ABSTRACT
Because of the legal restrictions on PCR technology and the expanding interest in nucleic acid-based diagnostic tests, commercial companies have developed alternative methods of nucleic acid amplification. Nucleic acid amplification is achieved in two main ways: by target (viral genome) amplification, where a specific sequence of interest is amplified to detectable levels, or by probe amplification, where the nucleic acid probe itself is amplified to detectable levels. Target amplification systems, including PCR, self-sustained sequence replication (3SR), and strand displacement amplification (SDA) incorporate target-specific sequences into the amplification product that are distinct from the sequences of the primers used to promote the reactions. Alternatively, probe amplification sys-
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terns such as ligase chain reaction (LCR) and Q-beta replicase amplify the probe itself without incorporating target information different from that used to start the reaction. The differences in reaction cycles between PCR and LCR are shown in Figure I. Different amplification methods have been reviewed by Forghani and Erdman (41). To date, the majority of methods describing the detection of respiratory viruses by nucleic acid amplification have used PCR, and rapid developments using LCR for viral diagnostics or a combination of PCR and LCR are awaited.
