Two HIV tests prove better than one.

A combination of blood and urine tests for HIV detects more infections than either test alone, a new study shows.

Some HIV-positive individuals may produce antibodies to the AIDS virus in one bodily fluid but not another, occasionally enabling the virus to evade detection by a single procedure, researchers at Calypte Biomedical Corp. in Berkeley, Calif., report in the November Nature Medicine. After testing blood and urine samples from 11,334 people, they found evidence that 1,181 were HIV-positive. Of this group, 25 had discrepant tests--10 showed positive only in urine and 15 only in blood.

Thus, dual testing of people at risk of infection may ferret out some hidden cases of HIV, says study coauthor Howard B. Urnovitz, a microbiologist at Calypte.

In the same issue of Nature Medicine, researchers at the University of Milan in Italy examining heterosexual couples found several people who were exposed to HIV but who tested negative in blood screenings. Some of them produced HIV antibodies in urine and vaginal samples, however. This points to a "compartmentalized" immune response, in which HIV-specific antibodies in the vaginal mucus lining might prevent the virus from reaching cellular targets, the authors say.