A combination of PCT, NGAL and resistin improved discrimination compared with the individual biomarkers in the Alder Hey ED and PICU cohorts, and more so for differentiating between “definite bacterial” infections and “definite viral” infections based on the PERFORM algorithm than for “SBI” and “non-SBI.” In the PERFORM algorithm, children with a clinical phenotype resembling a viral infection, but with a high CRP level not clearly explained by the presence of a bacterial co-infection, will be classified in either the “viral syndrome” or “unknown” group. Here, RETN is linked to viral infectious disease.