If you make a diagnosis and administer the treatment, and the treatment fails to work, it’s the diagnosis that’s wrong.

I once had a 17 year old patient with low back pain referred after the GP had become exasperated by his failure to improve – oh, and by the way, his ESR was 57 (for the non-medical reader, a high ESR is indicative of inflammation, infection or malignancy).

He was very stiff so I made a confident diagnosis of ankylosing spondylitis, not least because his SI joints looked fuzzy on the X-ray.

After two trials of different non-steroidals for a month each I put him on phenylbutazone. This didn’t work either. It now became apparent that his pain w as quite localised to the L2 region – which was just off the top of his original pelvic X-ray – and further investigation confirmed that he had osteomyelitis in L2 and L3 presumably from the discitis between. Common? No.

Corollary: if a physical sign doesn’t fit the diagnosis, reconsider the diagnosis.

An Indian gentleman in his 70s presented with typical symptoms of polymyalgia (pain and early morning stiffness across the neck and shoulders) and a high ESR. His son said he had just returned from India and had been investigated for an intermittent fever, but his malaria tests had proved negative.

I gave him some steroids (prednisolone-EC 10mg daily) for a fortnight; he did not improve at all. I assumed I had not started him on a high enough dose, and doubled it for another fortnight. Nothing.

So I followed the corollary and wondered whether this was some sort of malignancy (myeloma and prostate cancer can both masquerade as PMR) and did some further tests – bloods and a bone scan. I was inspecting the latter when a message came round from A&E that he had been admitted with a paraparesis; he had lost power and sensation in both legs..

Have you remembered the fever?

The liver tests were right up the Swanee and the scan showed a hot spot at T3. Osteomyelitis (probably tuberculosis) was the diagnosis on biopsy at the neurosurgical centre.

Patients with polymyalgia respond dramatically to steroids and the ESR comes shooting down; if this does not happen, then either we have some other unrelated pathology (as above) or we are dealing with some other sort of inflammatory joint disease. Medicine is like buses, not trams. You need to be able to make detours when things are not right, not just grind to a halt.

While this is a rather medical post, I am prompted to add it having just read “The Monogram Murders” by Sophie Hannah – it’s a Hercule Poirot story in the spirit of Agatha Christie.  Poirot tries to make the nice-bit-dim policeman, Catchpool, do some thinking for himself, pointing out repeatedly that facts are facts and you cannot ignore them when they don’t fit the theory.  You must make the analysis fit the facts, not the other way about.  In both these above cases I failed to do that.  As a corollary to that I estimate that every doctor will make at least two mistakes a year that result, or might have resulted, in serious harm to the patient.  I have.