Screening saves lives, but is not perfect

The following piece, by Irish Cancer Society Head of Research Dr Robert O’Connor, was first published in the Sunday Business Post on 29 April 2018.

There has been much coverage of the cervical cancer screening programme as a result of the case of Vicky Phelan, who was given incorrect results following a smear test and who is now terminally ill. Tests such as smears are a vital part of identifying if a cancer is present, or if there are conditions which, if left untreated, might lead to a cancer.

We have made significant technological advances in recent decades that have improved the sensitivity and specificity of such tests. But no medical test and no cancer test is 100 per cent reliable, or without errors, and the fact is that while cervical screening saves lives, it is not infallible.

A prestigious Cochrane review published into studies of the test from around the world found that for every 1,000 women screened, around 20 will have precancerous changes. But the review concluded that the test would only identify 12 of the women, missing eight of them.

Generally there are two types of error in screening tests. A false positive test occurs when a test falsely finds something abnormal to be present when it actually isn’t.

False negative tests are where the result doesn’t find anything out of the ordinary, when in fact there is. A false negative can lead to a belief that nothing is wrong when, in fact, a cancer can get worse and might require treatment.

These mistakes occur for a number of reasons: administrative, technical or due to human error, where the tester doesn’t pick up positivity. We are all human, and even in these serious situations, mistakes do happen – although they are rare.

When it comes to cervical screening in Ireland, it is important to note that the vast majority of tests are accurate and error-free. Screening saves lives by detecting cancers early, when treatment is most effective.

Each year in Ireland, 300 women are diagnosed with cervical cancer. Thanks mostly to screening, almost half of them receive their diagnosis at stage 1 of the disease, with 95 per cent of these women surviving for at least five years post-diagnosis.

In the case of more than 6,500 women, advance precancerous changes will be found which will need treatment to reduce the chance of them becoming a cancer.

Almost all cases of cervical cancer are caused by strains of the HPV virus. During intimate contact with an infected partner, the virus can find its way to the cervix.

If a woman isn’t immune or vaccinated, the virus will infect cells and can cause those cells to start to change. Those changes can be subtle, but over time can become bigger if the virus doesn’t clear. It takes several years for these changes to start to become cancerous, and the smear allows intervention before a full cancer occurs.

The smear is a lifesaving cancer preventative test, but it also involves a number of inherent challenges. First, cells must be taken from the cervix, and a human being – usually two human beings – must examine the cells under a microscope and look for very subtle differences. This process requires very significant skill and training, but mistakes can be made.

Second, only the bottom of the cervix tissue is routinely accessible with the basic test and although it is much less common, the HPV virus can infect cells deeper inside the cervix, an area which is not readily accessible during a routine test.

New technology now allows for testing for the presence of the HPV virus itself. This forms the basis of the HPV test that CervicalCheck is currently transitioning to.

HPV tests are sensitive and mostly automated. They also have a higher chance of picking up these deeper HPV infections. However, HPV can be present without there being any abnormal changes in cells, so the test has a higher false positive rate.

Because of this, where the result is positive, the HPV test is used in conjunction with direct examinations to confirm if there are any abnormal cells or cancer present.

As in all parts of medicine, errors occur, and a culture of openness and continuous quality improvement is paramount to ensure that this lifesaving test continues to reduce the chances of someone dying from cervical cancer as much as is humanly possible.