Human immunodeficiency virus, or HIV, is an incurable sexually transmitted disease. Common symptoms of HIV are fatigue, fever, headaches, rashes, sore throat and swollen lymph glands, though patients with acute HIV may not have symptoms right away. In addition, some people may not display any symptoms when they are diagnosed with HIV, according to the National Institutes of Health. For sexually active people, getting tested for HIV is very important, according to both the CDC and National Institutes of Health.
The most accurate way to detect HIV is through a blood test. When a person has an HIV infection, he will have anti-HIV antibodies in his blood. However, blood tests are not 100-percent foolproof: while a person may be infected, it can take up to 3 months for an ELISA test to switch from HIV negative to HIV positive, according to the National Institutes of Health.
According to the National Institutes of Health, the ELISA test is the most effective way to detect an HIV infection. An ELISA test requires a blood sample: the doctor will draw blood from either the inside of the elbow or the back of the hand, where it is easier access to the veins. Before drawing blood, the doctor will use an antiseptic wipe to cleanse the area and may use an elastic band on the upper arm to help pool blood in the veins. After the blood is collected in an airtight vial, the presence of anti-HIV antibodies is tested.
Although the ELISA test is very accurate, false positive are possible. The largest issue with the ELISA test is that the antibodies may not appear until after three months; at risk people should be tested often, every 3 to 6 months, according to the CDC. Other conditions, like Lyme disease, syphilis and lupus, can result in a false positive on the ELISA test.
After the ELISA test is done, the doctor will perform a Western blot to confirm the results. The Western blot also uses a blood sample: if it comes back positive, then the patient has HIV. If the Western blot comes back negative and the ELISA test was positive, then it was a false positive on the ELISA test. However, it is possible for Western blot results to be inconclusive, which requires further testing. In addition, a negative Western blot does not indicate the absence of HIV, especially if the test was done during the "window period" before antibodies were detectable.
According to the National Institutes of Health, a blood differential can also be used to test for HIV, though this is not as accurate as the ELISA test and Western blot. Using a special dye on a blood sample, the doctor can determine if there is a decreased percentage of lymphocytes, which is a sign of HIV. However, because decreased lymphocytes can indicate other diseases, it is not an accurate testing method.