Bordetella pertussis adenylate cyclase activity in nasopharyngeal aspirates for rapid diagnosis of whooping cough in relation to culture and serology

The diagnosis of pertussis: which method to choose?

Despite the introduction of routine vaccination against pertussis for more than a half century, leading to a drastic decline in the number of reported cases, pertussis continues to be an important respiratory disease afflicting unvaccinated infants and previously vaccinated children as well as adults in whom immunity has waned. The diagnosis of pertussis is challenging and accurate laboratory identification of Bordetella infections remains problematic. Common laboratory diagnostic methods used for pertussis diagnosis include culture, direct-fluorescent-antibody testing (DFA), serology and polymerase chain reaction (PCR). Culture of Bordetella pertussis is highly specific but fastidious and has limited sensitivity. DFA provides a much more rapid result, but has the disadvantage of poor sensitivity and specificity. Serology is not useful in infants. In older persons, it is hampered by the limitations of paired sera and it provides mainly a retrospective diagnosis. Such limitations of conventional diagnosis testing have led to the development of PCR assays. Notwithstanding its lack of standardization, PCR has been found to be more sensitive and more specific than other methods. In this report, we aimed to review current knowledge about the available diagnostic methods and tests that accurately diagnose pertussis.

Nested PCR optimized for detection of Bordetella pertussis in clinical nasopharyngeal samples

Several genes and sequences in Bordetella pertussis have been used as targets in diagnostic PCR assays. A previously developed single-step PCR assay for the detection of B. pertussis was based on an insertion sequence, IS480, that is present in about 70 to 80 copies in each genome. The diagnostic sensitivity, specificity, and reliability of this assay with aspirated and heat-treated samples from the nasopharynx of patients and their contacts was improved by the use of a nested PCR configuration. The nested PCR assay produced a 205-bp fragment with all of the 115 B. pertussis strains tested and was negative with all strains belonging to other Bordetella species (n = 44) as well as other bacteria commonly found in the upper respiratory tract (n = 115). The diagnostic value of the assay was verified by giving positive results for B. pertussis in all the 51 nasopharyngeal aspirates from culture-positive patients. The assay also detected 18 positive aspirates from a total of 196 culture-negative patients. A confirmatory cleavage of the 205-bp nested PCR product by MvaI gave in all cases two bands of 88 and 117 bp. In conclusion, this newly developed nested PCR assay was shown to be reasonably fast and uncomplicated, with an optimal sensitivity and a high degree of specificity for the diagnosis of B. pertussis in aspirated nasopharyngeal samples processed simply by heat treatment. The detection level in the nested PCR was about 10 bacteria per ml, or seven to eight insertion sequence copies per 10 microliters of boiled sample.

Amplification of DNA by the polymerase chain reaction for the efficient diagnosis of pertussis

The standard diagnostic methods for pertussis have several shortcomings. With the increased knowledge of the Bordetella pertussis genome a specific and conserved DNA sequence, present in about 70-80 copies in each genome, was selected for amplification with the polymerase chain reaction (PCR) technique in order to evaluate its diagnostic potential in children with suspected pertussis. The 400 basepair DNA sequence chosen was present and amplified in all 112 B. pertussis strains and in no other bacterial species examined. The specificity of the amplified material was documented by restriction enzyme cleavage. In nasopharyngeal aspirates a B. pertussis specific PCR product was visualized in 19/25 culture positive and in 5/50 culture negative children. In conclusion the present PCR assay for B. pertussis can be clinically useful and permit a specific diagnosis within 1 day after sampling. Further studies are requested to document its sensitivity, specificity and predictive value for positive and negative results.