e Current infectious disease molecular tests are largely pathogen specific, requiring test selection based on the patient's symptoms. For many syndromes caused by a large number of viral, bacterial, or fungal pathogens, such as respiratory tract infections, this necessitates large panels of tests and has limited yield. In contrast, next-generation sequencing-based metagenomics can be used for unbiased detection of any expected or unexpected pathogen. However, barriers for its diagnostic implementation include incomplete understanding of analytical performance and complexity of sequence data analysis. We compared detection of known respiratory virus-positive (n ؍ 42) and unselected (n ؍ 67) pediatric nasopharyngeal swabs using an RNA sequencing (RNA-seq)-based metagenomics approach and Taxonomer, an ultrarapid, interactive, web-based metagenomics data analysis tool, with an FDA-cleared respiratory virus panel (RVP; GenMark eSensor). Untargeted metagenomics detected 86% of known respiratory virus infections, and additional PCR testing confirmed RVP results for only 2 (33%) of the discordant samples. In unselected samples, untargeted metagenomics had excellent agreement with the RVP (93%). In addition, untargeted metagenomics detected an additional 12 viruses that were either not targeted by the RVP or missed due to highly divergent genome sequences. Normalized viral read counts for untargeted metagenomics correlated with viral burden determined by quantitative PCR and showed high intrarun and interrun reproducibility. Partial or full-length viral genome sequences were generated in 86% of RNA-seq-positive samples, allowing assessment of antiviral resistance, strain-level typing, and phylogenetic relatedness. Overall, untargeted metagenomics had high agreement with a sensitive RVP, detected viruses not targeted by the RVP, and yielded epidemiologically and clinically valuable sequence information. L aboratory diagnosis of infectious diseases has historically taken a syndrome-based approach. Culture of appropriate specimens on a combination of relevant media or cell lines enables detection of certain common bacterial, viral, and fungal pathogens. However, culture requires experienced personnel, requires several days to weeks to yield a definitive answer, depends on viability and appropriate culture conditions, and has limited sensitivity. Molecular tests have superior turnaround times, sensitivity, and taxonomic resolution. However, only targeted pathogens can be detected, and differentiation of clinically or epidemiologically relevant strains or genotypes is limited. Moreover, molecular tests need to be updated when new species or strains are recognized to ensure that newly identified genetic variants can be detected.In contrast, next-generation sequencing-based metagenomic testing combines and extends many advantages of molecular tests and culture-based methods. Host-and pathogen-derived nucleic acids are sequenced without a priori knowledge of expected pathogens, allowing simultaneous detection of a virtually ...