Molecular Diagnosis: PCR, RT-PCR, and Nucleic Acid Amplification

Molecular diagnosis detects a virus by amplifying and identifying its nucleic acid. The polymerase chain reaction (PCR) and, for RNA viruses, reverse-transcription PCR (RT-PCR) copy a target genome sequence millions of times so that even very small amounts of viral genome become detectable, giving these methods high sensitivity and specificity and making them the mainstay of modern viral diagnosis.

Definition

Molecular diagnosis is the detection and, where needed, quantification of viral nucleic acid by amplification techniques such as PCR, RT-PCR, and isothermal methods, which copy a specific genome target to detectable levels.

Scope

This topic covers nucleic-acid amplification techniques used in virology: conventional and real-time PCR, reverse-transcription PCR for RNA viruses, quantitative measurement of viral load, and isothermal alternatives such as loop-mediated amplification. It explains the principles and interpretation at a reference level and does not provide assay protocols or clinical management advice.

Core questions

• How does amplifying a target sequence make a virus detectable from a tiny starting amount?
• What additional step does an RNA virus require compared with a DNA virus?
• How is real-time PCR used to quantify viral load, and what does the cycle threshold mean?
• When are isothermal or multiplex formats preferable to conventional PCR?

Key concepts

• Polymerase chain reaction (PCR)
• Reverse transcription (RT-PCR)
• Primers and target specificity
• Thermal cycling and exponential amplification
• Real-time (quantitative) PCR
• Cycle threshold and viral load
• Multiplex amplification
• Isothermal amplification (e.g., LAMP)

Mechanisms

PCR uses two short primers that flank a chosen region of the viral genome, a thermostable DNA polymerase, and repeated cycles of heating and cooling. Each cycle denatures the DNA, anneals the primers, and extends new strands, doubling the target so that it accumulates exponentially. For RNA viruses, a reverse-transcription step first converts the RNA genome into complementary DNA before amplification. Real-time PCR monitors product accumulation cycle by cycle using fluorescent reporters, allowing quantification: the cycle threshold at which signal rises above background relates inversely to the amount of starting template, giving an estimate of viral load. Multiplex designs amplify several targets at once, and isothermal methods such as loop-mediated amplification achieve amplification at a constant temperature, reducing instrument requirements for decentralised testing.

Clinical relevance

Nucleic-acid amplification provides sensitive and specific detection of active viral infection and supports viral-load monitoring used to follow infections and treatment response. This entry describes how the methods work and how results such as cycle threshold are interpreted in principle, including the point that genome detection does not by itself prove infectious virus is present; it is descriptive of methodology and not a basis for individual diagnostic or treatment decisions.

Epidemiology

Real-time RT-PCR became the reference method for confirming SARS-CoV-2 infection, and the rapid publication of validated assays early in 2020 enabled global scale-up of molecular testing, illustrating how nucleic-acid amplification underpins outbreak detection and surveillance.

History

The polymerase chain reaction was conceived by Kary Mullis and first applied to genetic diagnosis by Saiki and colleagues in 1985, with the method formalised by Mullis and Faloona in 1987. Real-time quantitative PCR, described by Heid and colleagues in 1996, added continuous quantification, and isothermal methods such as loop-mediated amplification, introduced by Notomi and colleagues in 2000, broadened where amplification could be performed.

Key figures

• Kary Mullis
• Randall Saiki
• Christian Drosten

Seminal works

• saiki-1985
• mullis-faloona-1987
• heid-1996
• corman-2020

Frequently asked questions

What is the difference between PCR and RT-PCR?

PCR amplifies DNA, so it is used directly for DNA viruses. RT-PCR adds a reverse-transcription step that converts a virus's RNA genome into complementary DNA first, allowing RNA viruses such as influenza and coronaviruses to be detected.

Does a positive PCR result mean infectious virus is present?

Not necessarily. PCR detects viral genome, which can persist after a virus is no longer replicating or infectious. Establishing the presence of infectious virus requires culture or other functional assays interpreted in clinical context.