Pertussis, commonly known as whooping cough, is a highly contagious respiratory infection caused by the bacterium Bordetella pertussis. This illness can be particularly severe in infants and young children, often leading to serious complications. Accurate and prompt laboratory diagnosis is important for patient management and controlling disease spread. Testing involves multiple considerations regarding the method chosen and the timing of its use.
Available Testing Methods
Laboratory confirmation of pertussis relies on several methods, each with advantages and limitations.
Bacterial Culture
Bacterial culture, specifically for Bordetella pertussis, has historically been considered a definitive diagnostic test. This method involves growing bacteria from a collected specimen on specialized agar, such as Bordet-Gengou or Regan-Lowe. While highly specific, culture sensitivity is lower than other methods and decreases significantly after the first two weeks of cough onset. Results typically take 7 to 10 days.
Polymerase Chain Reaction (PCR)
Polymerase Chain Reaction (PCR) testing detects the genetic material (DNA) of Bordetella pertussis. PCR is more sensitive than culture and yields results faster, often within hours to a day. This makes PCR useful for diagnosing pertussis during the early cough stage and the first few weeks of paroxysmal cough. However, high sensitivity can lead to false-positive results, especially in low-prevalence settings or if the individual recently received a pertussis vaccine.
Serological Testing
Serological testing measures the body’s immune response by detecting antibodies against Bordetella pertussis. These tests look for immunoglobulin A (IgA), immunoglobulin G (IgG), or immunoglobulin M (IgM) antibodies. Serology is most useful later in the illness, four weeks or more after cough onset, when bacterial detection methods like culture and PCR may no longer be positive. The delayed antibody response means serology is not suitable for acute diagnosis but can confirm recent infection.
Importance of Specimen Collection
The accuracy of any pertussis test result depends on the quality of the collected specimen. For bacterial culture and PCR, the preferred specimen type is a nasopharyngeal aspirate or a nasopharyngeal swab. Proper collection technique is important to ensure an adequate amount of bacterial cells are obtained from the posterior nasopharynx. Incorrect collection, such as obtaining an anterior nasal swab, may result in an insufficient bacterial load, leading to a false-negative test result.
After collection, the specimen must be promptly placed into appropriate transport media. For culture, a specialized medium like Regan-Lowe or Amies charcoal preserves bacterial viability during transit. For PCR, a universal transport medium is often suitable. Rapid transport to the laboratory maintains sample integrity and prevents degradation of bacterial DNA or loss of viable organisms, ensuring an accurate result.
Understanding Your Test Results
Interpreting pertussis test results requires careful consideration beyond a positive or negative readout. A positive PCR result needs evaluation with the patient’s clinical symptoms, such as the characteristic paroxysmal cough, and any epidemiological context. This includes a known pertussis outbreak in the community or contact with a confirmed case. False positives can occur with PCR, particularly if the test is performed too late in the illness, due to detecting residual DNA from a past infection or vaccine.
A negative test result does not always definitively rule out pertussis. False negatives can arise if the specimen was collected too late in the illness, beyond the optimal window for bacterial detection, or if the collection technique was inadequate. Therefore, a negative result in a patient with strong clinical suspicion, especially if testing was performed late, warrants further clinical assessment. Consulting a healthcare provider is recommended to integrate test results with the patient’s overall health and guide treatment decisions.
References
1. Centers for Disease Control and Prevention. (2024). Pertussis (Whooping Cough): Diagnostic Testing. Retrieved from https://www.cdc.gov/pertussis/about/diagnosis-testing.html