in daily routines, age-related memory impairment, and urinary tract infections.

“The urinalysis is not a very exciting medical test,” stated Dr. Fishbinder. “It doesn’t matter that it’s cheap, fast, and most likely to provide an explanation for strange behavior in hospitalized patients. It’s really not as elegant as the testing involved in a chronic anemia or metabolic encephalopathy work up. I keep it in my back pocket in case all other tests are negative, including brain MRIs and PET scans.”

Nursing staff at Richmond Medical Hospital report that efforts to inform hospitalists about foul smelling urine have generally fallen on deaf ears. “I have tried to tell the hospitalists about cloudy or bloody urine that I see in patients who are undergoing extensive work ups for mental status changes,” reports nurse Sandy Anderson. “But they insist that ‘all urine smells bad’ and it’s really more of a red herring.”

Another nurse reports that delay in diagnosing urinary tract infections (while patients are scheduled for brain MRIs, nuclear scans, and biopsies) can lead to worsening symptoms which accelerate and expand testing. “Some of my patients are transferred to the ICU during the altered mental status work up,” states nurse Anita Misra. “The doctors seem to be very excited about the additional technology available to them in the intensive care setting. Between the central line placement, arterial blood gasses, and vast array of IV fluid and medication options, urosepsis is really an excellent entré into a whole new level of care.”

“As far as medicine-induced mental status changes are concerned,” added Dr. Fishbinder, “We’ve never seen a single case in the past 10 years. Today’s patients are incredibly resilient and can tolerate mixes of opioids, anti-depressants, anti-histamines, and benzodiazepines without any difficulty. We know this because most patients have been prescribed these cocktails and have been taking them for years.”

Patient family members have expressed gratitude for Dr. Fishbinder’s diagnostic process, and report that they are very pleased that he is doing everything in his power to “get to the bottom” of why their loved one isn’t as sharp as they used to be.

“I thought my mom was acting strange ever since she started taking stronger pain medicine for her arthritis,” says Nelly Hurtong, the daughter of one of Dr. Fishbinder’s inpatients. “But now I see that there are deeper reasons for her ‘altered mental status’ thanks to the brain MRI that showed some mild generalized atrophy.”

Hospital administrators praise Dr. Fishbinder as one of their top physicians. “He will do whatever it takes to figure out the true cause of patients’ cognitive impairments.” Says CEO, Daniel Griffiths. “And not only is that good medicine, it is great for our Press Ganey scores and our bottom line.”

As for the nursing staff, Griffiths offered a less glowing review. “It’s unfortunate that our nurses seem preoccupied with urine testing and medication reconciliation. I think it might be time for us to mandate further training to help them appreciate more of the medical nuances inherent in quality patient care.”

Dr. Fishbinder is in the process of creating a half-day seminar on ‘altered mental status in the inpatient setting,’ offering CME credits to physicians who enroll. Richmond Medical Hospital intends to sponsor Dr. Fishbinder’s course, and franchise it to other hospitals in the state, and ultimately nationally.

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