Advantages of PCR Testing.
- Valuable for detecting specific pathogens that are difficult to culture in vitro or require a long cultivation period
- Significantly more rapid in providing results compared to culturing
- Enables earlier informed decision making
- Rapid diagnosis of bacteremia, particularly for low levels of bacteria in specimens
- Useful in detecting cases in extra pulmonary specimens which may be missed by smear
and/or culture - Valuable screening tool
- PCR is still considered an adjunct test for certain diagnostic tests that still rely on
- smear and culture, such as tuberculosis
Limitations of PCR Testing.
- PCR testing alone may be limited as a diagnostic tool
- Still need culture for testing for drug/antibiotic susceptibility and genetic typing
- Post treatment diagnosis may be challenging
- PCR detects dead organisms that may be shed for weeks after the patient stops showing symptoms. Unclear regarding persistence of infection. Detecting dead organisms at this stage may have no clinical relevance
- PCR results should not be used as the sole basis of a patient treatment management decision. All results should be interpreted by a trained professional in conjunction with review of the patient’s history and clinical signs and symptoms.
- False positive and false negative results
- False negative results can be caused by:
- Improper sample collection/transport
- Insufficient amount of specimen
- Degradation of nucleic acids (typically RNA) during shipping or storage
- Specimen collected prior to onset of symptoms or late in illness
- Quantity of organisms is below detection limit
- Non-homogeneous distribution of the organism of interest
- Presence of amplification inhibitors in the specimen
- Laboratory processing/testing errors
- False positive results can be caused by:
- Detecting contaminants introduced during specimen collection, transport or processing
- Detecting organism’s representative of normal flora near specimen
- collection site, acid fast bacilli in water and contaminants in lab
- Mislabeling
- Specimen mix-up
- False negative results can be caused by:
Interpreting the results.
- A negative result.
- A negative result means that there is no evidence of DNA or RNA of the target
organism in the specimen tested. If no other etiology is identified and a specific
infection is still clinically suspected, additional specimens should be collected
and tested.
- A negative result means that there is no evidence of DNA or RNA of the target
- Positive result.
- A positive result indicates detection of DNA or RNA and confirms infection, but
does not necessarily mean viable organism of interest is present or that the
patient is contagious
- A positive result indicates detection of DNA or RNA and confirms infection, but
Reasons for discrepant results between PCR and culture.
- PCR Positive and Culture Negative
- PCR is generally more sensitive than culture for detecting organisms of interest
- Subclinical colonization
- PCR detects non-viable organisms
- Administration of antibiotics; injured organisms
- False positive PCR results
- PCR Negative and Culture Positive
- Presence of compounds in specimen that inhibit amplification of nucleic acids
- Laboratory testing error
- False negative PCR result
