Degrées du Loû and colleagues  present HIV surveillance data from pregnant women attending antenatal clinics (ANC) in Abidjan, Côte d‚Ivoire. To control for the recognized differences in fertility between HIV-infected and non-infected women , they estimated the HIV prevalence after adjusting for age and a measure of fertility. Overall, their crude and adjusted results were similar. Whereas we agree that ANC data are likely to be biased, we are concerned that the method of adjustment used will only partly take account of the fertility differences associated with HIV.
Adjustment using a relative inclusion ratio (RIR) was recently applied by Nicoll et al.  in the United Kingdom, and was based on fertility patterns by HIV status estimated for the general population. In Abidjan the HIV status and fertility results used for calculating the RIR were those of women attending ANC. This implicitly assumes that the ratio in fertility rates between HIV-infected and HIV-uninfected women attending ANC is the same as in the general population, but as these women were all pregnant this will not be the case. In the ANC non-sexually active women and primarily and secondarily infertile women will be excluded and subfertile women will be under-represented. On average, the fertility of women seen in the ANC will be higher than that seen in the general population, and the discrepancy between the fertility rates in the ANC and in the population is likely to be larger for HIV-infected women than for HIV-uninfected women because subfertility and infertility are more common after HIV infection. In addition, the live birth rates given are based on parity, including births before HIV infection, and do not reflect the current risk of pregnancy. The use of information from ANC attenders to calculate the RIR will result in incomplete adjustment of the results with respect to fertility differences, and therefore in false reassurance of the accuracy of ANC surveillance data as a guide to the prevalence of HIV in the population.
Another problem arising from the use of ANC attenders to estimate the HIV prevalence in the general population is that there may be differences in the use of antenatal services by HIV-infected and HIV-uninfected pregnant women. Several studies have shown the risk of HIV infection to be associated with socioeconomic status, which is also a determinant of the use of antenatal services . Adjustments made to improve estimates of general population HIV prevalence would ideally take account of this potential selection bias as well as fertility differences.
A simple adjustment method for ANC HIV surveillance data would be very useful, but unfortunately realistic adjustments cannot be made without information from women outside the ANC.
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