People with occasionally high blood pressure are more at risk of stroke than those with consistently high readings, research suggests.Current guidelines focus on measuring average blood pressure levels to spot and prevent the chance of a stroke.
But research suggests doctors should no longer ignore variation in test results and give drugs that produce the most steady blood pressure levels.
The Stroke Association called for national guidelines to be overhauled.
In the first of the series of studies published in The Lancet, UK and Swedish researchers looked at the variability in blood pressure readings at doctors' checks.
What we're saying is, don't discount that one-off high blood pressure reading
Professor Peter Rothwell, study leader
They found those with fluctuating readings at different GP visits had the greatest risk of future stroke regardless of what their average blood pressure reading was.
A review of previous trials also found that the differences in effectiveness of several blood pressure drugs could be explained by how well they kept blood pressure on an even keel.
Some drugs, in particular beta blockers, were shown in a separate study in The Lancet Neurology, to increase variation in a patient's blood pressure.
Professor Peter Rothwell of the Department of Clinical Neurology at the University of Oxford, who led the research, said the findings have major implications for how GPs spot and treat people at high risk of stroke.
"At the moment, the guidelines for GPs say not to believe a one-off unusual reading, to bring the patient back and measure again, and as long as it's not consistently high, there is no need to treat.
"What we're saying is don't discount that one-off high blood pressure reading."
He added that GPs would also need to make sure they prescribe the most effective drug combinations - ideally one that lowers blood pressure but also stabilises it.
It is not know exactly why occasional spikes would increase a person's risk of stroke but it is thought it puts undue stress on the system.
We urge GPs to read this research to help them prescribe the best treatment for people at risk of stroke
Joe Korner, Stroke Association
"If you get rapid fluctuations that can cause turbulent flow of blood which can cause damage and stiffening in the arteries," said Professor Rothwell.
He said anyone with high blood pressure who tests themselves at home might want to mention to their GP if they spot variations in their results.
The National Institute for Health and Clinical Excellence's guidelines on high blood pressure is in the process of being rewritten and these latest studies will be taken into account.
Joe Korner, director of communications at The Stroke Association said people who have occasional high blood pressure readings - known as episodic hypertension - are often not treated.
"With this new research it is now important that the clinical guidelines about treating high blood pressure are reviewed.
"In the meantime we urge GPs to read this research to help them prescribe the best treatment for people at risk of stroke."
Experts stressed that those already prescribed medication for high blood pressure should not worry or stop taking their pills.
Professor Peter Weissberg, medical director at the British Heart Foundation said: "Current practice is not wrong, but this might add a new measure to help doctors make decisions on who to treat for hypertension and which drug to use."