PCR testing: direct detection of organism genetic material
PCR (polymerase chain reaction) copies a chosen stretch of a pathogen's DNA or RNA in a sample until it can be detected, so it identifies the pathogen directly.
Remember
PCR looks for the pathogen's own DNA or RNA, so it identifies the infection directly, without needing the pathogen to grow or the patient to make antibodies.
Memory aid
PCR is a photocopier for genetic material
Think of PCR as a photocopier: even a tiny amount of a pathogen's DNA or RNA can be copied until it is enough to read.
PCR stands for polymerase chain reaction. It is a laboratory technique that picks out a short, chosen stretch of a pathogen's genetic material (its DNA or RNA) in a sample, such as a throat swab, and copies it many times until there is enough to detect. This copying is called amplification.
Each chosen stretch is designed to match the genetic code of one particular pathogen, so a positive result for it identifies that pathogen directly. Many bacterial and viral infections can be diagnosed this way, and PCR is especially helpful when a pathogen is difficult or impossible to grow in culture.
Because PCR looks for the pathogen itself, it does not rely on the patient producing antibodies. That matters in an immunocompromised patient, meaning someone whose immune system is weakened, because tests that depend on the immune response can fail there.
The same technique can also be built to look for resistance genes, the stretches of DNA that code for resistance to particular antibiotics. One test can then name the pathogen and warn which antibiotics may fail, so clinicians can move from broad, best-guess antibiotics to a narrower, targeted drug.
PCR has limits too. Cost and availability vary, and not every test is fast.
Key facts
- Target
- PCR finds a chosen stretch of the pathogen's own DNA or RNA in the sample.
- Method
- It copies that stretch many times until it can be detected.
- Specificity
- Each chosen stretch is designed to match one particular pathogen, so a positive result for it identifies that pathogen directly.
- Advantage
- It works for pathogens that are hard to grow in culture and does not depend on the patient making antibodies.
- Resistance genes
- PCR can be built to look for the stretches of DNA that code for resistance to particular antibiotics.
- Limits
- Cost and availability vary, and not every test is fast.
Exam traps
- Identifying a pathogen by its appearance after staining is the Gram stain, which shows colour and shape but often cannot name the exact species. PCR does not look at stains.
- Measuring the body's antibody response is serology, which tests the immune system's reaction rather than the pathogen. PCR does not depend on that response.
- Identifying a pathogen by whether it grows in culture is how culture-based tests work, not PCR, which copies genetic material and can detect pathogens that are no longer able to grow.
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