The newly-designed PneumoResp RDT presented in this study was shown able to detect rapidly S. pneumoniae antigens with excellent clinical sensitivity and specificity in non-invasive respiratory specimens from children suspected of respiratory infection. Moreover, the test was used to give an early measure of the S. pneumoniae load in these samples in order to anticipate the treatment of the most serious infections while sparing anti-pneumococcal therapy in case of negative result.
Although the rapid detection of S. pneumoniae antigens is currently performed in many body fluids, including urine, cerebrospinal fluid and pleural fluid [3,7,11,12], the specimens from the respiratory tract have been excluded from this clinical practice. A few years ago, a sputum antigen kit was developed in Japan and tested in adult patients for the rapid diagnosis of pneumococcal pneumonia with an excellent sensitivity, exceeding largely that of the urinary tests [18–20]. However, this diagnosis strategy has remained confidential and was never tested in children known to exhibit a high level of pneumococcal asymptomatic carriage [3,7,21].
The main strengths of this study can be summarized as follows. First, the present RDT is highly specific for S. pneumoniae, notably with regard of the other members of the mitis group. The only cross-reactive results in this group were obtained with S. pseudopneumoniae, a species considered to be a respiratory pathogen and having a similar susceptibility profile to antibiotics than S. pneumoniae [13,22–24]. Cross-reaction was also observed with Parvimonas micra, which was recently reported for another RDT directed against S. pneumoniae [25]; this agent is an anaerobic gram-positive bacterium of the oral microbiota that could be responsible for opportunistic infections of the respiratory area. Second, the sensitivity and NPV of the PneumoResp RDT were excellent with regard to semi-quantitative culture and PCR assays targeting virulence pneumococcal genes, which allowed ruling out all the negative results for S. pneumoniae at day 0 without risk of missing highly positive samples. Finally, the RDT used on 1:100 diluted samples was able to identify at day 0 all the presumptive cases of pneumococcal pneumonia identified in this study.
This exploratory work has some limitations. First, S. pneumoniae is the only microorganism that has been analysed. Although this germ is frequently involved in bacterial pneumonia, other respiratory bacteria such as Haemophilus influenzae or Moraxella catarrhalis, but also “atypical bacteria” (especially Mycoplasma pneumoniae and Chlamydia pneumoniae) and viruses need to be taken into consideration, notably in children [3,26–30]. Although not reported in this study, these agents were sought for in all the cases of ascertained pneumonia. Second, it is a retrospective study that was merely dedicated to explore the performances of the test; consequently, the diagnosis strategy that emerged with this two-step RDT would need to be validated prospectively. Third, we assessed that a high pneumococcal load in upper respiratory secretions (≥107 CFU/ml or equivalent by PCR) was predictive of an invasive pneumococcal infection; even if this threshold has been recommended in upper respiratory secretions by European guidelines [4,5], other guidelines discouraged their use [3]. However, the good correlation between quantitative results obtained by culture and PCR (see Figure 1) indicates that these measures may have a clinical pertinence.
The PneumoResp kit, by allowing a two-step analysis, first on undiluted secretions and, if positive, on 1:100-diluted secretions, can be used as a semi-quantification tool. In the conditions of the second step, the sensitivity of the RDT was close to the threshold of 107 CFU/ml considered significant for recognizing the involvement of this bacterium in the aetiology of a pulmonary parenchymal infection [4,5]. In accordance with the overall results of the study, an algorithm depicted in Figure 4 summarizes the benefit of the PneumoResp RDT for managing active pneumococcal infections in children. When the RDT is negative on undiluted secretion, the probability of active pneumococcal infection is very low and other aetiology must be looked for. When the RDT is positive on diluted secretion, this probability is high and an anti-pneumococcal treatment must be discussed. In intermediate situations (positive RDT on undiluted secretion and negative on diluted one), the semi-quantitative culture remains essential for deciding between carriage and active infection, as well as the implementation of other tests such as molecular multiplex approaches seeking at the same time the respiratory viruses (the first cause of pneumonia in children) and intracellular atypical bacteria which are difficult to grow and need a different antimicrobial treatment [26-, 30].