It was a scene I remember all too clearly even though nearly five decades have hurried past. I was sitting on a bus in the city centre. Directly in front of me sat a woman whose neck I can picture perfectly even after all these years. It was the suspicious looking lesion just below her hairline that had caught my attention.I felt it could well be a malignant melanoma. What should I do? Should I tell her? I was a medical student at the time. Just as I was pondering my next move the bus stopped and she got off.

I remember going back to medical school and discussing it over coffee with a group of friends and colleagues and none of us could completely agree as to what I should have done. What would you have done?

It is a situation that I have often thought about over the years, but one thing has definitely changed. In those days I seemed to be absolutely certain about my diagnosis. These days I would be nowhere near as sure. In the words of Bob Dylan, ‘but I was so much older then, I’m younger than that now.’

As I have written all too often, general practice is an extraordinarily uncertain science and art, and I sometimes felt that the more experience I had, the less certain I became. Some years ago I recall going to an educational session on the diagnosis of pigmented lesions. The dermatologists running the meeting showed us slide after slide of lesions that looked malignant but were actually benign, and lesions that looked benign and were actually malignant. To this day, I have no idea what their educational objectives for the evening were other than to increase our referral rates, because I certainly found my diagnostic confidence seriously challenged.

I don’t know about you, but minor skin lesions were typically one of the most common ‘while I’m here, doctor’ consultations, often being shown just as the patient is about to leave the room. ‘Could you have a quick look at this?’ they ask, and unless I was 100% certain that it really was a wart or a harmless papule, the simple fact is that all too often I honestly was not sure.

I definitely never retained the diagnostic certainty that I had as a medical student on that bus. At least with dermatological lumps and bumps, biopsy or excision can address the uncertainty once and for all.

Wouldn’t it be wonderful if we could surgically excise all those other causes of medical uncertainty and put them under the microscope? Wouldn’t it be marvellous to have a simple histology report that confirmed the diagnosis was unhappiness rather than depression? What would we give for a blood test that gave 100% certainty that this patient’s symptoms really were a cold rather than a chest infection?

It never fails to astonish me that the most common conditions are the very ones that we are the least certain about.

It’s the commonplace problems that can be challenging – a phrase that you probably never ever heard during your medical education.

However clever our technology might become, universal diagnostic certainty is unlikely ever to happen. One thing that we can all be certain of is that there will always be an infinite variety of human experiences and a finite number of diagnoses.

We are going to be stuck with uncertainty for a very long time to come.