Even by the standards of news media that insist on a Daily Breakthrough, this week has been medically miraculous - particularly if you're worried about Alzheimer's. Statins may help to treat it. So might blood pressure pills. So, too, a drug normally used for rheumatoid arthritis. Ditto one that untangles knotted neurones.

Interesting stuff. But maybe not as interesting as the revelation that, if you think you have Alzheimer's, you probably don't. Whereas if you're adamant you don't, you possibly do.

The explanation? Imagine this scenario. You're an hour late for your appointment because you're sure it was booked for midday, when I should be slurping coffee, not 11am, when I was busily typing “did not attend” into your notes. You last saw me four weeks ago and insist that I was a female doctor who gave you tablets the name of which escapes you. And you've forgotten why you booked to see me in the first place.

“Ah, that's it,” you exclaim. “My family's worried I've got Alzheimer's. But I know I haven't.” Two things occur to me. First, I mustn't forget to refer you to the local memory clinic. And secondly, this is a classical case of a patient being “in denial”.

Denial comes in various shapes and sizes, from the morbidly obese who blame their glands, to adolescent diabetics who would rather let their sugars reach coma levels than admit to their condition.

Even more common in general practice is the opposite of denial: insisting that you have a certain disease. This phenomenon is so rarely aired that it doesn't even have a name. So let's go with being “in insistence”.

Here's how it might work with Alzheimer's: the patient “in insistence” that she's dementing in fact isn't. Why? Because patients with Alzheimer's typically lose insight into their problems. That's why they're dragged disbelievingly to the GP by frazzled relatives. Whereas patients who insist that they've got it will inevitably have some harmless cause for their mental state, such as having to cope with the kids during the school holidays.

Not convinced? OK, try, “I'm depressed, doctor.” I guarantee the diagnosis won't be depression. It will be a reaction to being dumped last week by your boyfriend - something we GPs refer to as “unhappiness”. Patients “in insistence” want the quick fix of happy pills, whereas what they actually need is a shoulder to weep on.

As for patients with real depression, they never use the “D” word. They'll admit to feeling tired, sleeping poorly, going off sex or not caring that the football season's about to start. Anything but depression, in fact - possibly because they're scared of the treatment and so don't want their doctor leaping to the correct conclusion.

Not that these unwritten rules are limited to dementia or depression. Whatever your symptoms, the more certain you are about the diagnosis, the greater your chances of being hopelessly wrong. Convinced you've got flu? It's a cold. Certain it's ringworm? Eczema. Reckon it's chicken pox? Insect bites. Think it's insect bites? Chicken pox.

Even on those rare occasions when patients “in insistence” are spot on, it does them no good. Because doctors universally adhere to the golden rule that the punter with diagnostic conviction must be incorrect. Plus GPs have a professional obligation to resent and ignore patients who think they know a bit about doctoring.

The message is clear. Being in denial isn't good, but being in insistence is worse. So if you're convinced, say, that you've got diabetes, you should casually and innocently volunteer the classical symptoms, but bite your dehydrated tongue at the crucial moment, and let your GP think he's reached his own brilliant diagnosis. If he fails to take the bait, prompt him by whipping out a urine sample.

And if he still looks blank, tell him you're concerned it's Alzheimer's - and that he should book himself into the memory clinic.

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