Benefits and risks of screening tests
A wide range of screening tests are available. Some tests are considered to have a benefit, whereas the benefit of other tests is not so clear. We describe which criteria a screening test must fulfill for it to be worth having, explain important terms, and shed light on the scientific background.
Diagnostic tests are usually done to find out what is causing certain symptoms. Screening tests are different: they are done in people who do not feel ill. They aim to detect diseases at an early stage, before any symptoms become noticeable. This has the advantage of being able to treat the disease much earlier. However, treating a disease at an early stage only makes sense if it leads to a better health outcome than treating it at a later stage.
Another kind of screening is called opportunistic screening: this is where someone visits a doctor for a particular reason and, while they are there, they are offered an additional test, such as blood pressure measurement. In population-wide screening, everyone who fulfills the criteria for the test is invited to have a voluntary test. One example of this kind of screening in Germany is an x-ray examination of the breast (mammography) in women between the ages of 50 and 69, to screen for breast cancer.
The medical tests used for screening purposes are often not suitable for making a final diagnosis. Instead, many tests are used to detect any abnormalities first, which are then looked at more closely in other tests. This is the case for stool tests done in bowel cancer screening, for example. The stool tests look for occult (hidden) blood in a person’s stool, which could be a sign of bowel cancer. If the test results are abnormal, the bowel is examined closely with an endoscope. This is the only way to find out whether the traces of blood are indeed being caused by cancer, or by a non-cancerous condition such as hemorrhoids or an inflammation of the lining of the bowel.
Screening tests are not always done to look for a disease. Some screening tests aim to detect risk factors for certain diseases. In Germany, members of statutory health insurance funds who are aged 35 and over are entitled to a general check-up called the “Gesundheits-Check-up,” for example. This includes a blood and urine test, as well as other things. The findings are not always clear. They sometimes lie in a gray area between normal and abnormal results.
Can screening prevent diseases?
Screening tests are often advertised with slogans like “prevention is better than cure.” But most of them cannot influence whether someone will get ill. Many people wrongly mistake screening for “prevention,” and some think that having regular screening tests can protect them from a disease. But that is not the case: screening tests usually cannot prevent diseases. Sometimes doctors also tend to be too optimistic when it comes to the benefits of screening.
A screening program can only be considered to be a “preventive” measure if it aims to determine and influence risk factors, or detect and treat abnormal changes that could later develop into a disease. One example of this is endoscopy of the bowel, which makes it possible to detect and remove intestinal polyps that could later develop into cancer.
So preventive treatment can stop diseases from developing in some people. One disadvantage, however, is that many people then have treatment although they would never have got the disease. This is because many abnormal changes do not necessarily develop into a more serious condition, or might even go back to normal again by themselves, without causing any health problems.
Regardless of whether or not you have screening tests: if you have worrying symptoms, it is important to take them seriously and have them checked out by a doctor.
Can screening have harmful effects too?
Every screening test can be harmful too. For example, x-ray examinations expose the body to radiation, and endoscopy of the bowel can lead to bleeding or (in rare cases) serious injuries. So it is important that both the benefits and harms of a screening test are assessed in studies before introducing it at a large scale. That way, researchers can find out whether the benefits of the screening test outweigh the risks.
The statutory health insurance funds in Germany usually only pay for screening tests that have been proven to do more good than harm.
In Germany, some screening tests are offered as what are called individual health care services (in German: Individuelle Gesundheitsleistungen, or IGeL for short). In most cases, there is not enough good research to be able to say whether the benefits of these screening tests outweigh the risks. People have to pay for IGeL screening tests themselves. The statutory health insurance funds provide information (in German) about certain IGeL services on the website of the Medical Review Board of the Statutory Health Insurance Funds (Medizinischer Dienst des Spitzenverbands der Krankenkassen, MDS).
When might it be worth having a screening test?
The World Health Organization (WHO) has defined criteria for evaluating population-wide screening tests, which may help you decide whether or not to have such a test. The WHO criteria include the following:
- Screening should be done only for diseases with serious consequences, so that screening tests could potentially have clear benefits to people’s health.
- The test must be reliable enough, and not harmful in itself.
- There must be an effective treatment for the disease when detected at an early stage – and there has to be scientific proof that that treatment is more effective when started before symptoms arise.
- Neutral information should be made available to the public, to help people decide for themselves whether or not to have a screening test.
The WHO points out that detecting a disease early does not automatically have a benefit. If early diagnosis and treatment does not lead to an improved health outcome, detecting a disease earlier only makes people worry and have treatment for longer – unnecessarily, because they do not benefit from earlier treatment.
When is screening not beneficial?
Let us take the following (imaginary) example, which was developed by the Max Planck Institute for Human Development in Berlin. The diagram below shows two groups of people who all died of cancer at the age of 70 – regardless of whether or not they had done screening tests. In the group that did not have screening, the cancer was detected at the age of 67, three years before their death. In the group that had screening, on the other hand, the cancer was already detected at the age of 60.
If you only look at the number of people who were alive five years after their cancer was diagnosed, screening seems to be highly effective. In reality, however, all of the people died at the age of 70 – with or without screening. So having a screening test did not make them live longer, they just learned that they had cancer earlier. This means that it increased the time they spent living as cancer patients by several years.
If the screening tests had been effective, the people who had screening would have lived longer, so they would have died when they were older than 70, or their cause of death would have been something other than the disease that was detected through screening. Apart from that, screening might also have other benefits, such as an improvement in quality of life. The important thing is that screening should have benefits that outweigh the risks.
How are the benefits of screening tests assessed?
Researchers use different scientific methods to evaluate screening tests. For example, they do studies to find out
- how reliable the test is,
- whether a patient benefits from starting treatment earlier rather than later,
- which groups of people benefit from early diagnosis and treatment, and
- how the benefits compare to the risks.
Reliability of the screening
Screening usually consists of two steps. The aim of the first step is to look for signs of the disease in question. The results of a screening test are considered to be “positive” if signs of the disease are found, and ”negative” if no signs of the disease are found. The second step involves doing further tests to enable a more accurate diagnosis. But this is only done if the first test reveals abnormal findings – in other words, if the result is positive.
Caution: The terms “positive” and “negative” can be very confusing when it comes to screening, since their meaning is completely different from what we associate with them. You would not be too happy about a “positive” screening result, because this means abnormal findings. A “negative” result is good news: there were no abnormal findings.
The ideal screening: reliable and precise
If there were a perfect screening program, all participants would get a correct and reliable result in the first test. This means that:
- All of the people who have a positive test result really would be ill (this is called a “true positive” result).
- There would be no positive test results in people who are not ill (no “false positive” results).
- All of the people who have a negative test result would not be ill (they would have a “true negative” result).
- People who are ill would not have a negative test result (there would not be any “false negative” results).
Sensitivity and Specificity
But no test is that perfect. In scientific terms, a test that very reliably recognizes people who have the disease as “true positive” has a high “sensitivity.” This means it does not miss any people who are ill.
A test that clearly classifies people who do not have the disease as “true negative” has a high “specificity.” This means that the examination is not precise enough and only catches people who already have the disease.
The following (imaginary) example illustrates this:
Imagine ten people who do not have any symptoms. The two people highlighted in a dark color have cancer:
If there were a perfect screening test and all ten people were tested with it, all people drawn in a light color would have a “negative” test result (-) = “no cancer.” The two highlighted people would have a “positive” test result (+) = “cancer found.” The results of the screening tests would look like this:
If the test were unreliable, the result would be different. Let us assume that the screening test shows a positive result in only one of the two people who have the disease, and leads to a false alarm in half (4 out of 8) of the people who do not have cancer. In our example, this would mean that one case of cancer would not be detected, and four people who do not have cancer would wrongly get a positive test result:
An inaccurate test like this would cause unnecessary worry for the four people with false positive test results. Waiting for the diagnosis without knowing can be very distressing. Further tests might be needed to confirm the diagnosis too. This might include something like taking a tissue sample, and could lead to more stress or complications.
Screening could also have disadvantages for the person who had no signs of cancer in the test, although they actually have cancer. They will not get treatment as early as they could, and may have a false sense of security. Also, doctors might not respond properly to symptoms because they might rule out cancer as a possible cause. The person might then get worse care than if they had not had the screening test.
Reliable studies are needed to assess whether a test is accurate enough to be used in screening. Yet sometimes tests that often have false positive results can be useful too. They help narrow down the group of people who need further tests.
It is important to be aware that an initial positive test result does not necessarily mean that you really are sick. In skin cancer screening, for example, the great majority of suspicious moles and spots turn out to be harmless when further tests are done.
Early or late treatment
Detecting a disease before it causes symptoms is not always beneficial for the person affected by it. In order to benefit from an early diagnosis, there has to be an available treatment that is more effective when started early – that is, before any symptoms occur and you would have gone to see a doctor anyway. Or an early diagnosis has to benefit the person in a different way. For example, some people would like to know whether they are at greater risk of getting a hereditary disease, so that they can make certain decisions early, or take certain measures.
To be able to assess the benefits and harms of treatments, conclusive comparative studies are needed – particularly a kind of study called randomized controlled trials. In this kind of study, volunteers are randomly assigned to one of two or more groups. One of the groups gets the treatment to be tested, and the other group gets another treatment, no treatment or a fake treatment (placebo).
Studies like this can also be used to compare early treatment with later treatment. If the studies show that patients who get treatment earlier have a better health outcome than those who are treated later, this suggests that screening has a benefit.
Who might benefit from early treatment
Not all people have the same risk of a disease. For example, the risk of many diseases depends very much on age. Whether or not a screening test is worth having will greatly depend on the overall risk of getting the disease, and also on a person’s own specific risk. If only 20 out of 10,000 people have a certain undetected disease without any symptoms, it might not necessarily make sense to test all 10,000 people in order to find the 20 people with the disease.
But if 500 out of 10,000 people who are older than 60 have the disease in question, it might make sense to screen all people over the age of 60. Large statistical surveys are needed to identify the groups of people with a higher risk of certain diseases.
Balance of benefits and harmful effects
To be able to assess the advantages and disadvantages of screening, as well as the balance of benefits and harmful effects, the screening test itself needs to be tested in studies. Here it is not only the accuracy of the test that is of interest, but, more importantly, whether the people who have screening also have benefits to their health as a result.
The benefits and harmful effects of a screening test can only be weighed up if researchers do randomized controlled trials in which participants are observed throughout the entire screening chain – starting with the first test and continuing during the following tests and treatments.
Here one group regularly has screening tests, while the other group does not. At the end of the study the results of the two groups are compared, for example in terms of their disease-specific life expectancy and the number of treatments and complications they had.
Who can help you decide?
Sometimes it is easy to decide whether or not to have a screening test. If that is not the case, it may help to get advice, for example from a doctor. They can help individuals to weigh up the advantages and disadvantages of a certain test for themselves. Scientific, objective information is needed to do this. In Germany, some health insurance funds also publish information or decision aids about screening tests.
It is important for all information to be neutral and based on scientific facts. Information that is based on high-quality studies is a good basis for making decisions. It should include both the benefits and harmful effects of screening tests, and express the extent of benefits and harmful effects in clearly understandable numbers. It can also be helpful to find out how other people make their decisions, and how they cope with the associated worries.
Here are some questions you can ask if you are thinking about having a screening test:
- How likely am I to get the disease at my age?
- Would detecting the disease early benefit my health in the long term? For instance, is there any scientific proof that it might help me live longer?
- What adverse effects might be associated with the screening test, with possible tests following screening, and with the treatment? How common are they?
- How common are false positive test results (false alarms) and false negative test results (undetected diseases)? Will I have to have other tests if the test results are positive?
- How long will I have to wait for the test results?
- How often will I have to have a screening test in order to benefit from screening?
In Germany, screening is usually voluntary. It is only rarely compulsory; for instance, if there is an epidemic of a dangerous contagious disease.
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Petticrew MP, Sowden AJ, Lister-Sharp D, Wright K. False-negative results in screening programmes: systematic review of impact and implications. Health Technol Assess 2000; 4: 1-120.
UK National Screening Committee. Criteria for appraising the viability, effectiveness and appropriateness of a screening programme. London: NHS. June 2009. [Accessed on September 10, 2009].
Wilson JMG, Jungner G: Principles and practice of screening for disease. Public Health Papers Number 34. Genf: WHO. 1968.
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