Can you explain if it's possible for an hiv test to be wrong?

False results...HIV: Screen: ELISA Confirm: Western Blot (WB). False (+) ELISA check this...http://www.virusmyth.com/aids/hiv/cjtestfp.htm A (+) ELISA test is always followed by a WB test. A (+) WB confirms an HIV infection. A (-) WB test means the ELISA was false (+). An indeterminate WB needs further testing. Window period can mean (-) test. Repeat tests & checking HIV RNA levels clarifies this.

Yes if done too soon: False results of HIV tests are exceedingly rare as long as enough time has passed since the last possible exposure. That time is 6-8 weeks for the standard "third generation" antibody tests, although official advice for these tests is for final testing at 3 months. For the "fourth generation" (also called DUO or combo) test for both antibody and p24 antigen, results are conclusive by 4 weeks.