For suspected invasive bacterial infections, diagnostic testing should include blood cultures and appropriate culture of specimens from the suspected infected site. Not all specimens may be obtained prior to antibiotics but optimally a blood culture can be obtained at the time of intravenous access. In cases where antibiotics are started due to clinical instability, or when there is a requirement for coordination for surgically accessed cultures, cultures should still be obtained at that time. In certain cases, polymerase chain reaction testing may be helpful to guide therapy (e.g., cerebrospinal fluid, synovial fluid, pleural fluid). Diagnostic testing should be considered for suspected systemic viral infection that may mimic bacterial sepsis, and may allow more timely initiation of antiviral therapy and discontinuation of antibiotics if bacterial infection is excluded. In neonates where bacterial or viral sepsis cannot be differentiated based on the clinical presentation, and both antibiotics and antivirals are initiated, blood cultures should be prioritized and cultures from additional sites (e.g., cerebrospinal fluid) and polymerase chain reaction testing (e.g., herpes simplex virus) should be obtained as soon as is clinically feasible.