If my PCR (Polymerase Chain Reaction) sensitivity for Bordetella pertussis stops at 10^4 copies/ml, am I losing clinical significance?

From the Guidelines

A PCR sensitivity threshold of 10^4 copies/ml for Bordetella pertussis detection may indeed result in missing some clinically significant infections, and for optimal clinical utility, laboratories should aim for detection limits of 10^2-10^3 copies/ml. This sensitivity level is suboptimal as B. pertussis can cause symptomatic disease at lower bacterial loads, particularly in partially treated patients, those with prior vaccination, or in later stages of infection when bacterial shedding decreases 1.

Key Considerations

• The clinical presentation should guide management decisions, and empiric treatment with macrolides (azithromycin 500mg on day 1 followed by 250mg daily for 4 more days; or clarithromycin 500mg twice daily for 7 days; or erythromycin 500mg four times daily for 14 days) may be warranted in highly suspicious cases despite negative PCR results 1.
• If using this less sensitive assay, consider supplementing with additional diagnostic methods such as culture or serology in suspected cases with negative PCR results.
• The reduced sensitivity is particularly problematic because pertussis can progress from the catarrhal to paroxysmal stage while bacterial loads decline, potentially leading to false negatives during the period when patients are most likely to seek medical attention.

Diagnostic Approaches

• PCR is a rapid and highly specific test for Bordetella spp and has a sensitivity as high as 80 to 100% 1.
• Although widely used in the United States, PCR assays have not been standardized, and the analytical sensitivity, accuracy, and quality control of PCR-based B. pertussis tests can vary widely among laboratories 1.
• Serologic studies, including IgG and IgA titers of the antibody to PT, filamentous hemagglutinin (FHA), pertactin, and fimbriae, are available, but their use is limited by the need for paired sera and the potential for past infection or previous immunization to affect results 1.

Treatment and Prevention

• Treatment with erythromycin (or trimethoprim/sulfamethoxazole when a macrolide cannot be given) is necessary, and the recommended dose is 40 to 50 mg/kg/d in children and 1 to 2 g per day in adults for 2 weeks 1.
• Newer macrolides such as clarithromycin and azithromycin are also active against B. pertussis and have a better side-effect profile 1.
• Acellular pertussis vaccines are effective in preventing pertussis, and their use is recommended for adolescents and adults 1.

From the Research

PCR Sensitivity and Clinical Significance

• The PCR sensitivity for Bordetella pertussis stopping at 10^4 copies/ml may not necessarily lose clinical significance, as the detection of the bacteria is still possible at this concentration 2.
• A study comparing PCR findings with serology, culture, and clinical characteristics of cough illnesses found that PCR is more sensitive than conventional culture for the diagnosis of pertussis, with a sensitivity of 61% and specificity of 88% 2.
• Another study evaluated the use of real-time PCR for detection of B. pertussis and found that the sensitivity was 1 to 10 CFU/ml, which is equivalent to approximately 10^4 copies/ml 3.
• The clinical significance of detecting B. pertussis at 10^4 copies/ml is still important, as it can help initiate treatment and prevent transmission of the disease 4, 5.

Comparison of PCR and Other Diagnostic Methods

• PCR has been shown to be more sensitive than culture for the diagnosis of pertussis, with a higher detection rate in clinical specimens 2, 3, 5.
• Real-time PCR has been found to be superior to conventional PCR and culture for the diagnosis of B. pertussis, with a higher sensitivity and specificity 3, 5.
• The use of multiple real-time PCR assays can improve the detection rate of B. pertussis and help discriminate between B. pertussis and other Bordetella species 3.

Factors Influencing PCR Results

• The sensitivity of PCR can be influenced by factors such as the quality of the specimen, the transportation and storage of the specimen, and the age and vaccination status of the patient 5.
• The use of internal controls and duplexing with other PCRs can also affect the sensitivity and specificity of the PCR assay 3.