Molecular Diagnostics
PCR (Polymerase Chain Reaction)
Exponential DNA amplification — the bedrock of molecular diagnostics
Polymerase chain reaction amplifies a specific DNA segment exponentially through repeated thermal cycling. Each cycle has three steps — denaturation (94-98°C, separates strands), annealing (50-65°C, primers bind), extension (72°C, Taq polymerase synthesizes). After n cycles, ~2^n copies. Invented by Kary Mullis in 1983 (Nobel Prize 1993). Variants: RT-PCR for RNA (HIV, SARS-CoV-2), qPCR/real-time PCR for quantification, digital PCR for absolute counts, multiplex PCR for multiple targets simultaneously. Clinical uses span infectious disease, oncology (BCR-ABL, EGFR mutations), genetic testing, forensics, and minimal residual disease monitoring.
- Invented byKary Mullis, 1983 (Nobel 1993)
- Key enzymeTaq polymerase (Thermus aquaticus, hot springs)
- Cycle stepsDenature 95°C, anneal 50-65°C, extend 72°C
- Amplification~2^n copies after n cycles
- Detection limitSingle-copy DNA in real-time PCR
- Cycle threshold (Ct)Lower Ct = more starting template
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Why PCR matters
- Infectious disease diagnosis. SARS-CoV-2, HIV, HCV, TB, STIs all rely on PCR detection.
- Viral load monitoring. HIV and HCV treatment success measured by quantitative PCR.
- Cancer diagnostics. Detects driver mutations and minimal residual disease.
- Genetic testing. Single-gene disorders, BRCA, cystic fibrosis carrier screening.
- Prenatal diagnosis. NIPT detects fetal aneuploidy from maternal blood.
- Forensics. DNA fingerprinting from minute samples (hair, semen, touch DNA).
- Pandemic response. Rapid PCR scaling defined COVID-19 testing infrastructure.
Common misconceptions
- Positive PCR means active infection. Detects nucleic acid; can be from non-viable organisms or past infection.
- Lower Ct equals more contagiousness. Correlates but cycle thresholds aren't standardized between labs.
- PCR is always quantitative. Standard PCR is endpoint detection; only qPCR/dPCR quantify.
- One PCR test rules out disease. Sample collection errors and PCR inhibitors cause false negatives.
- PCR amplifies any DNA. Requires specific primers — won't detect unanticipated organisms.
- RT-PCR means real-time. RT actually means reverse transcription; real-time is distinct (often combined as RT-qPCR).
Frequently asked questions
Why does Taq polymerase enable PCR?
PCR requires high heat (95°C) to denature DNA, which would destroy regular DNA polymerases. Thomas Brock isolated Thermus aquaticus from Yellowstone hot springs in 1969; its DNA polymerase functions at 70-80°C and survives 95°C. Before Taq, PCR required adding fresh enzyme each cycle — impractical. Taq's heat stability enabled automation and commercial thermal cyclers. Modern engineered enzymes (Pfu, Phusion) add proofreading for higher fidelity.
How does real-time qPCR quantify DNA?
Uses fluorescent probes or intercalating dyes (SYBR Green) that emit light proportional to amplicon quantity. Ct (cycle threshold) is the cycle at which fluorescence crosses a baseline — inversely related to starting template. Each Ct unit corresponds to ~2x more starting DNA. Standard curves convert Ct to absolute copy number. SARS-CoV-2 testing reports Ct values; lower Ct (< 25) suggests high viral load and contagiousness. ΔΔCt method compares gene expression between samples.
What's RT-PCR vs qPCR vs RT-qPCR?
RT-PCR = Reverse Transcription PCR — RNA first reverse-transcribed to cDNA by reverse transcriptase, then amplified. Used for RNA viruses (HIV, HCV, SARS-CoV-2) and gene expression. qPCR = quantitative real-time PCR — measures amplification in real time without endpoint gel. RT-qPCR combines both — quantitative measurement of RNA, the standard for viral load monitoring. Confusing because 'RT' has been used for both reverse transcription and real-time.
How is PCR used clinically for infectious disease?
PCR detects pathogen DNA/RNA before serology (which requires immune response). Examples: HIV viral load (treatment monitoring, < 20 copies/mL undetectable goal), HCV (sustained virologic response criterion), HSV CSF (encephalitis), CMV blood (post-transplant), Chlamydia/gonorrhea NAAT, MTB (Xpert MTB/RIF detects TB and rifampin resistance in 2 hours), and SARS-CoV-2 nasopharyngeal/saliva testing. Sensitivity and specificity often > 95%.
How does PCR detect cancer mutations?
Targeted PCR amplifies regions with known mutations, then sequencing or allele-specific probes identify variants. Examples: BCR-ABL fusion in CML (monitoring tyrosine kinase inhibitor response), EGFR mutations in lung cancer (predicting erlotinib/gefitinib response), KRAS in colorectal (anti-EGFR therapy decisions), BRAF V600E in melanoma. Liquid biopsy uses PCR to detect circulating tumor DNA (ctDNA) at < 0.1% allele frequency, enabling non-invasive monitoring.
What is digital PCR?
Digital PCR partitions a sample into thousands of tiny reactions (droplets or wells), each containing 0 or 1 template molecule. After amplification, count positive vs negative partitions. Poisson statistics give absolute copy number — no standard curve needed. Advantages: detects rare variants (1 in 10,000), tolerates inhibitors, gives absolute quantification. Used for ctDNA detection, copy number variants, rare allele detection in transplant rejection (donor-derived cell-free DNA).
Why do PCR results sometimes give false positives or negatives?
False positives: contamination from amplicon carryover (use UNG/UDG enzyme to degrade prior products), reagent contamination, primer-dimer artifacts, non-specific amplification. False negatives: PCR inhibitors in sample (heparin, hemoglobin, urine), insufficient sample, target mutation in primer site, RNA degradation. Quality controls: positive control (must amplify), negative control (must not), internal control (validates extraction). Cycle threshold variability between labs requires calibration.