Working out the risk factor

Medical Matters:"We see things not as they are but as we are." - Kant

Most of us are aware that travelling by car is more dangerous than taking a plane journey. Yet, while many people admit to some trepidation before boarding an aircraft, how often have you met someone who says they are afraid of getting into a car?

Both the perception and communication of risk are central to medical science. Whether it is a decision to embark on treatment or to undergo a clinical investigation, consideration must be given to the accuracy of the test and the side effect profile of a drug or medical intervention.

Take the unfortunate case of Rebecca O'Malley, the Tipperary woman who underwent a mastectomy and chemotherapy for breast cancer 14 months after she was told that a breast lump was benign. Although all the facts have yet to be established, it appears that a human error was made in the interpretation of a tissue sample following a fine-needle biopsy of the lump.

There may also have been a systems failure because, contrary to best practice guidelines, the tissue sample was assessed by a pathologist working at a separate hospital from the one where Mrs O'Malley underwent clinical assessment. If she had had a triple assessment by a multidisciplinary team in a specialist breast unit, medical experts said the risk of a misdiagnosis would have been minimised.

Note the use of the word minimised. Not avoided, prevented or stopped. In other words, even in a best case scenario, certainty cannot be guaranteed.

As modern consumers, there is a tendency for us to assume that medical tests are 100 per cent accurate. But they are not: every single blood test, X-ray and biopsy result can be either falsely positive or falsely negative.

A person with a false positive result does not have the disease even though the test suggests they do. This means they are likely to undergo further investigations that are unnecessary or even dangerous.

False negative results occur in people who have the disease, but the test suggests they are disease-free. At a minimum, it means a delay in the accurate diagnosis of the disease; it may even result in the need for more extreme treatment than would have been the case if the correct diagnosis had been made in the first place.

The National Quality Assurance Group for Symptomatic Breast Disease Services acknowledges this reality when it suggests thresholds that would be acceptable when cells from a fine-needle breast biopsy are being analysed. The minimum acceptable false negative rate is less than 6 per cent while the minimum acceptable rate for false positive results in a specialist centre is less than 1 per cent.

This is an honest acknowledgement that even in conditions of best practice, up to one in 20 women may be told that they do not have breast cancer following a fine-needle biopsy, even though in reality they have the disease. (The group also advised a greater use of more invasive core biopsies to reduce the risk of a misleading result.)

Research confirms the fallibility inherent in the interpretation of test results. A group of pathologists was asked to microscopically assess 1,000 slides containing biopsy material from the cervix (neck of the womb) and to then repeat their analysis some time later.

Each pathologist agreed with himself some 89 per cent of the time, while senior and junior pathologists agreed with each other in just half of cases. The study found the doctors did particularly well in distinguishing clearly cancerous tissue, but were less able to accurately identify borderline or pre-cancerous lesions.

Yet, the perception is definitely out there that once a doctor tells you a test is normal, that this is absolutely the case. When more than 1,000 British women were asked what it meant to be told that a cervical smear test was normal, just 50 per cent correctly understood it to mean there was still a small but residual risk of cervical cancer.

The researchers recommended that the reporting of a normal smear be accompanied by a sentence stating this result means a low risk of having or developing cancer of the cervix in the next five years.

Although it might seem that the careful use of statistics would help to address the problem, evidence suggests otherwise. In one study, participants rated death rates of 1,286 out of 10,000 as more risky than rates of 24.14 out of 100.

So clearly we have a problem with both risk perception and risk communication. There is some evidence to suggest that patients prefer illustrated bar charts as a way of absorbing risk information. Others have argued that comparing health risks with everyday risks would help.

But I suspect that it is individual patient perceptions that are crucial, rather than bold figures or professional opinion. The most important determinants of how we interpret risk for ourselves may have to do with how we perceive the risk, the burden it places on us and the timeframe over which we will have to live with the risk. Acceptable risk is very much in the eye of the beholder.

Dr Muiris Houston is pleased to hear from readers at mhouston@irish-times.ie but regrets he is unable to reply to individual medical queries.