The first assays capable of detecting free circulating HIV particles were the HIV p24 antigen EI As. Quantitative measurement of plasma HIV RNA (viral load) have now superseded these EIAs. Measurement of viral load is based on amplification of viral nucleic acids or of the probe-binding signal (e.g. branched DNA tests). Results are reported as copies of virus per ml, and the new generation tests can detect 20-50 copies per ml. Measurement of viral load is now standard in industrialized countries for staging and monitoring response to antiretroviral therapy. However, several factors have limited their use so far in developing countries. These include the expense of the complex equipment necessary and the need for rigorous laboratory conditions, quality control and highly trained staff.
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