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It well established that in a pregnant HIV infected woman, the virus could be transmitted to the unborn child during pregnancy, birth or through breast milk. Approximately 90% of HIV infected infants and young children acquired the infection through mother-to-child transmission, which occurs in approximately 25%-35% of pregnant mothers who did not receive any medication during pregnancy or childbirth. Breastfeeding increases the risk of transmission to about 50%.
Making an accurate diagnosis of HIV in young  children of infected mothers is often difficult and challenging. Most of this difficulty results from a natural process that occurs in the last 3 months of every pregnancy. During the last three months of pregnancy, all mothers transfer antibodies through the placenta into the baby’s blood stream. The ability of mothers to transfer antibodies to their unborn child has been known to medical scientist for 100s of years. This phenomenon is taken into account when trying to diagnose infections in infants such as syphilis, hepatitis B and C infection, or when deciding on the appropriate time to administer certain immunizations.
Antibodies are substances our immune system makes as a response to specific infections. They help in fighting infections and can be found in body fluids such as blood, saliva, genital fluid etc. In older children and adults, the detection of antibodies in body fluids implies contact with that specific infection through natural infection or immunization. Antibodies can also be got through transfusion with blood or blood products.
Almost all infants born to infected mothers have antibodies to HIV floating around their blood stream. These antibodies were made by the mother's immune system, crossed the placenta to the baby's bloodstream during the last 3 months of pregnancy and can persist until the baby is 18months old. So, although one can easily make a diagnosis of HIV infection in older children and adults by testing blood for antibodies to HIV, these antibody tests cannot be use to establish a diagnosis of HIV in infants. A positive HIV antibody test in an infant born to an infected mother simply states “my mother is HIV positive”. In this scenario, further testing is required to determine if this infant who has been exposed to HIV in the mother’s womb is “exposed but uninfected” or “exposed and infected”. Note that HIV antibody test maybe falsely negative if the infant was born prematurely i.e. before the last 3 months of pregnancy. Other scenarios resulting in a falsely negative antibody test are (1) if the mother was acutely infected with HIV during childbirth and therefore had not yet made antibodies or (2) the mother is severely immunodeficient from HIV infection that she cannot make antibodies.
The best way to determine if an infant born to an HIV infected mother has acquired the infection is to do one of several tests that can detect the presence of HIV viral particles or genes in the blood of the infant. In developed countries, the test of choice is called HIV DNA PCR and detects the presence of HIV genes (DNA) in human blood cells. The beauty of this test is that it gives a yes or no answer to the question “Is this exposed child HIV infected”? However, because the majority of mother-to-child transmission occurs during childbirth, a negative test result after 4months of age is needed to be certain the infant is exposed but uninfected. Other tests that can be used to make a diagnosis in infants are HIV RNA PCR (commonly referred to as “viral load”), p24 antigen tests and HIV culture. Some developing countries use the “viral load” test to diagnose infants because it is easier and cheaper than the HIV DNA PCR test. Infants who are infected at birth usually have a high viral load over the first 1 to 2 months of life, so this test should be able to detect infected infants. However, machines used to determine viral loads have a minimum cut-off point such as <50copies/ml or <400copies/ml below which they are not able to count any more viral copies. This creates a dilemma because one is not sure if <50copies/ml or <400copies/ml means absolutely no virus detected, 245copies/ml, 32copies/ml e.t.c. The other tests are not used routinely for a variety of reasons.
An alternative to trying to establish an early diagnosis with the above tests is to do the HIV antibody test at ³18months when we expect all the antibodies got from the mother to have disappeared. If positive, the antibody test should be repeated at 24months to confirm the child is infected and now making it’s own antibodies. Some experts criticize this approach because they believe it is beneficial to initiate HIV medications early in infants who are infected. How early? No one is sure at the moment, as the studies are ongoing.
Of course any the infant regardless of age that becomes ill with an HIV associated illness when less than 18months old can be tentatively diagnosed on clinical grounds with confirmation testing done when feasible. If making a diagnosis on clinical grounds is based on opportunistic infection in the infant, one should be cautious because some congenital immunodeficiency disorders also lead to the same opportunistic infections that occur in HIV. A child with congenital immunodeficency disorders has a defect in one or more part of the immune system that he/she was born with due to a variety of reasons usually genetic.
In summary, making a diagnosis of HIV in infants born to infected mothers can be challenging and antibody cannot be used to make a diagnosis in the first 12-18months of life. This article did not consider breastfeeding, which can complicate interpretation of HIV test . It would be helpful to provide a forum for doctors where they can receive advice from experts regarding HIV diagnosis.
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