As genetically divergent strains of the HIV virus become increasingly prevalent in the United States, the ability of clinicians to effectively monitor progress of anti-viral drug therapies depends on the capacity of viral load assays to accurately quantitate, or measure, these variant HIV strains, according to research reported at the 16th Conference on Retroviruses and Opportunistic Infections (CROI).

A study by the Global HIV Surveillance Programme at Abbott showed significant variation in the ability of diagnostic tests to accurately monitor viral load in patients infected with differing strains, or subtypes, of the HIV virus. The study evaluated HIV-infected specimens using the Abbott RealTime HIV-1 test and the Roche COBAS Taqman HIV-1 test.

According to the study, the Abbott RealTime HIV assay quantitated HIV more accurately than the widely used COBAS Taqman assay in an analysis of 317 seropositive samples from seven countries: Argentina, Brazil, Cameroon, Saudi Arabia, South Africa, Thailand and Uganda. The samples included six different HIV subtypes and eight circulating recombinant forms. Only 18 samples were subtype B, the most common HIV strain in the United States.

The research showed the Taqman assay failed to detect five infected samples and underquantified viral load in 38 samples, compared to the Abbott assay.

"The differences in viral load quantitation between assays is likely due to the impact of HIV genetic variation on assay performance," said John Hackett, lead author from the Abbott HIV Global Surveillance Programme. "Since viral load measurement is critical for optimal patient management, clinicians must recognize that HIV diversity can influence the accuracy and reliability of assay performance."

The Abbott RealTime HIV-1 test is used in conjunction with clinical presentation and other laboratory markers as an indicator of disease prognosis, and as an aid in assessing viral response to antiretroviral treatment as measured by changes in plasma HIV-1 RNA levels. The RealTime assay is not intended for use as a donor-screening test for HIV-1 or as a diagnostic test to confirm the presence of HIV-1 infection.

Further evidence of the growing impact of diverse HIV subtypes for monitoring AIDS patients in the United States was reported by the University of Maryland School of Medicine. The study showed the prevalence of non-subtype B strains in the Baltimore metro area is about two percent, based on tests of 2,200 HIV patients. However, in the Maryland portion of the Washington D.C. area, 13 percent of the positive samples sequenced were non-B. The majority of non-B subtypes (80.8 percent) were from recent immigrants from Africa, of which 62.5 percent were women.

The Abbott RealTime HIV-1 assay is among the most sensitive viral load tests available, with a broad dynamic range capable of quantitating HIV-1 in plasma down to as few as 40 RNA copies per milliliter (mL) and up to as many as 10 million copies per mL.