The working array arranged in 96-well plate format was subsequently tested for specificity and potential cross-reactivity with human DNA and with each of the targeted viruses. None of the primer sets selected for a particular target virus in the array produced non-specific cross-reaction toward the other viruses. Comparative performance of the array was also evaluated through the testing of 17 clinical specimens from the United States patient population. All 17 samples were correctly identified in our PCR array with a high sensitivity to contain HIV-1, HCV, or HBV. We found that a combination of several primer sets targeting each virus in the array allows for the detection of different variants of the virus; however, it makes the absolute quantification with uniform DNA/RNA standards a challenge. Quantification by the assay is not always possible when the genetic group of the viral isolate being tested is different than the assay standards. This is one of the reasons why in certain cases the commercial assays underestimate viral loads by up to 1-10 log 10 copies per ml [37] [38] [39] . Nevertheless, relative quantification can be done using all primer sets selected for the array, and the absolute quantification can be performed with DNA/RNA standards using the primers targeting the most conserved genome areas. There are several commercial qualitative multiplex NAT assays now available on the market simultaneously targeting three most important blood-borne viral pathogens (HIV-1, HCV and HBV) [40] . One of them was recently approved by U.S. FDA for screening of blood and organ donors (http://www.fda.gov/BiologicsBlood Vaccines/BloodBloodProducts/ApprovedProducts/LicensedProduc tsBLAs/BloodDonorScreening/InfectiousDisease/ucm306073.htm).