Influencing factors associated with cycle threshold values of nucleic acid amplification tests in nasopharyngeal swabs of children with pertussis

Objective To explore the influencing factors of cycle threshold(Ct) values of nucleic acid amplification tests of Bordetella pertussis(BP) in nasopharyngeal swabs of children with pertussis.Methods The children with pertussis who were positive for BP nucleic acid in nasopharyngeal swabs and met the clinical diagnostic criteria in the inpatient department of Shanghai Children's Hospital from May 2021 to July 2022 were included in the study. The clinical data of children were collected retrospectively, and the differences of Ct values among different factor groups were analyzed.Results A total of 42 children with pertussis were included, and the average Ct values of nucleic acid detection was 29.8±4.1. The median age was 3 months, the minimum age was 1 month, and the maximum age was 9 years and 8 months. Children aged 1-5 months accounted for 64.3%, children without pertussis vaccine accounted for 57.1%, children exposed to family respiratory tract infection accounted for 45.2%, children with spastic cough accounted for 71.4%, and the proportion of peripheral blood lymphocytes ≥ 60% accounted for 78.6%. The Ct values of the group aged 1-5 months was lower than that of the group aged over 6 months (28.9±4.0 VS 31.5±3.8, p<0.05). The Ct values of the group without pertussis vaccination was lower than that of the vaccinated group (28.7±4.1 VS 31.2±3.7, p<0.05). The Ct values of the group exposed to family respiratory tract infection before onset was lower than that of the group without contact history (28.1±3.4 VS 31.2±4.1, p<0.05). The Ct values of the spastic cough group was lower than that of the non-spasmodic cough group (28.6±3.7 VS 32.8±3.4, p <0.01). The Ct values of the peripheral blood lymphocyte ratio ≥ 60% group was lower than that of the peripheral blood lymphocyte ratio < 60% group (29.0±3.7 VS 32.7±4.2, p<0.05).Conclusion Children with pertussis less than 6 months old, not vaccinated with pertussis vaccine, contacting families with respiratory tract infection, having spastic cough and peripheral blood lymphocyte ratio ≥ 60% may have lower Ct values of nucleic acid amplification tests and may have higher infection load, which should be paid attention to by clinicians.