You’re right, dried blood outside the body does not pose a high risk for HIV infection because normally, exposure to air dries the fluid (blood) that contains the virus, and that destroys much of the virus very quickly. Typically for HIV, there needs to be direct blood-to-blood contact for transmission to occur.
You’re asking about Dried Blood Spot Testing (DBST) which is one way to test for viruses like Hepatitis C and HIV.
To do this, 5 drops of blood are collected onto a special filter paper through a finger prick (similar to the finger poke used in diabetes monitoring). This paper is treated with chemicals specifically designed to absorb proteins and other genetic material and to inactivate blood-borne pathogens as it dries. Once the blood drops are collected, the paper is dried for several hours and sent to a lab in special packaging to keep it dry. While the amount of blood may be small and poses less of a bio-hazard risk than other testing methods, universal blood and body fluids precautions are used for all specimens to prevent any accidental exposure to any other bacteria or viruses that may be in the blood spots that we don’t know about.
Once at the lab, they test it using methods that are very similar for blood-in-a-tube specimens. First they use a hole punch to take one of the 5 drops of blood and soak it overnight in a buffer solution that allows the antibodies to come off. (Antibodies are produced by the immune system whenever you’re infected by any virus, even the cold or flu). Using the same method, the lab will test the sample to see if HIV antibodies are there. If there are no HIV antibodies present the sample is reported as ‘negative’ for HIV.
If the test is not negative, in other words there is a ‘signal’ present, they don’t yet call this HIV positive. The reason is: there is a chance that the ‘signal’ could be a false-positive. This happens with all forms of testing and happens when the amount of HIV in a population is low.
So, if an antibody signal is present, they do a second completely different test in which they look for the genes of the HIV. Here a larger ‘hole punch’ from one of the five dried blood spots are exposed to a chemical solution and activation process in order to isolate the genetic material. The lab is then able to gain information from genetic components of any viruses present. This will confirm the presence of HIV or not and can even be helpful in assessing which treatment options may be best.