Article: What you should know about screening tests

There are more and more screening tests being offered nowadays. Some are considered useful, while others are controversial. Screening tests are offered for a variety of different diseases. Each test should be examined as to what benefit and what harm it may have. As with practically any medical intervention, possible advantages and disadvantages need to be weighed before deciding whether to have a screening test or not.

In this article we would like to let you know about the chances and risks of screening tests to help you make your decision: How do I know whether having a screening test has an advantage or disadvantage for my health? What questions can I ask to judge a screening test? Why do statutory health insurance funds in Germany not pay for all screening tests?

We will explain the criteria a screening test must fulfill for it to be a good idea. And we will explain terms often used in connection with screening tests and shed light on the scientific basis. Before we start: Screening tests can usually not give absolute certainty. They cannot prevent diseases and are no guarantee of not getting a particular disease. But it might help you make the best decision for yourself if you understand what screening tests can do and what they cannot do.

What is a screening test?

Diagnostic examinations are usually done to find out what is causing certain symptoms. Screening tests are different: they are for people who do not feel ill. They aim to detect diseases at an early stage when they are not yet causing any symptoms. It makes sense to start treatment early if this leads to better results than starting it later.

There are many different kinds of screening. If all newborns are offered the same tests (for example screening tests for hearing impairment), this is one type of screening. Another kind of screening is called opportunistic screening: a person visits a doctor for a particular reason and on this occasion is offered an additional test, for example measuring his or her blood pressure. In a population-wide screening on the other hand, all citizens fulfilling the criteria for the test are invited to a voluntary examination. An x-ray examination of the breast (mammography) as a screening for breast cancer in women between the ages of 50 and 69 is an example of this kind of screening.

The medical tests used in screening tests are often not suited for making a definite diagnosis. They are often used to detect any abnormalities first, which are then looked at more closely in other examinations. This is the case in full body examinations done in skin cancer screening, for example, after which suspicious parts of the skin are examined more thoroughly. In addition, screening tests are not always done to look for a disease: Some screening tests are about detecting risk factors for certain diseases: In Germany, for example, members of statutory health insurance funds over the age of 35 can have a “health check-up” to have their blood and urine values checked, among other things. Sometimes the test results are not clear and are on the border between normal and abnormal.

Can screening prevent diseases?

Screening tests are often advertised with slogans like “preventing is better than curing”. But they cannot influence whether someone gets ill. Many people wrongly use the term “prevention” for screening, and some think that having regular screening tests can prevent them from getting a disease. But this is not the case: screening tests cannot prevent diseases.

The term “prevention” only makes sense in cases where a specific screening program aims at detecting and influencing risk factors (high blood pressure, for example) or at detecting and treating preliminary stages of a disease (changes in the cells of the cervix, or bowel polyps, for example). Preventive treatments can prevent diseases in some people. But they also have disadvantages: Many people are treated who would not have gotten the disease. This is because many preliminary stages do not necessarily develop into a serious condition or can even go back by themselves without causing any problems.

Every screening test and preventive examination can be harmful too. In an x-ray examination, for example, the body is exposed to radiation, and during an endoscopy of the bowel, the bowel can be injured. It is therefore very important to test benefit and harm of a screening test in trials before introducing it at a large scale. This way researchers can find out whether the benefit of a screening is greater than its harm. No evidence of benefit and possible harms are reasons for statutory health insurance funds in Germany not to pay for all screening tests. To find out which screening tests German health insurance funds do pay for, click here.

In Germany, some screening tests are offered as what are called individual health care services (in German: individuelle Gesundheitsleistungen, or IGeL for short). These are screening test that have not yet been sufficiently proven to have a benefit. If you are making a decision about one of these screening tests offered it is a good idea to get independent information and to think carefully about whether for you its possible benefit is greater than its possible harm. You have to pay for an IGeL screening test out-of-pocket. You can find information on IGeL by the statutory health insurance funds here (PDF file, in German).

When is it worth having a screening test?

The World Health Organization (WHO) has defined principles for evaluating population-wide screening tests, which may help you in your personal decision in favor of or against such a test. The WHO principles include these points:

• Screenings should be done only for diseases with serious consequences so that people taking part in a screening can have a clear health benefit.
• There must be a sufficiently reliable test method that is not itself harmful.
• There must be an effective treatment proven to be more successful when started before symptoms occur.
• There should be neutral information available for everybody to decide individually whether to have a screening test or not.

The WHO points out that detecting a disease early does not automatically have a benefit. It must be proven that early diagnosis and treatment lead to better results. If this is not the case, an early diagnosis unnecessarily prolongs the time someone is both worrying and having treatment – without having any benefit from it. This may sound trivial, but we should always keep this in mind. Sometimes doctors also tend to be too optimistic when it comes to the benefit of screening.

Let us take the following (imaginary) example researchers led by the Max Planck Institute for Human Development in Berlin have developed. The diagram shows two groups of people who all died from cancer at the age of 70 – regardless of whether or not they had done screening tests. In the group without screening the cancer was detected at 67 years, three years before their death. In the group with screening, on the other hand, cancer was already detected at the age of 60. If you only look at the number of participants who were alive five years after their cancer was diagnosed, screening seems to be highly effective. In reality, however, all people died at the age of 70 – with or without screening. So in this case screening did not make their lives longer, it just led to earlier diagnoses.

 If the screening had been effective, people who participated would live longer than 70 years: they would benefit from the screening. The benefit of a screening might also be judged by quality of life or other factors – the important thing is that the screening has a benefit.

How can you find out whether a screening test has a benefit?

Researchers use different scientific methods to evaluate screenings. For example they do trials to find out

• how reliable the testing method is (test accuracy),
• whether a patient has a benefit from starting treatment earlier rather than later,
• which groups benefit from early diagnosis and treatment, and
• how benefit and harm relate to each other.

Test accuracy

Screening usually has two examination stages. In a first step, the screening test finds out whether there are signs of the disease in question or not. If the test does not find any signs, no further action is taken. If the test does find a sign, you can usually decide whether more examinations should be done to find out more. After these additional examinations the doctor can make or rule out a diagnosis. A screening test result is called “positive” if there is a sign of a disease, and “negative” if there are no signs.

If there was a perfect screening program, all participants would get a correct and reliable result in the first test. This means:

• All people who have a “positive” test result really are ill (this is called a “true positive” result).
• There would be no “positive” test results in people who are definitely not ill (there would be no “false positive” results).
• All people who have a “negative” test result are definitely not ill (they have a “true negative” result).
• There would be no ill person with a “negative” test result (the screening program would find all people who have the disease, which means that there would be no “false negative” results).

But no testing method is this perfect. In scientific terms, a test that very reliably recognizes people who have the disease as “true positive” has a high sensitivity. A test that clearly classifies people who do not have the disease as “true negative” has a high specificity. You can read more about test sensitivity and test specificity in the glossary by clicking on these terms.

The following (imaginary) example illustrates what this means:

Imagine 10 people who do not have any symptoms. The two people highlighted in a dark color have cancer:

If there was a perfect screening test and all 10 people would be tested with it, all people drawn in a light color would have a “negative” test result (-) = “no cancer”. The two people highlighted would have a “positive” test result (+) = “cancer found”. The results of the screening would look like this:




If the test was unreliable the result would look different. Let us assume that the screening test shows a “positive” result in only 50% of all people who have the disease, and produces a false alarm in 50% of the people who do not have cancer. In our example, this would mean that one case of cancer is not detected and four people who do not have cancer wrongly get a “positive” test result:





Such an inaccurate test would cause unnecessary worry for the four people with “false positive” test results. Waiting for the diagnosis without knowing can be very distressing. In addition, a certain diagnosis may require further examinations like taking a tissue sample, for example. This can be associated with additional stress or complications. “False positive” results can also lead to unnecessary treatments.

The person who had no signs of cancer in the test, although he or she does have cancer, does not get any treatment, and probably has a false sense of security. Doctors might also not react to any symptoms because they exclude cancer as their possible cause. The person might then get worse care than would have been the case without the screening.

Reliable trials are needed to assess whether a test is accurate enough to be used in screening. Yet sometimes testing methods that often have “false positive” test results can be useful too. They help narrow down the group of people who need further examinations. It is important that you are aware of this: A first “positive” test result does not mean that you really are sick. In skin cancer screening, for example, the great majority of suspicious areas of skin turn out to be harmless in the following examinations.

Early or late treatment

Detecting a disease before it causes symptoms does not always have a benefit for the person affected by it. To benefit from screening a treatment has to be available that is more effective when started early – that is, before any symptoms occur and you would have gone to see a doctor anyway. Or there must be other reasons why an early diagnosis has a benefit for the person affected – for example, some people would like to know whether they will get a hereditary disease or not to be able to make certain decisions early, or to take certain measures.

To find out how effective treatments are, conclusive comparative trials are needed, particularly randomized controlled trials. In this kind of trial, volunteers are randomly assigned to 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). More than one trial is usually needed to draw reliable conclusions from the data.

These trials can also be used for comparing early treatment to later treatment. If the trials show in the end that patients who got treatment earlier have better results than the ones 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. The risk of many diseases depends very much on age. This has to be taken into account in screening tests. How high the risk of getting the disease is will have a great effect on whether it is worth taking part in a screening. 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 out of 10,000 people who are older than 60 years, 500 have the disease in question, it might make a lot of sense to screen all people over the age of 60 years. Very large statistical surveys are needed to identify the groups of people with a higher risk of certain diseases.

Balance of benefit and harm

A screening test can have a benefit

• if there is a reliable testing method,
• if it is known who has a high risk of getting the disease, and
• if trials have shown that earlier treatment is more successful than a later one.

But to be sufficiently sure the screening test itself needs to be tested in trials.

Randomized controlled trials are also done to assess the benefit and harm of a screening method. In this case one group takes part in screening regularly, while the other one does not.

This kind of screening trial is about testing the benefit and harm of the screening. It is not concerned with the question how many of the test results are “false positive”, but about whether the screening also has health benefits for people participating. The benefit and harm of a screening test can only be weighed up if trials are done that observe participants throughout the entire screening chain – starting with the first test and continuing during the following examinations and treatments.

How can my doctor help me decide about having a screening test?

Decisions are easy to make for some tests, but this is not always the case. It may help to get advice, for example from a doctor. Experts can help to individually weigh the advantages and disadvantages of a certain examination. Independent and science-based information like that found on this website can also help. 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 not based on high-quality trials, that does not talk about the benefit and harm of screening tests and that does not express the extent of benefit and harm in clearly understandable numbers is not a good basis for making decisions. In addition, it can be helpful to find out how other people come to their decisions and how they cope with the anxieties associated. You can read some interviews with patients here.

Here are some questions you can ask if you are thinking about screening:

• Is the probability of getting the disease without noticing it so high at my age that a screening seems to make sense?
• Would I have health benefits in the long term if a certain disease was detected early? Is there any evidence, for example, that I might live longer?
• What adverse effects might be associated with the screening test, with possible examinations following the screening and with treatment, and how frequent are they?
• How frequent are “false positive” test results (false alarm) and “false negative” test results (undetected diseases)? Are there any more examinations if I have a “positive” test result?
• What other decisions will I have to make when I get the test result?
• How long do I have to wait for the test result and who can I talk to if I have questions or concerns?
• How often do I have to do a screening test to have a benefit?

Screening tests are usually voluntary. Only if there is an epidemic of a dangerous contagious disease, for example, may testing be compulsory. In Germany, taking part in screening tests is voluntary. Since 2008, however, in some screening programs consultation is obligatory for certain groups of people. This means that you should have your doctor inform you once about a screening test and its advantages and disadvantages. After that you can decide whether you want to do it or not. People who refuse to have their doctor inform them about these screening tests will have higher co-payments if they do get a certain type of cancer later. To find out for which tests this is the case and which screening tests are paid for by statutory health insurance funds in Germany click here.

Regardless of whether you have a screening test or not: If you have symptoms that worry you it is important to take them seriously. A screening cannot prevent a disease from occurring. Even the best screening tests do not guarantee health. All of us have to live with the fact that we may get sick. We should not be overly worried or rack our brains about diseases we do not have and probably never will.

You can find more information on screening tests here. If you want to stay up-to-date with our latest publications on this topic, you can subscribe to our newsletter here.

Author: Institute for Quality and Efficiency in Health Care (IQWiG)

© IQWiG (Institute for Quality and Efficiency in Health Care)