Abnormal Result

Every two years, people in my age group are regularly invited to take a test for bowel cancer.  It's all wonderfully simple.  You do it yourself in the privacy of your home and post off a few small samples of poo, which are then tested for traces of blood that are invisible to the naked eye. 2% of tests will yield a positive result and this will trigger an investigation to find out what is causing the bleeding.

My test last week was inconclusive, so I was invited to do a re-test.  Today I learned that this second result is abnormal, so I've been called into Gloucester Royal Hospital for a consultation, which will probably lead to a colonoscopy.

The letter I was sent clearly states that most abnormal results are not caused by cancer.  The statistics break down something like this:

• 50% of those having a colonoscopy will have a normal result.
• A further 40% will have polyps, which are then removed as this reduces the likelihood of cancer developing in the future.
• The remaining 10% will be found to have cancer. 

So there's a 90% chance that I don't have bowel cancer.  And if I do, early detection will have greatly increased the probability that it can be successfully treated.

But the statistics can be run the other way round. 17-20% of UK citizens will develop bowel cancer at some stage in their lives.  So, since only 10% of abnormal results are cancerous, the chances of an abnormal result at some stage in ones lifetime must be pretty high.  I'll leave others to work out the mathematical probability.