Background:  A heat-denatured, signal-boosted ultrasensitive p24 antigen (Up24) assay was compared with DNA polymerase chain reaction (PCR) in HIV-exposed infants in Ho Chi Minh City, where the predominant HIV subtypes are E and recombinant AE.

Methods:  Perinatally exposed infants ­≥36 weeks gestation and ≤2 months of age without AIDS-defining illnesses were eligible for enrollment. Blood samples were taken at 2 and 6 months of age. Samples were processed within 6 hours for immediate testing or serum/plasma storage at –50°C (Up24) or –70°C (DNA PCR). The Up24 assay was performed at a hospital clinical laboratory; no modifications were made to kit reagents or protocols. The DNA PCR assay followed an in-house, 3-gene amplification protocol and was performed at the Pasteur Institute, Ho Chi Minh City. All positive 2-month tests were confirmed by PCR; negative and indeterminate 2-month tests were repeated at 6 months of age. HIV status was established by ≥2 correlating PCR results on separate samples. Infants with ≥2 DNA PCR tests and ≥1 Up24 test were included in the analysis

Results:  Between April 2005 and March 2006, 202 infants met analysis criteria; 112 (55%) were male. Infants had varying levels of perinatal ART exposure:  pre- and post-natal (157 or 77.7%), post-natal only (31 or 15.3%), or none (14 or 6.9%). Post-natal ART were given for ≥7 days; 19 infants (9.4%) were ever breastfed (median 5.5 days, range 1 to 70). At 2 months of age, 12 (5.94%) were HIV⁺. There were 2 false positive DNA PCR results (sensitivity 100%, specificity 99%). The Up24 assay was 100% sensitive and 100% specific. At 6 months, 1 infection was diagnosed by both DNA PCR and Up24, resulting in an overall transmission rate of 6.44%. The median p24 level in positive samples was 63,845 fg/mL (range 14,556 to 544,511). Excluding the late infection case, of the 106 infants for whom 6-month Up24 testing was available, 100% matched the negative 2-month results.

Conclusions:  The Up24 assay was comparable to an in-house DNA PCR assay in diagnosing HIV-exposed infants at 2 months of age in Vietnam. The lack of breastfeeding, short post-natal ART exposure, and age at testing may be factors in the Up24 assay’s success. The ELISA-based platform and higher range of specimen storage temperatures make the Up24 assay a practical alternative to molecular testing in resource-limited settings.