For some medical tests, radioactive chemicals are injected right into the bloodstream. That's pretty safe, but it can, ahem, be cause for alarm when a person goes through a security screen—as a commercial pilot discovered last year.

The pilot's tale of woe is related in the new issue of the Lancet, a British medical journal. To check out a possible heart problem, the pilot was given a thallium scan, a test in which radioactive isotope thallium-201 is injected into the bloodstream. The radioactive isotope travels to the heart so the cardiologist can tell whether the heart muscle is getting enough blood.

Two days after the test, the pilot flew to Moscow, where he was pulled aside in the airport after setting off a radiation detector. After hours of questioning, security officials let him go. The same thing happened again four days later at the same airport. "After that, they sort of said, 'Oh, it's you again,'" says Richard Underwood, the cardiologist the pilot consulted at the Royal Brompton Hospital in London.

The pilot's case is only unusual because he set off the alarms so many times in one month, Underwood says. The problem has been around for a while. A 1986 New England Journal of Medicine article reported that two patients had been seized by the Secret Service after the thallium in their blood set off radiation detectors in the White House. But with new fears of terrorism, sensitive detectors are appearing in more places.

Underwood now offers to give patients a letter they can carry if they plan to travel within a month of the thallium scan, explaining what the test is and providing the phone number of someone at the hospital who could explain further. Patients who plan to travel or visit a federal building while radioactive should ask for a letter from their doctor, says radiologist Lionel Zuckier of New Jersey Medical School, who has calculated how long patients could set off hand-held radiation detectors after such tests. The thallium test is only one of many that use radioisotopes. For example, the test used to determine if cancer has spread to the bones might leave a patient radioactive for a week or so.

(Zuckier's website includes background information and a radiation calculator with a form that physicians can use to generate a letter for patients.)