Nucleic Acid Amplification Technologies

The polymerase chain reaction (PCR) has proved to be a powerful and versatile tool and has opened new avenues in molecular biol­ ogy. Alternative nucleic acid amplification techniques, such as the ligase chain reaction (LCR), nucleic acid sequence-based amplifica­ tion (NASBA), and transcription-mediated amplification (TMA), a variation of NASBA, are also now available. These techniques are all designed to amplify specific nucleic acid sequences in an expo­ nential manner, thus providing a basis for extremely sensitive diag­ nostic assays. However, despite the widespread and successful ap­ plication of genomic amplification techniques in biological research, they have not yet reached the point of routine use in clini­ cal laboratories. Thus, although the R&D investment in nucleic acid diagnostics is in excess of $250 million annually, clinical ap­ plications remain relatively modest. One of the principal reasons for this delay in clinical application has been the problem of acci­ dental contamination of negative clinical specimens with minute amounts of amplified products from a previous positive reaction. Carry-over contamination of amplicons can now be prevented by chemical means or the use of a closed reaction system. However, the current instrumentation is essentially modular in nature, com­ prising machines that perform the three essential steps of nucleic acid amplification technology: sample preparation, the amplifica­ tion reaction, and detection of products. Consequently, the test pro­ cedures are more complicated with somewhat lower sample throughput than the enzyme immunoassays currently performed in clinical laboratories.