Understanding Your Lab Results
Healthcare providers often recommend microbiological tests when an HIV-positive patient has symptoms of a specific infection. For example, a healthcare provider might order a Mycobacterium avium microbiology test for a patient experiencing unexplained fever, weight loss, and anemia. To test for this bacteria, the lab will try to grow out, or "culture," Mycobacterium avium complex (MAC) in a chemical mixture that contains the patient's blood, other body fluids, or tissue. If the test is positive, the healthcare provider will know to start treating the patient for MAC.
Some microbiological tests require body fluids other than blood. For example, a patient who has symptoms of pneumonia or tuberculosis may need to produce sputum (phlegm) so that microbiological tests can be conducted. Patients who are thought to have an infection in their brain may need to have microbiological tests of their spinal fluid, which requires a procedure called a lumbar puncture or spinal tap. Stool (feces) is used for various microbiological tests as well and is discussed below.
Unlike microbiological tests, which look for the presence of a disease-causing microorganism, serologic tests most often look for specific antibodies produced by the immune system. Also unlike microbiology tests, serology tests are extremely useful to determine if someone has been exposed to an infection before they show signs of disease or to indicate if an infection or disease was present some time in the past.
The HIV tests, known as the ELISA and Western blot assays, are both serologic tests. Neither test looks for the presence of HIV, but rather the antibodies produced by the immune system to fight HIV. Antibody testing has remained one of the cheapest, most reliable methods of diagnosing HIV. HIV antibody testing also allows patients to find out their status many years before they get sick, thus allowing them to start medications to treat HIV and prevent opportunistic infections early. A number of HIV-antibody tests are available to healthcare providers and patients, including standard tests requiring blood samples to be sent to laboratories, tests that can be performed on oral samples, tests that can determine the presence of HIV antibodies in 15 to 30 minutes, and home-collection kits with the results available by phone.
The following is a list of additional serologic tests recommended by healthcare providers for their HIV-positive patients:
Toxoplasma Serology: Toxoplasmosis is a serious infection of the brain. Between 15 to 40 percent of people living with HIV have antibodies to Toxoplasma gondii, the protozoan responsible for causing toxoplasmosis. If a patient with HIV is positive for Toxoplasma antibodies and has a T-cell count less than 100, he or she will need to take Bactrim or Septra (TMP/SMX) to prevent the infection from causing disease. TMP/SMX is also the antibiotic used to prevent Pneumocysti pneumonia (PCP) when the T-cell count is less than 200.
Syphilis Serology: Syphilis (Treponema pallidum) is a potentially fatal bacterial infection usually spread through unsafe sexual activity (vaginal, oral, or anal sex without a condom). If detected early and treated, syphilis can be successfully controlled. If left unchecked, it becomes much more difficult to treat and can result in serious disease of the brain and death. It's generally recommended that sexually active adults and teenagers be tested for syphilis antibodies every year.
Serology testing can be used to diagnose syphilis and to determine if the infection has responded to therapy. However, syphilis testing isn't always accurate. It's estimated that between 6 to 10 percent of people with HIV will test falsely positive; a much smaller percentage will test falsely negative. No one is sure why this happens, but it may have something to do with abnormalities in the immune system or the presence of yet another infection.
Hepatitis B and Hepatitis C Serology: Hepatitis B virus (HBV) and Hepatitis C virus (HCV) can both cause liver problems in HIV-positive and -negative people. Even though many people with HIV are also infected with HBV, only a small percentage will actually develop symptoms from the infection. HBV isn't usually fatal, but can cause liver problems in a small percentage of those infected. HCV, on the other hand, can cause serious liver disease in some patients infected with the virus.
Tests for HBV and HCV are both available, but their interpretation can be complicated. People who have hepatitis B surface antigen (HBsAg) present in the blood are either carriers of HBV or have active infection with the virus. People who have recovered from HBV (that is, have cleared the virus from their bodies) and those who have had the HBV vaccine will have hepatitis B surface antibody (HBsAb) present in the blood. In other words, in the case of HBV, having the antibody present means that the person is immune to HBV infection, either from having had it in the past or from having had the vaccine. HIV-positive patients who test HBsAg-positive require additional testing to determine the status of the infection. HIV-positive patients who test HBsAg-negative and HBsAb-negative should talk to their healthcare provider about getting vaccinated against hepatitis B.
The presence of antibodies to HCV in the blood means that a person has HCV infection unless proven otherwise. Tests that are similar to viral load tests for HIV can be done for HCV to establish the presence of the hepatitis C virus. So with HCV, having the antibody usually means that a person is infected with the virus rather than immune to it, like in the case of HBV. There is no vaccine for HCV at this time, and the infection is best avoided by not sharing needles or other drug paraphernalia and by practicing safer sex.