Which of the following is the most common source of contamination leading to false-positive results in PCR amplification?

Explanation

## Most Common PCR Contamination Source **Key Point:** Amplicon carryover contamination is the most frequent and clinically significant source of false-positive PCR results in diagnostic laboratories. ### Why Amplicon Carryover is the Biggest Problem 1. **Exponential amplification**: PCR products accumulate to ~10^9 copies per reaction 2. **Aerosol generation**: Opening tubes and pipetting creates fine aerosol droplets carrying amplicons 3. **Environmental persistence**: Amplicons are stable DNA fragments that persist on surfaces, pipette tips, and equipment 4. **Cross-contamination**: Even nanogram quantities of amplicon can seed subsequent reactions and be amplified exponentially ### Prevention Strategies | Strategy | Mechanism | |----------|----------| | **Separate laboratory spaces** | Physically isolate pre-PCR (extraction) from post-PCR (amplicon handling) areas | | **Unidirectional workflow** | Never move samples backward from post-PCR to pre-PCR zones | | **Dedicated pipettes & tips** | Use aerosol-resistant tips and equipment reserved for each phase | | **UV irradiation** | Expose work surfaces and equipment to UV light to degrade DNA | | **Enzymatic degradation** | Use dUTP/uracil-DNA glycosylase (UNG) to destroy carry-over amplicons | **High-Yield:** In diagnostic PCR (especially for infectious diseases and forensics), amplicon contamination is responsible for >90% of false-positive results. This is why "separate spaces" is a mandatory quality control measure in accredited labs. **Clinical Pearl:** A patient tested "positive" for a pathogen in a clinical PCR run, but repeat testing was negative — amplicon carryover from a previous high-load positive sample is the most likely explanation. **Mnemonic:** **SAFE PCR** = Separate spaces, Aerosol-resistant tips, Forward workflow only, Enzymatic/UV decontamination