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 in nasopharyngeal swabs of children with pertussis. Methods This study is a retrospective case analysis. The children with pertussis who were positive for nucleic acid by real-time fluorescence PCR in nasopharyngeal swabs and met the clinical diagnostic criteria in the inpatient department of Shanghai Children's Hospital from May 2021 to August 2023 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. T-test was used for comparison between the two groups, and F-test was used for comparison between multiple groups. Multiple linear regression was used to analyze the related factors of Ct values.Results A total of 50 children with pertussis were included, and the average Ct value of nucleic acid detection was 30.39±3.96. The median age was 3 months, the minimum age was 1 month, and the maximum age was 9 years and 8 months. There were 28 boys (56.0%) and 22 girls (44.0%). There were 34 cases (68.0%) aged 1-5 months, 27 cases (54.0%) without pertussis vaccine, 22 cases (44.0%) exposed to family members with respiratory tract infection before onset, 38 cases (76.0%) with spastic cough and 39 cases (78.0%) with peripheral blood lymphocyte ratio ≥60%. Nasopharyngeal swabs of 38 children were cultured, and 10 cases (26.3%) were positive. The Ct value of the group aged 1-5 months was lower than that of the group aged over 6 months (29.48±3.87 VS 32.32±3.55, P<0.05). The Ct value of the group without pertussis vaccination was lower than that of the vaccinated group (28.79±3.79 VS 32.26±3.35, P<0.01). The Ct value of the group exposed to family members with respiratory tract infection before onset was lower than that of the group without contact history (29.07±3.70 VS 31.42±3.91, P<0.05). The Ct value in the spastic cough group was lower than that in the non-spasmodic cough group (29.62±3.86 VS 32.81±3.39, P<0.05). The Ct value in the peripheral blood lymphocyte ratio ≥60% group was lower than that in the peripheral blood lymphocyte ratio <60% group (29.44±3.67 VS 33.75±3.15, P<0.0l). The Ct value in the positive culture group was lower than that in the negative culture group (27.16±3.17 VS 30.70±3.75, P<0.05). Multiple linear regression analysis showed that pertussis vaccination and positive culture of Bordetella pertussis were related factors of Ct value (P<0.05).Conclusion The Ct value of nucleic acid test is higher and the infection load is lower in children vaccinated with pertussis vaccine. Active vaccination strategy is helpful to the prevention and control of pertussis.