The World Health Organization has produced guidelines for the management of common illnesses in hospitals with limited resources. This series reviews the scientific evidence behind WHO's recommendations. The WHO guidelines, and more reviews are available at http://www.ichrc.org
This review addresses the question: What is the role of HIV antigen testing in infants <12-months old?
The WHO Pocketbook of Hospital Care for Children recommends HIV antibody tests (ELISA or rapid) from the age of 18 months. For children <18 months, they can be used to diagnose the exposed infant, but not necessarily the infected infant. The guidelines also state that all positive HIV antibody tests should be confirmed by virological tests as soon as possible; otherwise repeat antibody testing can be performed after 18 months of age (Pocketbook, p. 204).
With the increasing availability of anti-retroviral drugs in resource poor countries, there is a vital need to develop a straightforward, cost-effective laboratory method for the early diagnosis of infant HIV infection. Immunoassays for HIV antibodies currently provide a simple and cost-effective diagnostic test for children >12 months.
The Prevention of Mother to Child Transmission (PMTCT) programmes in low-resource settings therefore require all exposed children to be followed to 12 months of age or even older before their HIV infection status can be determined . In South Africa, over 280 000 children per year are being monitored for >12 months, 90% of whom will prove to be uninfected but will have received prophylactic treatment . These costly inefficiencies have been addressed in the United Nations Development Goals which seek to prioritize the need for a straightforward and inexpensive HIV diagnostic test in children. The current WHO guideline for diagnosis of HIV infection in infants <18 months is DNA/RNA virology, which is neither cost effective nor easily accessible.
HIV-1 p24 antigen testing, whilst more cost effective, was originally trialled with discouraging results. In 1996, a new amplification boosted procedure was established, the Ultrasensitive p24 assay, which was proposed as being both cost and resource efficient. This review intends to answer the question: can the newer antigen marker linked techniques be effectively used in the diagnosis of HIV in the infant population?
The Cochrane database and the Medline database of the US National Library of Medicine were searched for reviews and randomized trials The PubMed clinical search strategy used was ‘HIV Antigens’. The search was limited to ‘human’, ‘published in the last 10 years’, ‘All infant’, ‘Preschool child’ and ‘Child’. The search was conducted on 20 May 2009.
Papers were excluded if they did not relate to the Ultrasensitive p24 assay techniques, diagnosis, non-comparative, if they failed to clearly define our comparison groups and identify/control for known confounders or if the sample sizes were too small. Papers were included only if extraction of data relating to infants specifically <12-months old was possible. Methodological quality of included papers was at least type 2b according to the criteria of the Oxford Centre for Evidence-Based Medicine.
Our search criteria retrieved 155 results including 12 reviews. All abstracts were read: if there was any doubt as to the relevance of the article, the full text was sourced. Citations listed in relevant trials were also hand searched and reviewed. The exclusion criteria applied left a total of seven papers for review.
Ultrasensitive p24 antigen assay performance
The primary outcome assessed was the sensitivity and specificity of the Ultrasensitive p24 antigen assays for the diagnosis of HIV in infants aged between 0 and 12 months. Positive and negative likelihood ratios were calculated where possible. Table 1 summarizes the findings.
aThere are no false positive's or false negative's in these data.
bAuthor stated that the data set was too small for accurate analysis.
cAuthor only gives raw data for children who tested positive for HIV.
Ultrasensitive p24 antigen assay cost
A detailed analysis by Sherman et al.  highlights the huge cost inefficiencies of the currently employed PMTCT programme of prophylactic treatment until antibody diagnosis at 18 months. The study suggests that this cost could be reduced by 25% if a diagnosis was reached earlier (using PCR virology) so that only those infected were treated.
Further, the cost of the Ultrasensitive p24 antigen test is considerably less than PCR virology. Zijenah et al.  have calculated that in Zimbabwe, the total cost of the Ultrasensitive p24 antigen technique, including equipment costs, reagents and personnel training, to be US$10 per test. This is in contrast to US$50 per DNA PCR test.
A review by Creek et al.  comparing the prices of DNA PCR across Africa found the lowest costs to be in Rwanda at US$14 per test, and the highest to be in South Africa at US$50 per test. George et al.  state that the average cost of the Ultrasensitive p24 test across countries where HIV subtypes A, B, C, D and E predominate was found to be only US$7 per test.
Sutthent et al.  found that in Thailand, the cost of in-house DNA PCR analysis to be US$30, and the corresponding cost for the Ultrasensitive p-24 antigen test was only $3.
These figures highlight the major cost advantage of this technique above all others currently employed for the diagnosis of HIV in infants <12-months old.
Majority of the studies have indicated that the Ultrasensitive p24 assay technique produces sensitivities >91.7% for infants of <12-months old. Two studies report slightly lower sensitivities of 85% and 87%, but the sample size used in these reports was significantly smaller. It would appear that the sensitivities associated with samples taken from infants within in the first 10 days of life are significantly lower than those taken after this period. If these samples were excluded, the sensitivity of the Ultrasensitive p24 assay technique would be further improved. These findings strongly support the use of the Ultrasensitive p24 assay technique in the screening of HIV in infants from 10 days to 12-months old.
The specificities associated with the Ultrasensitive p24 assay technique were extremely high with the majority being >98.5%. This highlights the useful role of the Ultrasensitive p24 assay technique in ensuring that only a very low proportion of HIV-negative infants get exposed to unnecessary treatment.
The positive likelihood ratios range from 32 to 74, thus providing strong evidence for the validity of a positive HIV diagnosis based on the Ultrasensitive p24 assay technique. The majority of negative likelihood ratios are <0.1 thus providing strong evidence for the validity of a negative HIV diagnosis based on the Ultrasensitive p24 assay technique.
The Ultrasensitive p24 assay is significantly costlier, resource effective as well as accurate than its alternatives. Another advantage of p24 testing is that it is relatively quick in producing results, taking ∼6 h .
This review has shown that the new Ultrasensitive p24 assay is as accurate as PCR virology, significantly cheaper and less resource demanding when used to diagnose HIV in infants of <12 months old.