What the reaction actually does
PCR (polymerase chain reaction) is a cycling amplification method. Each cycle has three parts: denaturation, where heat separates the DNA strands; annealing, where short primers bind the sequences that flank the target; and extension, where a heat-stable polymerase copies the region between the primers. Repeating the cycle doubles the target each time, so the reaction converts an undetectable amount of nucleic acid into a measurable signal. Detection is fluorescent, and the number of cycles needed to cross the detection threshold gives a rough sense of how much target was present at the start.
Culture and PCR answer different questions
Culture
- Requires living, replicating organisms
- Takes one to several days
- Yields an isolate for identification and susceptibility testing
- Fails if the patient received effective antibiotics before collection
- Cannot grow organisms with no available culture system
PCR from a direct specimen
- Requires only intact nucleic acid, living or not
- Turns around in hours
- Yields a presence or absence result for defined targets
- Still works after antibiotics have sterilized the culture
- Detects unculturable organisms if a target exists
Multiplex panels: breadth in one specimen
A multiplex panel runs many primer sets against a single specimen, grouped by syndrome rather than by organism. A respiratory panel, a gastrointestinal panel, and a meningitis/encephalitis panel each cover the pathogens that commonly cause that syndrome. This is efficient when the differential is broad and the specimen is limited, but it fixes the menu in advance. A pathogen not on the list is not detected, and the result is reported as negative for the targets tested, not as absence of infection.
Reading a negative panel
A negative multiplex result means none of the panel targets were detected. It does not mean the specimen is sterile, that no organism is present, or that the patient does not have an infection. The correct next step depends on whether the clinical picture still points to an organism outside the panel's coverage.