For the last six weeks, Walter Reed National Military Medical Center has been engaged in a highly unusual effort to identify an individual who is H.I.V. positive but was wrongly informed that he or she was H.I.V. negative after a mix-up of blood samples taken at the hospital.

The mistake occurred in late October when the military’s flagship hospital, in Bethesda, Md., sent 150 blood samples to a contract laboratory for analysis. One sample tested positive for H.I.V., hospital officials said, but it was wrongly labeled with the name of a patient who subsequent tests showed was not infected.

A hunt is now underway to identify the infected person, who may be in need of treatment and could be unknowingly infecting others through unprotected sex or the sharing of needles.

Hospitals are supposed to have strict safeguards to ensure the integrity of laboratory specimens because the consequences to patients are potentially life-threatening. Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University, said the episode raised questions about the adequacy of the hospital’s procedures. “How sure are they that this didn’t happen before?” he asked.