Every year, about 50 US blood donors test positive on the standard HIV screening tests, even though they are not infected with the human immuno deficiencyvirus, researchers report. A certain pattern of test results may give a false-positive reading in as many as 1 in every 100,000 cases, according to the researchers, Dr. Alan E. Williams, of the American Red Cross Greater Chesapeake and Potomac Region in Baltimore, Maryland, and colleagues.