This retrospective study reported the use of mNGS for the diagnosis of pneumonia in patients with cancer and compared it with conventional laboratory tests. First, the sensitivity of mNGS for pathogen identification was significantly higher than that of conventional tests, especially for pathogens that are difficult to culture and require prolonged incubation. Second, mNGS requires less time to obtain results (≤ 30 hours) compared with sputum culture (usually 3–5 days). In addition, mNGS was more effective than conventional tests at detecting coinfecting pathogens. Finally, mNGS had the advantage of predicting antibiotic resistance.
Compared with traditional assays, mNGS is an unbiased method for detecting all potentially infectious pathogens in a sample[13]. Previous studies have shown that mNGS has adequate accuracy and a significantly higher sensitivity for detecting pathogens[19, 20]. Moreover, mNGS is less affected by prior antibiotic exposure[15]. In addition to pathogen identification, mNGS provides clinical microbiome analysis, human host response analysis and drug resistance prediction[13]. Therefore, it is valuable in the identification of pathogens causing pneumonia, particularly in cases of unexplained or mixed infection[14]. Immunocompromised patients with cancer are more susceptible to severe pneumonia, mixed infection, and pneumonia caused by pathogens that are difficult to detect using conventional tests[1, 4, 5]. Therefore, mNGS may be a crucial method for identifying pneumonia pathogens in patients with cancer. This retrospective study confirmed this finding. In terms of bacteria, mNGS detected additional Streptococcus pneumoniae, Klebsiella pneumoniae, Staphylococcus epidermidis, Acinetobacter baumannii, Bilophila wadsworthia, Bacteroides heparinolyticus, Gordona bronchialis, Sphingomonas spp., Gordonia polyisoprenivorans, and Ralstonia mannitolilytica, which are difficult to identify using conventional tests. In terms of viruses, mNGS detected additional Epstein-Barr virus (EBV) in Patients #1, #3, #6, #9, and #12; human herpesvirus 1 in Patient #1; small double-stranded RNA virus in Patients #5 and #10; influenza A virus in Patient #12; human papillomavirus 8 in Patient #7; and molluscum contagiosum virus in Patient #10. In terms of fungi, fungal culture of Patient #3 yielded only Aspergillus flavus, whereas mNGS identified Saccharomyces cerevisiae. mNGS also detected Candida glabrata in Patient #2, Candida parapsilosis in Patient #8, and Candida albicans in Patient #12. The results showed that mNGS can rapidly detect more pathogens, regardless of the type of infection. This could help guide timely antibiotic adjustment and improve the prognosis of pneumonia in patients with cancer.
In our study, the most common pathogens detected by mNGS were Streptococcus pneumoniae and influenza A virus, both of which are common in non-cancer patients with pneumonia. Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa are common pathogens that cause nosocomial infections in the general population[21–24]. In addition, mNGS detected a variety of uncommon pathogens, such as Bilophila wadsworthia spp., Bacteroides heparinolyticus, Gordona bronchialis, Sphingomonas spp., Gordonia polyisoprenivorans, Ralstonia mannitolilytica, and small double-stranded RNA virus.
Pathogens in patients with cancer may differ from those in healthy individuals. Our study showed that mNGS can detect several opportunistic pathogens that are difficult to detect using conventional tests, such as opportunistic bacteria (e.g., Bilophila wadsworthia spp., Klebsiella pneumoniae, Leptospira hepatica) and latent viruses (e.g., EBV). These pathogens are relatively harmless to healthy individuals but may cause infection in immunocompromised patients[25–27]. Our study confirmed that patients with cancer are susceptible to infection by these pathogens, possibly because of their immunocompromised status and frequent hospital treatments. Owing to the characteristics of opportunistic pathogens, it is necessary to determine whether they are pathogenic. In our study, the condition of Patient #1, who had Bilophila wadsworthia spp. detected on mNGS, improved after metronidazole administration. Similarly, the condition of Patient #7, who had Klebsiella pneumoniae detected on mNGS, improved after switching to Cefoperazone Sodium and Sulbactam Sodium. Although mNGS detected EBV in 5 patients, it was considered non-pathogenic because more than 90% of adults are latently infected with EBV during their lives, and these patients had no symptoms of associated viral infections[28]. In addition, owing to the large number of colonizing microorganisms in the respiratory tract, distinguishing between infection and colonization may be challenging. In our study, mNGS detected Candida albicans in Patient #9, whose symptoms did not improve with the addition of antifungal medication but improved with steroid therapy following a diagnosis of immune checkpoint inhibitor-associated pneumonitis[24]. Therefore, we considered Candida albicans to be a respiratory colonizer. However, clinicians should be aware of the possibility of fungal infections after long-term steroid therapy.
In general, mNGS distinguishes between infection and colonization using quantitative or semi-quantitative statistical analyses. One study developed rule-based and logistic regression models to differentiate between lower respiratory tract infections and colonization, both of which had an accuracy of 95.5% in the validation cohort[29]. However, further studies on mNGS are required to better differentiate between infection and colonization. We suggest combining the patient's clinical presentation, blood test results, imaging findings, and empirical treatment outcomes to determine whether an opportunistic pathogen is pathogenic.
Pneumonia is more complicated to diagnose and treat in patients with cancer, especially in those who have received immunotherapy or radiotherapy[3, 30]. It is sometimes difficult to distinguish between immunological or radiological pneumonia and lung infection. Whereas traditional tests have the drawbacks of false-negative results and long culture times, mNGS can shorten the time to diagnosis and help with rapid decision-making regarding treatment options. In our study, Patient #11, who was diagnosed with radiation pneumonitis but had a negative mNGS result, showed rapid clinical improvement after the discontinuation of antibiotics and treatment with steroid therapy alone, which also prevented antibiotic abuse and flora destruction, as radiation pneumonitis tends to be complicated by fungal infections in the presence of prolonged steroid use[31, 32].
In this study, mNGS detected five ARGs and predicted pathogen resistance to guide clinical treatment. Compared with sputum culture, mNGS can detect drug resistance more rapidly and comprehensively. Consistent with previous studies[19, 33], mNGS was able to predict drug resistance in slow-growing or unculturable pathogens, and detect inactive pathogens after antibiotic treatment. Patients with hospital-onset lower respiratory tract infections (LRTI) have a higher burden of ARGs in their respiratory microbiome than patients with community-onset LRTIs[34]. By identifying ARGs, mNGS may help in the early identification of future secondary lung infections[35]. The mNGS test is particularly indicated for tumor patients with severe or complicated pneumonia, as well as for tumor patients whose etiology is unknown on conventional examination, and for whom empirical treatment has not been effective[36, 37].
This study has some limitations. First, it was a retrospective study with a small sample size, which may limit the generalizability of the results. Second, the mNGS specimens were sent to a commercial laboratory rather than a hospital microbiology laboratory, which may have reduced the sensitivity because the increased turnaround time reduced pathogen viability. In addition, mNGS is expensive and not currently covered by health insurance in China. This may have contributed to the bias in patient selection in this study. Prospective clinical studies are essential to investigate when to perform mNGS and to compare the diagnostic accuracy of mNGS with other methods.