Polymerase chain reaction (PCR) is a technique used to amplify a sample of DNA. PCR sensitivity refers to how effectively a sample is amplified. Sensitivity is both a blessing and a curse for people who use this technique to prepare DNA for analysis. A number of things can influence PCR sensitivity, and researchers have conducted a range of studies to learn about the things which can play a role in sensitivity, and how these things can impact the result of a test.
Put simply, in a PCR procedure, a sample of DNA is basically unzipped to create two strands, and then encouraged to replicate, creating multiple copies of the strand of DNA. This technique takes advantage of the way in which DNA replicates in the real world to create a large sample which can be used for analysis. Using PCR, people can amplify a small sample to look at a specific area of interest and to identify particular segments of DNA which may be a cause of interest or concern.
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The extreme sensitivity of this process means that PCR is very vulnerable to contamination. If the person administering the test contaminates the sample with DNA from her or his body, such as pet hair or a flake of skin, the sample will be skewed. PCR sensitivity is a major concern in labs because people want to avoid contaminated samples, if possible.
PCR sensitivity is also a blessing, however. The ability to amplify everything and anything in the sample means that a PCR can be used to find DNA which may only be present in trace amounts in a sample. For example, someone with a chronic condition like HIV could have a very low viral load, making it hard to actually find copies of the virus in the patient's blood. Thanks to PCR sensitivity, even very low viral loads are detectable, because a single copy of the virus will be amplified when it is run through PCR.
In forensic PCR analysis, PCR sensitivity can be critical, as it may allow a technician to identify foreign DNA in a sample which could provide a clue to a crime. Analysis of ancient DNA samples can also be complicated or made easier by sensitivity. The ability to amplify a tiny sample of DNA can be a blessing when only a limited sample is available, as when a forensic anthropologist has only a single tooth to work with when identifying someone, but it also means that a limited sample can be compromised beyond use by contamination.
Many things can influence PCR sensitivity. The precise process used in the lab is a factor, as are the compounds used to process the sample. This is why technicians must carefully document the process, so that someone studying the results can look for problematic techniques or chemicals used which might invalidate the results or call them into question.