HIV test strips that show a second line overnight have meaningless results.
Regardless of the type of HIV test strips used in the market, the results will not be meaningful if the reading time is exceeded. Generally, the results are more accurate when read 10-15 minutes after the drop of blood, depending on the instructions for use of the different test strips.
The second bar may appear when the test strip is oxidized or contaminated in the air, or when it is stored for a long period of time.
When the second bar appears, you can retest with a new test strip or have a blood test done at the CDC.
HIV test strips should be used according to the instructions, and if you are unsure, you can have your blood drawn at a regular hospital or CDC facility.