The expense of PCR testing limits diagnosis of HIV infection in infancy in low resource settings. The ultrasensitive p24 antigen assay has been proposed as an accurate substitute; however, its ability to detect different HIV viral subtypes remains to be determined. A sensitivity of 98.1% and specificity of 98.7% was obtained testing 203 samples from 24 HIV-infected and 66 uninfected infants born to HIV subtype C-infected women.
Antenatal clinic prevalence rates of HIV infection in South Africa were 25.9% in 2002, which means that more than one-fourth of children currently born in South Africa are vertically exposed to HIV. 1 In general guidelines for Prevention of Mother to Child Transmission (PMTCT) programs in low resource settings require all exposed children to be followed to 12 months of age or older before their HIV infection status can be determined by an HIV antibody test. The persistence of maternal anti-HIV antibodies and the cost and complexity of two HIV PCR tests precludes earlier testing. 2 In South Africa >280 000 HIV-exposed children per annum, of whom as many as 90% may be HIV-uninfected, must be followed for 12 months or more. A universal frustration of PMTCT programs in low resource settings is the lack of capacity to achieve this follow-up, thereby missing the opportunity to determine the HIV infection status of exposed children. 3, 4 This prevents access to available medical care known to improve the quality and quantity of life of HIV-infected children and their families (e.g. prophylactic medication, treatment of intercurrent infections, nutritional and emotional support). Assessment of the p24 antigen (Ag) assay for diagnosis of HIV infection in infants is listed among the urgent research priorities to improve pediatric HIV/AIDS care in developing countries. 5, 6
The value of the p24 Ag assay in this context has been limited by its poor sensitivity. 7, 8 Recently a technically improved version of the assay has been as sensitive and specific as HIV PCR tests that detect viral nucleic acids. This ultrasensitive p24 Ag assay has been proposed for use in diagnosis of pediatric HIV infection and in viral load monitoring of patients receiving antiretroviral therapy. 5, 8–14 A proviso for widespread introduction of the p24 Ag assay is the requirement for further validation of the assay in non-B viral subtypes. 5, 9, 13 Viral subtype C accounts for 99% of HIV infections in South Africa. 15 The performance of the p24 Ag assay for early infant diagnosis in HIV subtype C is described.