Overview
The polymerase chain reaction (PCR) cycle is the repeated three-step temperature sequence, denaturation, annealing, and extension, that copies a selected nucleic acid target so it can be detected.
Classification
| Step | Practical job |
|---|---|
| Denaturation | Separate DNA strands with heat |
| Annealing | Allow primers to bind the target |
| Extension | Polymerase synthesizes new DNA |
| Repeat | Increase target copy number for detection |
Morphologic Features
Not applicable, this entry covers thermal cycling steps, not morphologic identification.
Laboratory Characteristics
- Instrument temperatures and times are assay-specific
- Interrupted cycling is not a reportable patient result
- Ct/Cq values relate to signal during cycling and are assay-dependent
Reference Intervals
Not applicable as a universal numeric interval. Cycle programs and any Ct/Cq rules are defined by the validated assay.
Clinical and Laboratory Significance
Correct cycling is required to generate detectable copies of the selected target. Incorrect or incomplete cycling can cause false-negative, invalid, or unreliable results.
Differential Considerations
Distinguish incomplete/incorrect cycling or instrument failure from a true patient-negative result with a completed valid run and passing controls.
Comparison Tables
PCR cycle checkpoints
| Observation | Safe action | Unsafe action |
|---|---|---|
| Run interrupted mid-cycle | Follow SOP, do not report as negative | Call not detected anyway |
| Tempted to add extra cycles | Keep validated cycle count | Extend cycles to force a positive |
| Late amplification | Apply assay Ct/Cq rules | Assume severity from cycle number alone |
Classification Frameworks
Not applicable . World Health Organization (WHO) disease classifications do not define PCR cycle steps.
Laboratory Notes
- Denaturation → annealing → extension, then repeat
- Use only the validated assay thermal profile
- Interpretation follows the validated assay and laboratory procedure
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021. (Molecular diagnostics chapters, verify current edition.)
- Buckingham L. Molecular Diagnostics: Fundamentals, Methods, and Clinical Applications. 3rd ed. F.A. Davis; 2019. (Verify current edition.)
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute (CLSI). Consult current CLSI molecular method documents available through your laboratory, document numbers and editions change; do not treat any single CLSI title as universal.
- Institutional molecular laboratory SOPs, assay manufacturer instructions for use (IFU), and locally adopted accreditation requirements. Always verify current local practice.
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