Advantages and disadvantages of screening tests

A lot of research has been done on a number of screening tests and they were found to have the following potential advantages:

• They can prevent a disease (preventive testing): This is true of bowel or cervical cancer screening, which look for benign (non-cancerous) early signs of cancer. If it finds any and they’re remedied, the cancer can be prevented. So the screening means you don‘t suffer the mental stress of being diagnosed with cancer and you don‘t receive treatment that can sometimes have severe side effects.
• Although the tests can‘t prevent the disease, they can improve your chances of only needing low-impact treatment and of recovering. One example is breast cancer screening, which can detect breast cancer before it spreads to other parts of the body.

But even well-studied screening methods aren‘t perfect. For instance, they usually reduce a person‘s risk of dying of cancer by 30% to 60% but they don’t eliminate it completely.

Like a drug, screening has numerous possible side effects. But it‘s important to know about the potential disadvantages in order to make the right decision for you. It can also help you cope better with any disadvantages that affect you.

The disadvantages can be split into the following categories:

1. The screening test can have side effects or cause complications like injuries.
2. The test delivers an incorrect abnormal result (false positive result). This can cause unnecessary worry and stress.
3. The test detects the disease but the person’s chance of recovery doesn’t improve as a result.
4. There hasn’t been much research on the method concerned so it’s not clear whether it offers any advantages and whether they outweigh the potential disadvantages.

The following sections explain the disadvantages in these categories in more detail.

The potential side effects and complications depend on the test method used. Taking blood for a blood test can cause bruising, for instance. And a colonoscopy can cause injury to the colon (though this is rare).

In Germany, screening programs are only allowed to use x-rays and computed tomography (CT) scans if they‘ve been authorized by the Federal Office for Radiation Protection (Bundesamt für Strahlenschutz) because they pose a small risk of cancer. Currently, the only authorized test is the mammography examination used in breast cancer screening.

The word “ screening” comes from the technical term for sifting or sieving. The idea is to “sift” through a large group of people with no symptoms to find the ones that do have the disease.

That can be quite difficult because diseases like cancer are – thankfully – not that common. Experts on the screening program for breast cancer, for example, assume that around 8 out of 1,000 women aged 50 or over have the disease without knowing it.

So the aim of the screening is to:

• correctly identify people who do have the disease and
• correctly identify people who do not have the disease.

But, in reality, there are two types of mistake that can happen:

• The screening misses someone who does have the disease. The medical term for this is a “false negative result.”
• Sometimes the result is abnormal although the person doesn’t actually have the disease. This is referred to as a “false positive result.”

To rule out false positive results, the screening usually has two parts. The first one checks for abnormalities or signs of the disease in question. If it finds anything, a second part is carried out to determine whether the person does actually have the disease. An example would be a mammography examination that finds signs of breast cancer. This would be followed up by a biopsy, which is the only way to find out whether it is actually cancer.

The fact that tests can produce false-negative results means it‘s particularly important to take seriously any symptoms that could be a sign of a disease – even if you‘ve only just been for screening.

Getting a positive test result and a confirmed diagnosis can be quite perplexing. A diagnosis is always bad news – especially if it‘s cancer. On the other hand, providing a diagnosis is precisely the point of screening – it‘s meant to detect diseases earlier in order to improve the chance of recovery.

But cancers behave in different ways so early detection doesn‘t necessarily mean more effective treatment or, in an ideal scenario, complete recovery.

The aim of cancer screening is to identify tumors before they spread to other parts of the body (metastasis). If they do spread, it‘s very difficult to treat them and the chances of recovery are lower. But there are four different types of tumor, and screening is only really of benefit for the first one:

• Screening can improve a person‘s chance of recovery if the tumor is one that only grows in one place (locally) for a relatively long time (first type). A screening test during that time can detect the tumor and then it can be successfully treated.
• The second type of tumors, on the other hand, spread at an early stage (metastasis). Often, screening does not find them in time. This means that the diagnosis is made earlier but the chance of recovery doesn‘t improve. In other words, having a screening test doesn’t mean the person lives longer. They just learn that they have cancer earlier.
• Some tumors don’t spread at all or not until much later, which means they can still be successfully treated even if the person already has noticeable symptoms (third type). So screening doesn‘t affect the chance of recovery in these cases either.
• Some types of tumor grow so slowly that there are never any noticeable symptoms (fourth type). When screening detects this type of tumor, the result is what‘s known as “overdiagnosis” – a diagnosis that wouldn‘t have any impact on the person‘s health.

The problem is that when a tumor is found it‘s usually not possible to say for definite in advance what type it is or how common each type is. For instance, experts assume that half of the prostate tumors detected by screening would never be noticed if screening wasn‘t carried out.

Good-quality research on screening is necessary to assess the advantages and disadvantages. Research has been done on most of the screening tests that public health insurance covers.

Research on the advantages and disadvantages of screening isn‘t only about the accuracy of screening tests. The most important question is whether they have any health benefits for the people who go for screening.

The benefits and harmful effects of screening can only be weighed through comparative studies. They entail one group having regular screening tests, while the other group does not. At the end of a study, the researchers look to see if, for example, less people in the first group died of the disease or whether screening caused more complications than in the other group.

Randomized controlled studies, in which the participants are randomly assigned to a specific group, deliver the best results. Studies of this type have been conducted for numerous screening tests, like mammography and colonoscopy examinations.

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 usually screening services for which no good-quality research is available on the benefits and harmful effects of testing or for which good-quality studies haven’t found any advantages. They include ultrasound examinations used in ovarian cancer screening, for example. You have to pay for these tests yourself. The public 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).

As early as the 1960s, the Word Health Organization (WHO) drew up good practice requirements for screening programs in response to the disadvantages described above. The WHO criteria include the following:

• Screening should be done only for diseases with serious consequences, so that screening tests can 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 an early diagnosis followed by treatment doesn’t result in a better outcome than a late diagnosis, the early diagnosis merely extends the person’s anxiety – unnecessarily because they don’t benefit from earlier treatment anyway.