Target RNA or DNA

Solid support

■ Figure 7-31 Second-generation bDNA assays use extender probes that bind multiple amplifiers, increasing the signal intensity and improving limits of detection.

luminescence is measured in a luminometer. This system has a detection limit of about 50 target mol/mL.69

There are several advantages to this method. First, there is less risk of carryover contamination resulting in a positive test in the bDNA assay than in PCR.70 Second, multiple capture and extender probes can be incorporated that detect slightly different target sequences as occurs with different isolates of hepatitis C virus and HIV. By incorporating different probes that recognize slightly different sequences, multiple genotypes of the same virus can still be detected by the same basic system. Finally, requiring that multiple probes bind to the same target increases the specificity of the system. It is highly unlikely that all of the required probes would bind nonspecifically to an unrelated target and produce a signal. The bDNA signal amplification assay is currently available for the qualitative and quantitative detection of Hepatitis B Virus, Hepatitis C Virus, and HIV-1.6

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