HIV can be tricky to detect in its early phase. Most HIV testing technology tests not directly for the HIV virus but the reaction your body has to infection with the HIV virus. The window period is the time between infeciton with HIV and the ability for this infection to be detected.
At EraHealth we use the Alere Rapid HIV Test. This test can detect HIV infection as early as 12 days after HIV exposure. It's important to note that is some cases it can take up to 3 months before HIV infection can be detected. This is called "the window period".