Doctors should only test people for a new virus if they are very ill in hospital with a respiratory infection, have been in Qatar or Saudi Arabia and test negative for common forms of pneumonia and infections, the World Health Organisation said on Saturday.
The newly discovered virus from the same family as SARS has so far been confirmed in only two cases worldwide, one in a 60-year-old Saudi man who died from his infections, and another in a man from Qatar who is critically ill in a London hospital.
In updated guidance issued six days after it put out a global alert about the new virus, the WHO said suspected cases should be strictly defined to limit the need to test people with milder symptoms.
But it added anyone who has been in direct contact with a confirmed case and who has any fever or respiratory symptoms should also be tested.
The WHO said in a statement its new case definition was designed "to ensure an appropriate and effective identification and investigation of patients who may be infected with the virus, without overburdening health care systems with unnecessary testing."
The United Nations health agency said on Sunday a new virus had infected a 49-year-old Qatari who had recently travelled to Saudi Arabia, where another man with an almost identical virus had died.
The virus is from a family called coronaviruses, which also includes viruses that cause the common cold and SARS, or Severe Acute Respiratory Syndrome, which emerged in China in 2002 and killed around a tenth of the 8,000 people it infected worldwide.
A spokeswoman for Britain's Health Protection Agency, where scientists analyzing samples from the Qatari man found a match with the fatal Saudi case last weekend and reported their finding to the WHO, said on Saturday the 49-year-old was still in intensive care.
He is being cared for at St Thomas's hospital, where he has been connected to an artificial lung to keep him alive.
The WHO says there is so far no evidence to suggest the potentially fatal virus spreads easily from person to person. Scientists say the genetic makeup of the virus suggests it may have come from animals, possibly bats.
The WHO has been collaborating with laboratories such as the HPA and another lab in the Netherlands which were responsible for the confirmation of new virus.
"These laboratories have been working on the development of diagnostic reagents and protocols which can be provided to laboratories that are not in a position to develop their own, and these are now available," it said.
But it stressed only patients who fulfilled strict criteria - including having severed respiratory syndrome, requiring hospitalization, having been in Qatar or Saudi Arabia or in contact with a suspected or confirmed case, and having already been tested for pneumonia.
"The essence is that this is not for people with coughs and colds," WHO spokesman Glenn Thomas told Reuters.
Six suspected cases in Denmark last week turned out to be false alarms and Thomas said it was important "to alleviate the burden of testing" by ensuring health authorities and members of the public understand the criteria for a suspected case.