Health screening for all sounds an entirely benign idea that nobody could fail to welcome.

The reality, however, is more complex and more nuanced. The NHS already has admirable screening programmes, based on clear calculations of costs and benefits.

The reason they have so far been limited to breast, cervical and bowel cancer is not because the NHS has lacked the “vision” displayed by the Prime Minister.

Wider screening has often been proposed but on close examination has been shown to be poor medicine or a poor use of resources. That may be changing, but patients and taxpayers would be wise to look carefully into the mouth of this particular gift horse.

For effective screening, there must be a reliable test that detects the early stages of a disease. The test must be good enough not to throw up false alarms or to miss more than a small fraction of real cases.

There must be a treatment that can prevent the disease developing - or all that is achieved is telling people a little sooner that they are dying of something uncurable.

And the benefits of screening must exceed the risks. What risks are there, people may say, in simply getting the once-over from a GP or a practice nurse?

The risks are, first, that of creating an army of “worried well” — people prematurely categorised as diabetic, for example — whose psyche is altered by the diagnosis without any subsequent benefits.

Second, and more important, the risks are of exposing people to a battery of further, more invasive, tests to confirm the diagnosis. Screening may put people on an endless cycle of hospital visits, biopsies, anxiety and expense. The US medical system is notorious for doing this.

What the Prime Minister appears to be promising is not a nationwide invitation to be screened — such as occurs with breast and cervical cancer — but the right of patients to ask their GPs for such screening if they want it.

For most patients, this will be nothing new. GPs and practice nurses are only too willing to take blood samples to test for glucose tolerance, cholesterol levels and liver function, and to measure blood pressure, which gives advance warning of strokes. Patients at good GP practices have only to ask.

So what, exactly, is different? Mr Brown is offering patients the “right” to something that is already available. That could ginger up backsliding practices to do better, but if it depends on patients asking, it will mainly benefit the middle classes. They are far better aware of their rights, and more insistent on demanding them, than the poor, who probably need them more.

There is also no measure of cost-effectiveness. Will the National Institute for Health and Clinical Excellence (Nice) be asked to look at the issue before it is implemented?

And will the National Screening Committee’s advice be sought? In the past, it has been highly sceptical of the value of population-wide screening for diabetes and kidney disease, for example.

The truth is that the NHS has an outstanding record in implementing only proven screening programmes. In his desire to make a splash on the 60th anniversary of the NHS, Mr Brown appears either unaware, or dismissive, of this record.

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