Mammograms and PSA tests are becoming more routine these days – and they are supposed to help us know what is happening to our bodies so we can catch things early.
But do they just make us worry more?
If you are a woman you may have read that 90% of women with breast cancer get a positive result from a mammogram and thought that therefore 90% of women with positive results have breast cancer. This is not true!
The high false alarm rate, combined with the disease’s prevalence of 1%, means that roughly nine out of 10 women with a worrying mammogram don’t actually have breast cancer.
It’s a maths puzzle many of us would struggle with. An author in the USA, Gerd Gigerenzer has produced a book that tries to clarify things. He campaigns for risks to be expressed using numbers of people instead, and when possible diagrams like this:
Even so, Gigerenzer says, it’s surprising how few specialists understand the risk a woman with a positive mammogram result is facing – and worrying too. “We can only imagine how much anxiety those innumerate doctors instill in women,” he says. Research suggests that months after a mammogram false alarm, up to a quarter of women are still worrying about the result on a daily basis when they don’t need to.
Survival rates are another source of confusion for doctors, not to mention journalists, politicians and patients. These are not, as you might assume, simply the opposite of mortality rates – the proportion of the general population who die from a disease. They describe the health outcomes of people who have been diagnosed with a disease, over a period of time – often five years from the point of diagnosis. They don’t tell us about whether patients die from the disease afterwards.
Take prostate cancer. In the US, many men choose to be screened for prostate-specific antigens (PSA) which can be an indicator of the disease. In the UK, it’s more common for men to get checked only after they start experiencing problems. Consequently, they are diagnosed with prostate cancer later, and are less likely to survive for five years before dying – but this doesn’t mean that more men die.
Moreover, many men have “non-progressive” prostate cancer that will never kill them. While screened American men in this situation are marked as having “survived” cancer, unscreened British men aren’t. These two facts explain why five-year survival rates of prostate cancer are much higher in the US than in the UK (99% rather than 81%), while the numbers of deaths every year per 100,000 men are almost the same (23 in the US, 24 in the UK).
Again a diagram can be used to show that the risk of death is the same whether men are screened for prostate cancer or not:
So when it was stated that someone’s chance of surviving prostate cancer in the US was twice that of someone using the National Health Service, this was wrong. And when, in 1999, there was a furore about Britain’s survival rate for colon cancer (at the time 35%) being half that of the US (60%), experts again ignored the fact that that the mortality rate was about the same.
Patients have many misconceptions about health risks. Gigerenzer and his colleagues asked over 10,000 men and women across Europe about the benefits of PSA screening and breast cancer screening respectively. Most overestimated the benefits, with respondents in the UK doing particularly badly – 99% of British men and 96% of British women overestimated the benefit of the tests.
A quarter of British women went so far as to guess that 200 women out of every 1,000 screened have their lives saved by mammograms. But Gigerenzer says the real figure is about one woman in 1,000 – four out of every 1,000 screened women die from the disease, as opposed to five out of every 1,000 unscreened women. He says that this benefit has been represented as a “20% mortality reduction”, which might explain why many women in the UK seem to think that 20% of women are saved by undergoing the procedure.
Again, the culprit is the use of percentages rather than actual numbers to represent risk and benefit as in this diagram:
Perhaps the most notorious example of patients being misled about risk occurred in October 1995, when the UK’s Committee on Safety of Medicines warned doctors that a new, third-generation oral contraceptive pill doubled the risk of thrombosis. Thousands of women came off the pill, even though the risk had merely increased from a one-in-7,000 chance of getting the disease to a two-in-7,000 chance. The following year saw an additional 13,000 abortions in the UK.
It is thought to be better for a patient not to ask their GP what they would recommend but rather ‘If it were your mother or relative you were going to advise, what would you do?@
The answer may be very different. A 1993 study found that while the rate of hysterectomy among Swiss women was 16%, among female doctors and doctors’ wives it was 10%.
There are three other questions Gigerenzer advises patients to ask doctors to ensure they get all the facts:
- “What are the alternatives?”
- “What’s the benefit and what’s the harm?”
- “Please tell me this in terms of absolute numbers. If 100 take this medication and 100 people don’t, what happens after five years?”
If you can get answers to these questions then you have enough knowledge to make up your own mind about treatment … worth remembering next time you are lying awake at night worrying!