Accurate testing for babies

Appropriate tests to use for babies younger than 18 months differ from those for older children and adults because babies may have maternal HIV antibodies in their blood for up to 18 months after birth. Conventional antibody tests cannot distinguish between maternal and infant HIV antibodies . 

Studies of the loss of maternal antibodies show that most children lose their maternal antibodies by twelve months. However, the time taken has varied between studies, and a small proportion of uninfected children remain antibody positive at 15 months and 18 months.^{1 2}

If the child of an HIV-positive mother has negative antibody tests, this indicates the absence of infection. However a reactive ('positive') result in a child under 18 months could represent either persistent maternal antibodies or antibodies newly produced by an infected child.

An assay which amplifies HIV DNA by polymerase chain reaction (PCR) is considered the gold standard.³

The sensitivity of the test is low in the first few days of life, but increases to more than 90% by two to four weeks. In a meta-analysis, 38% of infected children had positive HIV DNA PCR tests by age 48 hours. No substantial change in sensitivity during the first week of life was observed, but sensitivity increased rapidly during the second week, with 93% of infected children testing positive by age 14 days. By age 28 days, HIV DNA PCR had 96% sensitivity and 99% specificity .⁴

A number of studies and meta-analyses have estimated the sensitivity and specificity to be as high as 90 to 100% by one month of age in non-breastfed babies (sensitivity and specificity improve with increasing age of the child).

An alternative is to use an RNA-amplifying assay, which may be more readily available because of the common use of these tests in the monitoring of HIV-positive patients. Studies of the use of RNA assays with infants have reported sensitivities of 25 to 50% within the first few days of life, to 100% by six to twelve weeks of age. Some studies show RNA tests to have as good (or better) sensitivity and specificity as DNA.

In some settings, RNA assays are used as confirmatory tests for infants who have an initial positive DNA PCR test. In addition to confirming infection status, the expense of repeat DNA PCR testing is spared and the RNA measurement will help guide treatment decisions.

A reactive ('positive') virologic test indicates likely HIV infection and should be confirmed by a repeat test on a second specimen as soon as possible after the first test result becomes available.

Prompt identification of a child’s positive status will allow for the early use of combination therapy. Similarly, timely confirmation that the baby is HIV-negative allows for unnecessary prophylaxis to be discontinued, and immunisations to be given.

It is important that any test used is sensitive to the mother’s virus, which may be of a rare subtype or may be HIV-2. HIV RNA assays may be more sensitive than HIV DNA PCR for detecting HIV non-subtype B .

Infants who were infected in the uterus may have detectable virus at birth, whereas infants infected during delivery may not have detectable virus until days or weeks later. Moreover, any child who is breastfed will have continuing exposure to the virus, and the timing of tests to exclude infection will need to be adjusted.

Use of antiretrovirals, either by the mother during pregnancy or by the baby as PEP, can also delay detection of both DNA and RNA, although RNA tests are somewhat more sensitive.^{5 6}

BHIVA guidelines recommend that tests are performed at one day, six weeks and twelve weeks. For infants at high risk of infection, an additional test may be performed at two to three weeks. If all these tests are negative, BHIVA suggests that the parents can be informed that the child is HIV-negative.

The sequence of tests is necessary because infection may take place during pregnancy, during delivery or from breastfeeding. In practice, the majority of infections are picked up by six weeks.

Moreover, at 18 months an antibody test should be performed.³ This will both confirm loss of maternal antibodies and provide an opportunity for later infections (possibly due to breastfeeding) to be detected.

For the test at 18 months, fourth-generation tests have sometimes been shown to continue to detect maternal antibodies (and so give false positive results). Older antibody-only tests should therefore be used.⁷

US guidelines recommend testing at 14 to 21 days of age, at one to two months, and at four to six months. Definitive exclusion of HIV infection in a non-breastfed infant is based on two or more negative virologic tests (one obtained at age ≥1 month and one at ≥4 months), or two negative HIV antibody tests from separate specimens obtained at age ≥6 months.⁶ 

The US guidelines also recommend that if the mother has not been tested antenatally or after delivery, the newborn infant should be tested with a rapid test. This may indicate that the child should begin PEP immediately. HIV status can be confirmed with a secondary test as soon as possible, and PEP either maintained or discontinued, as appropriate.