Virtual Medical Centre

Cancer Screening

• What is a screening test?
• What makes a good screening test?
• Which diseases are good to screen for?

What is a screening test?

A screening test is any investigative method that can be applied to a seemingly healthy patient for the purposes of diagnosing underlying conditions that may have otherwise gone unnoticed. The purpose of such a test is to allow for treatment of these conditions to be started as soon as possible, leading to better outcomes. A screening test is not meant to give a definite answer about a diagnosis, but rather is just there to indicate if there should be further tests. Usually a screening test will take place when a patient presents to their GP for another purpose. This is because the purpose of screening is to pick up people who are not yet showing symptoms of the disease rather than waiting until they show symptoms. There are some instances however, where formal screening may be organised centrally by the health department.

Screening for certain cancers in people without any symptoms is an excellent public health initiative that each year saves many lives. However, in some cases there is little evidence that screening improves the quality of life or chance of survival for people diagnosed with the condition. Also, some screening tests can lead to further, more invasive, test that can have some complications associated with them. The Western Australian Clinical Oncology Group and the Australian Cancer Council have looked at all the evidence surrounding these screening tests and come up with some recommendations for screening specific cancers.

What makes a good screening test?

The reason some screening tests are not used is because no test is perfect; especially ones that are designed to be as quick, cheap and easy as is necessary to be a screening test. To help explain this idea, here are a few examples of the terms used to describe tests and work out how suitable they are:

• Sensitivity: This is the percentage of people who have the disease that will be picked up by the test. The higher this is the better, as then it is less likely the test will not pick up someone who actually has the disease.
• Specificity: This shows how good a test is at showing that a person does not have a disease. Sometimes a test can come back saying that there is no disease, when actually there is; however, the higher the specificity, the lower the chances are of this occurring.
• Positive predictive value: This is the percentage of times a person who the test says has the disease, actually does have it. Sometimes a 'false-positive' result can occur, where the test shows that the person has the disease when they actually do not. The higher the positive predictive value of a test, the lower the chance is of this occurring.
• Negative predictive value: This is the percentage of times when if a person does not have the disease, the test will show that correctly. The higher the better, as the higher it is the more likely a negative test result is to be accurate.
• Acceptability: This is not a statistical term, but rather simply pointing out that for a screening test to be good it has to not only be statistically sound (in the ways described above) but also be cheap, quick, easy to do and not too unpleasant for the patient. If this is not the case, then no one will want to do the screening test and so the who effort will be wasted.

Which diseases are good for screening?

Some diseases are good ones to screen for, and others are not. Basically, there are a couple of things about a disease that make it a good candidate for screening tests:

• The disease, and how it progresses, should be well understood. It should also have a long time when it is just beginning to become a problem but is not yet spreading or causing major trouble, as this gives a good window for when screening can diagnose it.
• The disease should be common, and also associated with a high level of mortality (death) or morbidity (loss of quality of life). It is a waste of resources to check for a disease that only a tiny fraction of people will get because it will not find very much but will cost a lot of time, effort and money that could be put elsewhere.

There should also be good treatment available for the disease, otherwise what is the point in looking for it? A screening test should be to look for diseases which can have something done about them.The testing should be acceptable, safe and relatively cheap.