Pertussis is a highly contagious infectious disease that is still widespread in both children and adults. The problem with pertussis is that unvaccinated infants are at risk of severe illness and death. Moreover, people around the infants can be the source of infection due to the decreased effectiveness of the vaccine. Nevertheless, no tests can detect the infection early and efficiently enough to treat or prevent the disease [12]. All existing tests require specialized equipment and trained personnel, which makes it difficult to access the tests. In addition, pertussis is highly infectious, so it is essential to have a test that provides rapid results at the patient’s first visit. We need to improve the inadequate access to testing for infected people. Especially in primary care facilities that most infected people visit first, it takes time to obtain test results due to the lack of testing capability.
We developed an immunochromatographic antigen test to improve access to testing. The ICkit result can be checked in 15 min after dropping the sample without specialized equipment. The dilution sensitivity test confirmed that the ICkit could detect the antigens of B. pertussis strains. Furthermore, this was the first prospective clinical study using the ICkit for the diagnosis of pertussis. The usefulness of the ICkit was evaluated in 195 cases suspected of being infected with B. pertussis. Their baseline demographics and clinical symptoms (Table 2 and Table 3) were similar to those previously reported [1]. There were many inpatient cases in infants without vaccination, while there were many mild cases in adults. Antimicrobial treatment started before confirming the laboratory results due to its seriousness when inpatients and infants had probable pertussis. Several people were enrolled in the study after contact with people suspected of having pertussis. Almost all cases had a cough, but fewer cases with probable pertussis had a fever than with other infections. The duration of the cough was less than 28 days in most cases. For specific symptoms, paroxysms and post-tussive vomiting were common in children and adults. On the other hand, whooping and apnea episodes were seen in children, but rarely in adults.
The clinical performance of the ICkit was well correlated with its sensitivity of 86.4% (19/22) and specificity of 97.1% (168/173) compared to rPCR. The ICkit’s specificity compared to culture was high, but its sensitivity compared to culture was lower than that compared to rPCR (Supplementary Table S1). When comparing the ICkit and culture to rPCR, the sensitivity of the ICkit was higher than that of culture (Table 4). Since rPCR is the most sensitive of pertussis tests [26], the sensitivity of the ICkit, which correlated well with the sensitivity of rPCR, is considered to be higher than that of culture.
There were eight cases in which the results of rPCR and the ICkit did not match. In the three cases that were rPCR-positive and ICkit-negative, it was suspected that the results of the ICkit were false-negative due to an antigen amount below the detection limit of the ICkit, because the three cases had a smaller number of IS481 genes in the swabs, as shown in Fig. 2. There is a similar report on the relationship between quantitative rPCR and culture [27]. This result showed that it is crucial to secure enough antigen by proper sample collection to obtain optimal results with the ICkit. The CDC describes a precise method for collecting a nasopharyngeal (NP) swab that applies to direct tests for B. pertussis [28]. The doctors’ diagnoses also supported positives for pertussis in the three cases. In the five cases that were rPCR-negative and ICkit-positive, one of them seemed to be a false-negative rPCR result based on the doctor’s diagnosis. The other four cases were presumed to be false-positive results of the ICkit. False-positive results for immunochromatographic antigen kits have been reported to be caused by non-specific reactions of the kits when the specimen contains a large number of substances such as viscous substances and human anti-mouse antibodies (HAMAs) [29].
Moreover, it was not possible to determine if the false-positive results were caused by cross-reactivity with B. parapertussis and B. holmesii in the present study (Supplementary Table S2). Even if B. parapertussis or B. holmesii causes the infection, the treatment is the same as for B. pertussis infection. The doctor’s diagnosis also suggested false-positive results with the ICkit. Therefore, we must remember that the ICkit may show false-positive results if specimens contain viscous substances, HAMAs, B. parapertussis, or B. holmesii.
Essentially, the ICkit can be positive in the presence of the antigen regardless of age. In the present study, the ICkit detected the antigen in both children and adults similarly to rPCR (Fig. 3). However, the ICkit may produce false-negative results due to the disappearance of the bacteria if time has passed since onset or if patients take antimicrobials. Therefore, caution is needed regarding the timing of ICkit use. The CDC recommends the implementation of culture within two weeks and PCR within four weeks [10]. The period during which the ICkit detected the antigen was up to the 25th day, the same as rPCR in the clinical study (Fig. 4). Nakamura et al. reported that colonization tended to be less in adults than in children [30]. In addition, adults often have mild symptoms with atypical cough for pertussis and may delay seeking medical attention [31]. Therefore, we should ask for epidemiological information such as household infections and outbreaks to use the ICkit early after onset. In the present study, all ICkit-positive cases in adults were tested within two weeks of contact with infected cases. When the antigen has already disappeared over time, an antibody test should be an option for diagnosis [32, 33]. Although the results of the present study were limited to three positive cases of rPCR in adults, further consideration of how to use the ICkit in adults is needed.
The characteristics of the ICkit’s rapidity and convenience suggest three major points. First, the ICkit would prevent infected people from getting more severe illness. The ICkit would contribute to early diagnosis and increase the opportunities for administering a macrolide, which is the first-line agent for pertussis, in the early stage [34, 35]. If macrolides can suppress bacterial growth during the catarrhal phase, they can suppress the production of toxins that exacerbate respiratory symptoms. Based only on clinical symptoms, it is often difficult to distinguish B. pertussis infection from viral and other bacterial respiratory tract infections without any characteristic symptoms, which delays the administration of macrolides [1]. Both clinical symptoms and test results are important to identify and treat pertussis in the early stage. Second, the ICkit would help in antimicrobial stewardship [36, 37]. In the present study, about 30% of cases received antimicrobials before laboratory confirmation. The shorter turnaround time with the ICkit enables doctors to check the test results on the spot and to decide whether patients need to receive antimicrobials within the first consultation. If the test results rule out pertussis, and a viral respiratory tract infection is suspected, we will refrain from unnecessary antibiotics. Due to the side effect of hypertrophic pyloric stenosis, we must avoid unnecessary macrolides in infants without testing [38, 39]. Antimicrobial stewardship is essential, because antimicrobial-resistant strains of B. pertussis have already been identified [40–42]. Third, the ICkit would contribute to preventing spread of the infection. Delay in diagnosis and treatment increases the risk of transmission and further expands large-scale outbreaks [11, 43]. The ICkit will be useful in environments where culture and nucleic acid tests are not actively available or in urgent situations such as an outbreak. In such situations, the easy accessibility of the ICkit would be beneficial for checking whether people around the infected person are infected. In institutions that have difficulty performing tests, the ICkit would make it possible to identify B. pertussis that has been missed so far. The potential benefits of the ICkit would be maximized in low-resource countries with many infected people due to low vaccine coverage and where conventional testing for pertussis is difficult due to immature infrastructure [44, 45].
In conclusion, a new, rapid immunochromatographic antigen kit for B. pertussis was developed. Its procedure is simpler and faster than conventional tests, so that its result is available at the first visit of potentially infected people. Furthermore, the ICkit is a POCT that does not require any equipment and is available in any clinical setting. The dilution sensitivity test and the clinical study demonstrated that the ICkit could detect the B. pertussis antigen. The clinical performance of the ICkit was well correlated with the rPCR results and was demonstrated within the same time from cough onset as rPCR. However, to correctly interpret the ICkit’s results, we must use the ICkit with a complete understanding of the ICkit’s precautions, such as cross-reactivity, the method for collecting the specimen and, the timing of use. Easy and rapid detection of B. pertussis by the ICkit would help diagnose pertussis more rapidly and efficiently.