Screening

Screening refers to using an investigation to detect a disease which has not yet caused symptoms, so-called subclinical disease, with the aim of initiating management as early as possible, to improve the prognosis. Examples include:

• Regular mammography or cervical cytology in women
  • Used to detect early or precursor breast cancer or precursor stages for cervical cancer, respectively
• Faecal occult blood test in middle-aged/elderly
  • Used to detect early colorectal cancer or bleeding colon polyps
• Screening for inborn errors of metabolism and developmental dysplasia of the hip in newborns
• Non-invasive prenatal test (NIPT) for trisomies during pregnancy

Screening is indicated if the disorder has a high mortality or morbidity, and there is treatment available. Like most things in life, screening is only viable if it can be proven that the advantages weigh up for the disadvantages.

Screening tests

A screening test is most useful when it has high sensitivity. It would be great if a screening test also had high specificity, but there is often a trade-off between sensitivity and specificity. Therefore, most screening tests have low specificity. As a result, screening tests produce few false negatives but many false positives, and many more false positives than true positives.

Because many of those who have a positive screening test are false positives, positive screening test must always be confirmed by a more specific test.

Advantages of screening

Screening may have many advantages. The main advantage, and the intention behind screening, is that treatment can be started at an earlier time, before symptoms even appear, which usually improves the prognosis considerably, and may allow for curative treatment, which may not have been an option if the disorder was not diagnosed at the asymptomatic stage.

Most national screening programmes are backed up by evidence that they reduce mortality or morbidity. For example, breast cancer screening reduces risk of dying from breast cancer by approximately 20-30%.

Disadvantages of screening

Unfortunately, screening has many disadvantages as well.

Low specificity of tests causing worry and more testing

Because screening tests necessarily have low specificity, there will be many false positives. Testing positive on a screening test, especially for cancer, leads to considerable worry for a person. Considering that most people who test positive on a screening test is false positive, the positive predictive value is low as well, meaning that the chance of having the disorder when testing positive is low. This concept is very difficult for laypeople to understand.

Because screening tests must be confirmed by a confirmatory test, screening for a disease always leads to more testing. In many cases, these tests are invasive, either entailing radiation exposure (CT scan) or complicated patient preparation and discomfort during the procedure (colonoscopy). In some cases, for example with NIPT testing, the confirmatory test (amniocentesis or chorionic villus sampling) is invasive and increase the risk of harm (abortion of the foetus in this case), which may lead to harm for people who are actually healthy.

Early diagnosis may not always improve prognosis

It is reasonable to assume that early diagnosis always improves the prognosis, but that is not always the case. In many cases, the cancer would develop so slowly that the person would never have known of it, or possibly only developed mild symptoms. However, cancer diagnosis almost always leads to aggressive treatment, which has its own effects on quality of life.

This is especially important for prostate cancer, for example. Prostate cancer is relatively common in elderly, but research has shown that many who are treated for subclinical prostate cancer would never have developed symptoms of the cancer, and would rather have died peacefully, never knowing that they even had the cancer. Also important to consider that cancer treatment causes significant reduction in quality of life, which is especially unfortunate if the cancer would never have caused symptoms anyway.

South Korea started screening for thyroid cancer in the late 20th century. Up until relatively recently, research showed that, while the incidince increased significantly (more cases of thyroid cancer were discovered), the mortality remained the same. In other words, screening detected more cases and lead to more people being treated for cancer, which is a burden to both the healthcare system and the individual, but screening could not demonstrate a reduction in mortality, which is arguably one of the most important goals of screening. This shows that screening can lead to significant overdiagnosis.

In Norway, it is estimated that for every 6 case of breast cancer that is discovered due to screening, 1 of those cases are overtreated, meaning that the cancer in that one case would not have needed treatment if it wasn't screened for.

Disorders are often rare, which reduce the predictive value

As already established, pre-test probability influences the predictive value of a test. In asymptomatic people (the target population of screening tests), the pre-test probability is equal to the prevalence of the disorder. The disorder we screen for often have a very low prevalence; for example, breast cancer has a prevalence of 0,4%.

Because the disorders we screen for is so rare, the positive predictive value decreases.

A negative screening test does not guarantee absence of disease

Even though most tests used to screen populations have high sensitivity and therefore a low rate of false negatives, false negatives still occur, giving many people the impression that they do not have a disorder when they do. This may lead to them not seeking healthcare for new symptoms which are actually due to a disorder, because they think the screening test has cleared them.

Screening a population is expensive

For screening to be effective, it needs to screen many people, usually many thousands. This is expensive, money which could be used to research other fields of medicine.

References

‎