The pros and cons of screening tests

Dr David Howdijam *

Diagnostic tests are done to find out what is causing certain symptoms in a person. They are done to confirm, or determine the presence of disease in an individual suspected of having the disease.

Screening tests are different. They are done in people who are not ill or do not have any symptoms. The aim is to detect disease at an early stage, before the symptoms start showing. The advantage is that the disease, when detected, can be treated early before associated complications start developing.

Kinds of screening tests

There are many kinds of screening tests. Of these, the opportunistic screening constitute a major portion. Opportunistic screening is where someone visits a doctor for a particular reason and the person is asked to undergo additional tests other than those directly related to the reason of the visit, such as blood sugar. In many situations, a broad panel of tests are prescribed in order to screen for other health issues.

There's the population-wide screening where everyone who fulfils the criteria for the test is asked to undergo a voluntary test. An example of this could be a Hepatitis C Virus (HCV) screening test for a specific section of a population.

Are screening tests suitable for making a final diagnosis?

The medical tests done for screening purposes are not suitable for making the final diagnosis. Instead, screening tests are used to detect any abnormality first, which are then followed up with other confirmatory tests. Moreover, many screening tests, specially the rapid tests, are prone to giving false-positive or false-negative results.

An example of a screening test with its follow-up investigation is the case of stool for occult blood. In many situations, this test is done as a screening for bowel cancer. The test looks for occult (hidden) blood in the stool sample of a person, which could be a sign of bowel cancer.

When the result comes out abnormal, the bowel is examined further more closely with an endoscope and followed by biopsy. This is the only way to confirm whether the traces of blood are indeed caused by cancer, or by other non-cancerous conditions like haemorrhoids or an inflammation of the linings of the bowel.

Can screening tests prevent diseases?

It is seen that many people mistake screening for "prevention" and some even think that having regular screenings can protect them from a disease. But that is grossly mistaken. Screening tests do not prevent diseases as such. A screening test can only be considered "preventive" if it is able to determine the risk factors, or detect abnormal changes early that could latter develop into a serious condition.

An example of such preventive screening is the Pap test used for cervical cancer screening. The Pap test do not prevent cervical cancer but what is does is detect abnormal changes in the cervix early before they advance into cancerous conditions. In this case, preventive treatment can halt the disease from progressing further.

Are there disadvantages of screening tests?

Many abnormal changes do not necessarily develop into serious conditions and may even correct themselves with time without causing any health problems. Some people start worrying because of an abnormal screening test result and many contemplate going for treatment even though they might not have the disease. Detecting a disease early does not automatically have a benefit unless it leads to an early diagnosis and treatment for an improved health outcome.

What are the criteria for screening tests?

There are certain criteria that must be followed:

o Screening tests should be done only for diseases with serious consequences, so that they could have potentially clear benefits to people's health.
o The test must be reliable and precise.
o There must be effective treatment of the disease when detected early. There must be clear evidence that early detection leads to more effective treatment.
o Proper information must be made to the people for them to decide whether or not to go for the screening test.

The ideal screening test: reliable and precise

A screening test usually follows two steps. The first step aims at looking for signs of the disease in question. The results of a screening test are considered "positive" if signs of the disease are found, and "negative" if no signs are there. The second step involves doing further tests to enable a more accurate diagnosis. But this second step is done if the first test reveals abnormal findings or when the result is positive.

No test is perfect. A test that reliably recognises 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 correctly identifies people who do not have the disease as "true negative" has a high "specificity".

Ideally, all screening tests should have high sensitivity and specificity. An inaccurate test would cause unnecessary worry for people with false positive test results. On the other hand, a false negative result might cause a delayed treatment and give a sense of false security.