A no-template control appears at Cq 38. The consequence may be clear: additional investigation, repeat testing, or delayed results. The cause is less obvious. Is the signal coming from carry-over amplicons generated within the workflow, or contaminating DNA introduced with the reagents?
Once contamination is detected, identifying its source becomes the challenge. Different contamination sources can generate the same positive NTC result while requiring completely different corrective actions. Without a clear understanding of where contamination originated, troubleshooting becomes slower, more costly and less predictable.
The distinction matters because not all contamination is created equal. Carryover contamination originates from previous amplification reactions, while contaminating DNA may be introduced through enzymes, buffers or other assay components. Yet both can produce identical assay signals. Effective contamination control therefore depends less on reacting to contamination events and more on implementing preventive strategies that reduce contamination risk before it impacts assay performance.
Two Sources of PCR Contamination
Contamination reaches a molecular assay by two main routes, and robust testing has to defend against both.
1. PCR carry-over contamination: amplicons from previous reactions.
A single PCR can generate up to a billion copies of its target sequence. Whenever amplified material is handled, whether during routine workflow steps or through accidental tube opening, those copies can aerosolize and spread to pipettes, benches, consumables, and reagents.
Because PCR amplicons are abundant, stable, and readily re-amplified, even trace amounts can become a persistent source of false positive results. The challenge is not simply removing contamination once it has occurred. Carryover prevention must be established before contamination enters the workflow. If previous reactions were not designed with a dUTP/UDG strategy in place, contaminating amplicons remain fully amplifiable and cannot be selectively removed later.
As Rys and Persing described it, PCR contamination was historically "the greatest impediment to routine implementation of nucleic acid amplification."[1] Left unchecked, yesterday's positive result can become today's false positive.
2.Reagent derived contamination, including master mixes.
It is tempting to assume commercial reagents are free of contaminating DNA, but the evidence suggests otherwise. In their landmark 2014 study, Salter and colleagues showed that contaminating DNA is ubiquitous in commonly used extraction kits and laboratory reagents, varies between batches, and can critically impact results from low-biomass samples.[2] The lower the true signal, the greater the influence of background contamination. In sensitive clinical and microbiome applications, contaminating DNA in a master mix can generate results that do not originate from the sample itself.
Why the pressure is rising now
If contamination is an old problem, why does it feel more urgent today? The answer lies in the growing sensitivity of molecular assays. As detection technologies improve, even trace amounts of contaminating nucleic acid can influence results, creating greater pressure to control contamination across the workflow.
Increasing sensitivity
Advances in PCR chemistry, instrumentation and assay design have pushed detection limits lower than ever before, enabling earlier and more accurate detection across clinical, environmental and research applications. That sensitivity comes with a trade-off. Low-level carryover amplicons or reagent-derived DNA that would previously have gone unnoticed can now generate detectable signals.
Expanding applications
The 2025 revision of the MIQE guidelines notes that qPCR is now used across clinical diagnostics, agriculture, environmental monitoring, forensic science and regulatory testing. As molecular methods become embedded in more decision-making processes, the impact of inaccurate or inconsistent results becomes more significant, with consequences that can include misdiagnosis, unnecessary interventions and compromised public health measures.[3]
Why Quality and Regulatory Expectations Are Increasing
Regulatory pressure is real, even where the mechanism is still being contested. In May 2024 the FDA finalised a rule to phase out its long-standing enforcement discretion for laboratory-developed tests, many of them molecular assays; a federal court vacated the rule in March 2025, and the FDA formally rescinded it later that year.[4] The broader emphasis on assay robustness, reproducibility and quality control remains evident in guidance documents such as MIQE 2.0.[3]
For assay developers, this means treating contamination prevention as part of assay design and validation rather than relying solely on procedural safeguards during routine use.
How Laboratories Prevent Contamination
Two sources, two different controls. Physical separation of pre- and post-amplification areas, unidirectional workflows and no-template controls remain the foundation, but procedure alone is not enough, which is why biochemical safeguards matter.
It is worth being clear about what the no-template control can and cannot do. It tells you that something has entered the reaction. It does not tell you which of the two sources it came from, which is why the distinction has to be designed into the assay rather than diagnosed after the fact.
Carry-over Contamination Control
For carryover contamination, the dUTP/uracil-DNA glycosylase (UNG) system introduced by Longo and colleagues in 1990 remains the gold standard. By substituting dUTP for dTTP, every amplicon becomes effectively tagged, allowing UNG to degrade uracil-containing carryover products before the next reaction begins while leaving native template DNA unaffected.[5] The approach carries one prerequisite: every PCR in the workflow has to be run with dUTP in place of dTTP, or there is nothing for the enzyme to recognise.
Cod UNG completely changed the game by successfully enabling seamless carryover contamination control in one-step RT-qPCR. Traditional UNGs are hyperactive at reverse transcription temperatures, meaning they would instantly destroy your cDNA as it is being made. Cod UNG solves this by delivering total pre-PCR cleanup at room temperature, then permanently deactivating before the RT step even begins.
In addition to unlocking single-tube RNA diagnostics, Cod UNG delivers a massive advantage post-PCR: it completely safeguards your product integrity. Because the heat inactivation is 100% irreversible, the enzyme will never wake up to chew up your DNA upon cooling. This ensures your valuable dU-containing amplicons remain perfectly intact and stable for critical downstream applications like cloning, next-generation sequencing (NGS), and gel electrophoresis.
Learn more about the development of Cod UNG and why PCR needed a fish.
Reagent-Derived Contamination Control
Contamination originating from reagents and master mixes demands a highly specialized approach to remove contaminants while preserving primers, probes and other assay components. Champlot et al. validated a robust procedure utilizing a heat-labile, double-strand-specific DNase (HL-dsDNase). The strict double-strand specificity is critical, as it selectively degrades contaminating genomic DNA while leaving single-stranded primers and probes in the master mix untouched.[6]
Furthermore, because the enzyme is uniquely heat-labile, it can be fully inactivated by a mild heat inactivation step before adding sample and running the PCR, successfully preserving the assay's ability to amplify minute quantities of target DNA. This approach perfectly embodies the "prevent and destroy" strategy described by Borst et al.: preventing contamination from entering the workflow, and rapidly eliminating contaminating DNA when it does.[7]
The bottom line
As tests become more powerful, more widely used and more heavily scrutinised, contamination control is no longer simply a best practice. It is a prerequisite for generating results that are robust, reproducible and clinically meaningful.
Assay developers and laboratories that build contamination control into the whole workflow, from reagent preparation and master mix quality through to amplification and analysis, are better positioned to deliver results that clinicians can trust. Heat-labile dsDNase for reagent decontamination and Cod UNG for carryover prevention address contamination risk at different stages of that same workflow.
References
- Rys PN, Persing DH. Preventing false positives: quantitative evaluation of three protocols for inactivation of PCR amplification products. J Clin Microbiol. 1993;31(9):2356–2360. doi.org/10.1128/jcm.31.9.2356-2360.1993
- Salter SJ, Cox MJ, Turek EM, et al. Reagent and laboratory contamination can critically impact sequence-based microbiome analyses. BMC Biol. 2014;12:87. doi.org/10.1186/s12915-014-0087-z
- Bustin SA, Ruijter JM, van den Hoff MJB, et al. MIQE 2.0: revision of the Minimum Information for Publication of Quantitative Real-Time PCR Experiments guidelines. Clin Chem. 2025. doi.org/10.1093/clinchem/hvaf043
- FDA. Medical Devices; Laboratory Developed Tests. Final Rule, 89 FR 37286 (May 6, 2024); vacated by American Clinical Laboratory Association. FDA (E.D. Tex., March 31, 2025); rescinded via 90 FR 45134 (September 19, 2025).
- Longo MC, Berninger MS, Hartley JL. Use of uracil DNA glycosylase to control carry-over contamination in polymerase chain reactions. Gene. 1990;93(1):125–128. doi.org/10.1016/0378-1119(90)90145-H
- Champlot S, Berthelot C, Pruvost M, et al. An efficient multistrategy DNA decontamination procedure of PCR reagents for hypersensitive PCR applications. PLoS ONE. 2010;5(9):e13042. doi.org/10.1371/journal.pone.0013042
- Borst A, Box ATA, Fluit AC. False-positive results and contamination in nucleic acid amplification assays: suggestions for a prevent and destroy strategy. Eur J Clin Microbiol Infect Dis. 2004;23(4):289–299. doi.org/10.1007/s10096-004-1100-1
