The diagnosis of HIV is made through testing and it is done for a few key reasons: 1) to facilitate individual patient care, 2) to ensure safety of blood products for transfusion, and 3) to aid in national disease surveillance. From a public health perspective, awareness of the number of new and existing HIV infections in a particular setting is very important, especially in areas where a sudden increase in number of cases or a shift in the affected demographic may predict a possible epidemic.
The decision of a particular individual to have an HIV test depends on a risk assessment of self and environment. Simply put, a HIV test is strongly recommended if an individual engages in high-risk behaviour.
Widespread HIV testing and knowledge of HIV status will hopefully encourage earlier access to medical care and life-saving treatment, which then results in decreased disease transmission and avoids the increased risk of death and illness associated with a late diagnosis. It also permits individuals who test negative to receive information that could discourage future risky behaviours. Save for specific instances such as in blood and organ donation where testing is imperative for public safety, mandatory HIV testing is generally not widely accepted, as ethical and human rights issues preclude its recommendation. Universal testing is regarded as a process where everyone in a specific setting is offered a HIV test as part of routine care, but is also given the option to refuse the test. Also called an “opt-out” strategy, this is the norm in some countries among hospitalised patients, pregnant women, and prisoners. This facilitates early detection and treatment of individuals who would most likely be unable to submit themselves to a HIV test due to absence of symptoms often attributed to the disease. However, recent trials in some local hospitals show that uptake of the opt-out testing is quite low i.e. most individuals opt-out of universal testing.
For people who are at increased risk of acquiring HIV, such as those attending sexual health clinics, drug dependency services, antenatal clinics, and patients with tuberculosis, hepatitis B or C, and lymphoma, universal testing may have a role. HIV testing should also be routinely offered to those with sexually-transmitted infections, to partners of people living with HIV, men who have sex with other men and their female contacts, those with history of injecting drug use, and a history of sexual contact abroad with individuals from countries with a high HIV prevalence.
The HIV test is presently voluntary and confidential. Counselling is done prior to and after the procedure and informed consent is obtained. The result of a positive HIV test is given directly by the testing clinician to the patient, and the patients have a right to expect that the information exchanged will be held in strict confidence.
The sequence of laboratory markers demonstrated during HIV infection is predictably consistent, thereby allowing its classification into distinct laboratory stages:
- The eclipse period – the period after infection with HIV when no laboratory markers are consistently detectable.
- The window period – the interval between infection and detection of HIV antibodies which is most commonly picked up by screening tests. The newer, more sensitive tests aim to shorten this duration to enable earlier detection.Acute HIV infection – also known as seroconversion, this usually happens two to six weeks after exposure or becoming infected. The symptoms of acute infection look similar to those of other viral illnesses and are often compared to those of the flu. The symptoms may last a week or two and then completely go away as the virus goes into a non-symptomatic stage. Some of these symptoms include fatigue, muscle ache, sore throat, skin rash and fever. HIV antibodies may not be detectable at this stage. • Established HIV infection – the stage of a fully developed antibody response sufficient to validate an HIV diagnosis using the confirmatory tests.
After HIV enters the body, low-level of HIV RNA is intermittently present in the blood, but the lack of markers make laboratory detection inconsistent. After approximately 10 days, intense replication of the virus occurs, and HIV RNA increases to high levels and becomes detectable by nucleic acid tests (NAT). HIV p24 antigen is subsequently expressed after 4-10 days. Thereafter, the appearance of HIV antibody occurs and persists throughout the course of infection.
Figure 1. Sequence of appearance of laboratory markers for HIV-1 infection
Adapted from Centers for Disease Control and Prevention and Association of Public Health Laboratories. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. Available at http://stacks.cdc.gov/view/cdc/23447. Published June 27, 2014. Accessed 13/10/2014.
Tests for HIV are of two types: one involves drawing blood and sending it to a laboratory facility, while another is done immediately at the site, which is termed point-of-care (POC) testing. A combination of a repeatedly reactive screening test, followed by a highly specific confirmatory assay is the current approach to diagnosis.
The screening test, which is presently a fourth-generation enzyme-linked immunosorbent assay (ELISA), tests for both HIV antibody and p24 antigen. Older third-generation ELISA only detects HIV antibody. The reactive specimen is then confirmed by undergoing either HIV-1 indirect immunofluorescence assay or the Western Blot test to validate results. The Western Blot has 99% specificity, making it currently the practical gold-standard confirmatory test. HIV screening assays have evolved over time to become more reliable in detecting HIV infection, thereby limiting the occurrences of false-positive and false-negative results. The turnaround time for initial results of third-generation ELISA takes about 1-3 hours and is more sensitive in distinguishing early infection compared to earlier generation tests and rapid antibody tests. Fourth-generation ELISAs, in comparison, are even more sensitive during early HIV infection and has a quicker turnaround time of less than 30 minutes. It has also been found to decrease the window period of HIV to an average of 16.8 days, compared to 21.4 days with the third-generation assay. False-positive ELISA results are seen in chronic alcohol use, rheumatic disease, congenital bleeding disorders, syphilis, neurocysticercosis, multiparity, and recent infections with dengue, malaria, and hepatitis B. False-negative results may occur in very early HIV infection due to low antibody production, the presence of an underlying neoplasm, use of chemotherapy, and extensive blood transfusion. A person who tests positive for HIV is expected to remain positive, and lifelong sero-positivity is generally the rule, but exceptions can be seen in late infection, profound immune deficiency, rapid disease progression, and early treatment with HIV antiretroviral therapy. It must be emphasized that such exceptions are very far and few in between.
Opportunities for early treatment are missed during the early acute phase of HIV, due to false-negative HIV antibody test results. Detection of HIV nucleic acids by amplification, using NAT, enables earlier detection compared to later-generation ELISAs, and reduces the window period further to 10-12 days. These tests however act only as diagnostic adjuncts and do not replace serologic standards. Its high cost is also a barrier to wide-spread utilization. The second type of test exhibits the convenience of HIV testing, and is typified by rapid test kits and oral-based kits. These tests can be completed in 15-30 minutes, permitting early intervention. The simple, user-friendly rapid assay makes use of either a single drop of blood from a finger prick or a gum swab, and is useful in resource-poor areas, sites outside of a healthcare setting, or for specific populations in which a delay in HIV diagnosis will be a great disadvantage, such as in pregnant women, STD clinic clients, and emergency room patients. Compared to standard screening tests, point-of-care testing is less sensitive and specific, thus positive results must still be confirmed by serologic tests.
The United States Centers for Disease Control and Prevention has recently come up with updated recommendations for HIV testing, which include tests for HIV antigens and HIV nucleic acids as a result of findings of studies involving populations at high risk for HIV that demonstrate that antibody testing alone might miss a considerable percentage of HIV infections detected by current tests. This new algorithm holds several advantages, such as a more accurate laboratory diagnosis of acute HIV-1 infection, an equally accurate diagnosis of an already established infection, a more accurate diagnosis of HIV-2 infection, fewer indeterminate results, and a faster turnaround time. The need for confirmation with the Western Blot is no longer required with the latest algorithm. It recommends starting with an assay that detects HIV-1 and HIV-2 antibodies, as well as the p24 antigen. Specimens that test positive on this initial assay are then differentiated into HIV-1 and HIV-2. Specimens that are negative or indeterminate on the antibody differentiation assay, proceed to NAT. Through this method, acute infection is less likely missed, and HIV-negative persons are reassured of their uninfected state.
Still, no diagnostic assay or algorithm can be completely accurate in all cases of HIV infection, and inconsistencies in the tests may come about during an individual’s work-up, necessitating further validation.
After receiving the results of a positive HIV test, discussion between the clinician and patient will revolve around certain issues such as addressing immediate concerns of the patient, onward referral to a HIV physician for continuity of care, identifying a social support system, provision of information necessary to ensure continuing healthcare, and discussion of partner disclosure and prevention strategies.
The initial evaluation with the attending HIV physician will include comprehensive history-taking, physical examination, and laboratory tests. Tests specific to HIV include CD4 T-Lymphocyte count with percentage to determine the current immune status, quantitative plasma HIV RNA levels, antiretroviral resistance testing, and a co-receptor tropism assay. An array of routine lab tests with blood draws, urinalysis, tuberculin skin test, chest radiograph, and a pap smear for women will complete the baseline work-up. The extensive initial work-up aims to stage the disease, identify other co-infections and co-morbidities commonly associated with HIV, and address non HIV-related conditions.
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- Dewar R, Goldstein D, Maldarelli F. Diagnosis of Human Immunodeficiency Virus Infection. In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Disease Mandell, 7th ed (pp 1663-1686) Philadelphia: Churchill Livingstone.
- Centers for Disease Control and Prevention and Association of Public Health Laboratories. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. Available at http://stacks.cdc.gov/view/cdc/23447. Published June 27, 2014. Accessed October 20, 2014
- British HIV Association et al. UK National Guidelines for HIV Testing. Available at http://www.bhiva.org/documents/guidelines/testing/glineshivtest08.pdf. Published September 2008. Accessed October 20, 2014
- Centers for Disease Control and Prevention. Advantages and disadvantages of different types of FDA-approved HIV immunoassays used for screening by generation and platform. Available at http://www.cdc.gov/hiv/pdf/testing_Advantages&Disadvantages.pdf. Accessed October 20, 2014
- Aberg JA, Gallant JE, Ghanem KG, et al. Primary Care Guidelines for the Management of Persons Infected With HIV: 2013 Update by the HIV Medicine Association of the Infectious Diseases Society of America. Available at http://cid.oxfordjournals.org/content/early/2013/11/12/cid.cit665.full.pdf+html. Published online: November 13, 2013. Accessed October 20, 2014.