Introduction
Screening tests are routine tests that are done on people without complaint. They don’t feel ill. The purpose is to detect disease long before symptoms and to hopefully detect disease at a time when it can be easily treated.
On the other hand, diagnostic tests are usually done to delineate the current state of disease. The purpose of a diagnostic test could be to identify the size of a tumor or whether a tumor may be affecting the function of an organ system.
Why screening becomes complex
Screening to detect disease early becomes a complex question.
Is the test itself safe? Is the test cost-effective? Many times testing to detect disease is not the end of the question and it leads to additional tests, perhaps even biopsies. So the path is at times complicated and detailed.
Screening is designed for people who feel fine but may have an early variant of a disease that is known to progress. A pap smear looks for cervical cellular changes before cancer develops. A colonoscopy searches for polyps or abnormal tissue known to progress into colon cancer.
These tests differ from diagnostic exams, which are ordered to explain symptoms that already exist.
A screening test is a narrow tool designed to find disease early enough to intervene and make a difference in survival. I have become a fan of Total Body MRIs. They aren’t ionizing radiation. They’re done with a high tech magnetic device. This consideration of doing your entire body to screen for abnormalities has only developed for the masses with the purchasing power of large associations or groups. One such entity is Function Health. Last year they offered that Total Body MRI for $500. I saw recently the price had increased to about $1,000. To put it in perspective that test a couple of years ago would have been thousands if you could find a center that would do it. That test has been usually done to look at specific areas due to suspected disease.
The idea of doing your entire body to detect abnormalities like cancer wasn’t a consideration due to the cost of adding all of those small areas. Very expensive. Most insurance companies will even now balk at covering the cost of that test without a specific diagnosis and the cost benefit of screening for cancer hasn’t been proven with population studies. What do you do with the results? Read on for the other side of the discussion.
Different disease paths
There are a few problems you face when screening. Sometimes you detect a very fast and aggressive disease process. They may be quite advanced when detected.
Other tumors may grow much more slowly, stop growing, or even regress. It might never cause symptoms or shorten life.
The problem is that we’re not that good at distinguishing which path a cancer might follow. Very difficult to predict.
The cascade effect after an abnormal result
You might imagine that we could cause problems trying to answer these questions.
Medicine tends to spring into action when it doesn’t have an answer. There can be a dozen different actions to take when the doctor is unsure of the result.
Naturally, this gets treated as a potential threat. The truth may be that we don’t know the future behavior of that small nodule. It could be an aggressive cancer detected early. This test was done to reassure but instead became the first step in the following action sequence. A scan led to a procedure. And your worries don’t get answered for quite some time. The decisions are not driven by illness. This is a risk-aversion protocol. Determine early if you have an early lurking cancer.
For many routine screening tests, false alarms are far more likely than life-saving discoveries.
Mammograms and call-backs
With routine mammograms, for example, about 10% are called back. That means that in 10 years you would likely be called back at least once. Many end up being false alarms. Up to 17% might have a biopsy to later hear that it was benign.
There are real costs associated with this. There are often follow-up tests and there’s time away from work, etc. There are out-of-pocket expenses many times. Even after being told that there’s no disease, that it’s benign, the anxiety can persist for some people indefinitely. They can’t get beyond thinking that maybe there really is something wrong.
Overdiagnosis
Overdiagnosis can lead to big problems. The test wasn’t wrong. It detected something real. Its discovery leads to surgery, radiation, and years of follow-up. The problem was that we couldn’t really predict the future action of that group of cells. It had the potential to be a bad actor.
How often does this happen? Estimates vary widely depending on age, screening frequency, and definition. Even conservative analyses find that overdiagnosis is not uncommon.
National Cancer Institute estimates that 20 - 50% of screen-detected breast cancers represent overdiagnosis. These are real tumors that would not have caused symptoms or shortened life if left undiscovered. Both age and screening method can affect the outcome. The estimate is about one in seven women screened every two years, between ages 50 and 74, may be detected to have one of these cancers.
The benefits are frequently emphasized while false positives and overdiagnosis receive far less attention.
For patients, once you begin the process of screening it isn’t easy to get off the train.
Clinicians might understand the limits of screening. The system pushes clinicians to advocate that patients pursue a definitive diagnosis. Clinicians are scored on how many of their patients have completed screening exams, vaccination, etc.
Doctors aren’t faulted for ordering too many tests. They are condemned for missing something. It’s no wonder so many tests are performed.
Our system promotes tests and procedures. The system gets filled with testing healthy people and the capacity to treat the ill is diminished. That’s a paradox to consider.
When does screening help?
Screening works best when you’re dealing with a disease that has a very long latent or quiet phase before symptoms appear. There’s also the need to have effective treatment that meaningfully impacts the outcome. The person screened should also have enough added life expectancy to benefit from the treatment.
One example of this is lung cancer screening. Screening is most impactful for a very narrow group, and that group is long-time, heavy smokers. The screening test is an annual CT scan to detect aggressive tumors early. This doesn’t benefit non-smokers or light smokers but only the heavy smokers.
When evidence changes screening recommendations
Sometimes the evidence changes the recommendations for screening. What was once an annual pap smear has been reduced for low-risk women without any uptick in cancer deaths.
Mammography and prostate-specific antigen (PSA) testing are used to screen symptom-free adults for breast and prostate cancer. The mortality gains for average-risk adults are small. There is a higher rate of overdiagnosis.
Marketing and relative vs. absolute benefit
The marketing of these screening exams might be factually true, but deceptive when you see “20 percent drop in deaths.” For example, if 10 people would die, and screening saves 2 of those 10, that is marketed as a 20% drop. However, that does not explain the benefit in absolute, real-world numbers. To save those two lives, thousands of people were likely screened, making the test’s value for the average individual much smaller than the “20 percent drop” suggests. I hope I explained that clearly. I remember studying that in medical school many years ago and it threw me for a loop in the beginning.
Screening isn’t useless. But the benefits most people expect are often larger than the benefits received.
And, of course, marketing gets involved. Many of the stories used to market the screening tests use very pronounced stories without describing the full scientific prediction basis. The headline might be “My life was saved by getting my mammogram and finding my cancer early.” Hard to be rational when emotions and fear are triggered.
Screening is a choice
Deciding whether to be screened is a choice, not a mandate. It should be discussed and debated. Consider the family history, genetics, lifestyle, and past abnormal results. For people at average risk, the benefit is often far smaller than cultural messaging suggests.
As people age or develop serious illnesses, the chance of benefit drops. Finding a slow-growing cancer late in life may cause more harm than good.
Screening can find abnormalities early, but it cannot reliably predict which ones will become dangerous. Understanding what a test can and cannot tell you makes results easier to interpret.
A test is useful only if acting on it improves outcomes. If a positive result mainly leads to more testing or treatment with unclear benefit, waiting may be reasonable.
Some people want every available data point, even if it means more follow-up. Others know that uncertainty would create anxiety rather than clarity. Screening is not a measure of diligence or virtue. It’s a tool. The right choice is the one that fits your goals, your tolerance for uncertainty, and what the test can realistically offer.
Questions to ask before getting screened
Be rational and reasonable. Get answers to questions like: What’s my personal risk for this disease? How often does this test lead to false alarms? What happens next with a false alarm? You’d be surprised at how many times I have cancelled performing a test just by going through the process of “what if” with a patient. Explaining that a positive test usually gets scheduled for surgery often elicits a response of “whoa, hold on a minute. I don’t want that.”
Ask, how would a positive result change my care? Does the test itself have risks? What happens if I wait? Am I making the right choice consistent with my goals?
At the end of the day, we’re talking about screening exams.
It isn’t the same as being proactive to prevent disease. Skipping or delaying a screening test is not the same as ignoring health. It detects disease earlier, but it does not prevent it.
The small, daily choices we make have a much bigger impact on our health. If you’re obese or smoke there are more important choices to discuss.