Types of PCR: RT-PCR, qPCR, Multiplex, Nested, Digital PCR Explained

Why So Many Types of PCR?

Since Kary Mullis pioneered the polymerase chain reaction in 1983, the technique has evolved into dozens of specialised variants. Each type of PCR modifies the core chemistry — changing the polymerase, the detection method, the primer strategy, or the reaction format — to address specific experimental needs. Choosing the right PCR type is crucial for obtaining reliable, reproducible results.

1. Conventional (Endpoint) PCR

Conventional PCR is the original format. After thermal cycling, products are analysed by agarose gel electrophoresis with ethidium bromide or SYBR Safe staining. It is qualitative — it answers "is the target present or absent?" — and is widely used for genotyping, cloning, and presence-absence detection in microbiology and forensics.

Best for: Cheap, qualitative screening; gel-based analysis; molecular cloning; colony PCR.

2. Real-Time Quantitative PCR (qPCR)

qPCR monitors amplification in real time using fluorescent reporters. Two main chemistries exist: SYBR Green (intercalating dye that binds any double-stranded DNA) and TaqMan probes (sequence-specific hydrolysis probes). qPCR provides quantitative data — it measures the amount of starting template — and is the gold standard for gene expression analysis, pathogen load quantification, and GMO detection.

Key metrics: Cq (quantification cycle), amplification efficiency (90–110%), and melt curve analysis (for SYBR Green). For a detailed comparison of detection chemistries, see TaqMan vs SYBR Green.

3. Reverse Transcription PCR (RT-PCR)

RT-PCR combines reverse transcription (RNA to cDNA) with PCR amplification. It detects and amplifies RNA targets, making it essential for gene expression analysis, RNA virus detection (including SARS-CoV-2), and transcriptome profiling.

RT-PCR can be coupled with qPCR (RT-qPCR) for quantitative RNA measurement. Important: RT-PCR refers specifically to the use of reverse transcriptase, not to "real-time PCR" — a common point of confusion.

4. Multiplex PCR

Multiplex PCR uses multiple primer pairs in a single reaction to amplify several targets simultaneously. Multiplex PCR primer design requires careful Tm harmonisation, amplicon size differentiation, and cross-dimer avoidance across all primer pairs.

Applications include pathogen panel diagnostics, microsatellite analysis, and forensic STR profiling. Commercial multiplex kits can amplify 10–30 targets in one reaction.

5. Nested PCR

Nested PCR uses two sequential amplification rounds. The first round uses outer primers to amplify a larger region. The second round uses internal (nested) primers that bind within the first amplicon. This two-round strategy dramatically increases specificity and sensitivity.

Best for: Detecting low-abundance targets, ancient DNA, and challenging samples with degraded DNA. The trade-off is higher contamination risk due to tube opening between rounds.

6. Digital PCR (dPCR)

Digital PCR partitions the sample into thousands to millions of individual nanolitre- to picolitre-scale reactions. After PCR, each partition is scored as positive or negative (digital readout). Absolute target concentration is calculated using Poisson statistics without needing a standard curve.

For a detailed comparison, see Digital PCR vs Real-Time PCR.

7. Other Notable PCR Variants

Emerging PCR Technologies

Several newer PCR formats are gaining traction in research and clinical labs. Droplet digital PCR (ddPCR) partitions samples into 20,000+ nanolitre droplets for absolute quantification without standards. Digital PCR is particularly valuable for rare mutation detection in liquid biopsy, copy number variation analysis, and measuring gene expression differences smaller than two-fold. Another emerging format is digital multiplex PCR, which combines the partitioning power of dPCR with multi-target detection using colour-coded probes or melting temperature differences. Isothermal amplification methods such as LAMP and RPA compete with PCR for point-of-care applications by eliminating the need for thermal cyclers, though they typically have lower multiplexing capability.

How to Choose the Right PCR Type

1. What is your question? Presence/absence? → Conventional PCR. Quantification? → qPCR or dPCR. RNA target? → RT-PCR/RT-qPCR.
2. How many targets? One target → simplex PCR. Multiple targets → multiplex PCR.
3. What sensitivity do you need? Standard sensitivity → conventional or qPCR. Single-molecule sensitivity → dPCR or nested PCR.
4. What sample type? Pure DNA → standard PCR. Degraded DNA → nested or short-amplicon qPCR. RNA → RT-PCR.