Washington ? The White House is warning that anthrax field tests widely used since last fall’s attacks give fast but often incorrect results, prompting authorities to shut down buildings prematurely and hand out unneeded antibiotics.

In a memo being sent Monday to more than 250 federal agencies and to firefighters, police and local officials across the country, authorities say none of the commercially available field tests are reliable. They advise federal agencies to stop buying them and to cancel any contracts that are pending.

“This equipment does not pass acceptable standards for effectiveness,” said the memo from John H. Marburger III, director of the White House Office of Science and Technology Policy. “Field testing … is not recommended and should not be used.”

The advisory comes after an extensive study of the tests by the Centers for Disease Control and Prevention for the FBI. The study, the first of its kind, found that all tests on the market were prone to miss small amounts of anthrax and to detect anthrax when there was none there.

The memo advises authorities to send results to a CDC-approved lab, where they can get initial readings within six hours. A 17-page set of guidelines offers detailed suggestions for how to handle suspicious mail, warning agencies not to take “dramatic actions” before figuring out whether the threat is credible.

The guidelines also recommend that federal agencies stop routinely testing their mailrooms for anthrax, given that most mail is being irradiated, low levels of anthrax do not pose a significant risk and the tests used are not reliable.

The field tests which cost about $35 each are designed to quickly determine whether a suspicious white powder could be anthrax, and hundreds of thousands of them were sold during and after last fall’s attacks-by-mail.

But false results cause real problems, officials say.

In May, for instance, field tests indicated anthrax in the mailrooms of the World Bank and the International Monetary Fund. The World Bank shut down the ventilation system in the entire building and sent 1,200 workers home because it was too hot to work inside. The IMF gave about 100 people antibiotics.

In the end, anthrax was not confirmed at either location.

“Bad information is worse than no information,” said Dr. Michael Osterholm, a bioterrorism expert at the University of Minnesota who has been serving as an adviser to HHS.