Discriminating between viral and bacterial lower respiratory tract infection (LRTI) in children is challenging, leading to an excessive use of antibiotics. Myxovirus resistance protein A (MxA) is a promising biomarker for viral infections. The primary aim of the study was to assess differences in blood MxA levels between children with viral and bacterial LRTI. Secondary aims were to assess differences in blood MxA levels between children with viral LRTI and asymptomatic controls and to assess MxA levels in relation to different respiratory viruses.
Children with LRTI were enrolled as cases at Sachs’ Children and Youth Hospital, Stockholm, Sweden. Nasopharyngeal aspirates and blood samples for analysis of viral PCR, MxA and CRP were systematically collected from all study subjects in addition to standard laboratory/radiology assessment. Aetiology was defined according to an algorithm based on laboratory and radiological findings. Asymptomatic children with minor surgical disease were enrolled as controls.
MxA levels were higher in children with viral LRTI (n=242) as compared to both bacterial (n=5) LRTI (p<0.01, area under the curve (AUC) 0.90, 95% confidence interval (CI):0.81-0.99) and controls (AUC 0.92, 95% CI:0.88-0.95). In the subgroup of children with pneumonia diagnosis, a cut-off of MxA 430μg/l discriminated between viral (n=29) and bacterial (n=4) aetiology with 93% (95% CI: 78%-99%) sensitivity and 100% (95% CI: 51%-100%) specificity (AUC 0.98, 95% CI: 0.94-1.00). The highest MxA levels were seen in cases PCR positive for influenza (median MxA 1699μg/l, interquartile range (IQR): 732-2996) and respiratory syncytial virus (median MxA 1115μg/l, IQR: 679-2489).
MxA accurately discriminated between viral and bacterial aetiology in children with LRTI, particularly in the group of children with pneumonia diagnosis, but the number of children with bacterial LRTI was low.

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