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Laboratory evaluation in well-appearing febrile infants 22 to 60 days old at low risk for invasive bacterial illness

Laboratory evaluation in well-appearing febrile infants 22 to 60 days old at low risk for invasive bacterial illness
Initial tests (all patients):
  • CBC with differential (optional)
  • Procalcitonin
  • C-reactive protein*
  • Blood culture
  • Urine dipstick and microscopic urinalysis
Urine dipstick or microscopic urinalysis positive:Δ
  • Urine culture◊
  • 22 to 28 days old: CSF studies suggested:
    • CSF bacterial culture and Gram stain
    • CSF glucose and protein
    • CSF cell count
    • Enterovirus PCR if pleocytosis or increased seasonal prevalence of enterovirus infection
    • HSV PCR§
  • 29 to 60 days old: CSF studies not necessary prior to antimicrobial treatment
Fever >38.5°C (101.5°F) or any elevated inflammatory marker:¥
  • 22 to 28 days old: CSF studies recommended
  • 29 to 60 days old: CSF studies suggested
CBC: complete blood count; CSF: cerebrospinal fluid; PCR: polymerase chain reaction; HSV: herpes simplex virus; ANC: absolute neutrophil count; IBI: invasive bacterial illness; WBCs: white blood cells; HPF: high-powered field.
* If procalcitonin testing is rapidly available (ie, within 1 to 2 hours), then C-reactive protein testing is optional. If procalcitonin testing is not available, then CRP should be obtained and considered along with ANC and rectal temperature in determining risk of IBI.
¶ Urine specimens obtained by bladder catheterization or suprapubic aspiration are preferred for urinalysis. Specimens obtained by bag, spontaneous voiding, or stimulated voiding are acceptable for urine dipstick testing and microscopic urinalysis but should not be sent for urine culture because of high contamination rates. Refer to UpToDate topics on urine collection techniques in infants and children.
Δ Positive results consist of any leukocyte esterase present, >5 WBCs/HPF (centrifuged urine), or >10 WBCs/HPF (uncentrifuged urine).
◊ Urine culture should be obtained by bladder catheterization or suprapubic aspiration. The American Academy of Pediatrics Clinical Practice Guideline for evaluation and management of well-appearing febrile infants 8 to 60 days old recommends that a urine culture only be sent if a urine dipstick or microscopic urinalysis is positive. However, it is reasonable to send the urine culture on all well-appearing febrile infants 8 to 60 days of age in settings with documented low rates of specimen contamination and timely urine specimen processing.
§ HSV studies are indicated for well-appearing infant at high risk, including those with a history of maternal active genital lesions at birth, CSF pleocytosis, or other signs of HSV infection; additional blood studies and microbiology skin and blood specimens are also required. Refer to UpToDate topics on diagnosis of HSV infection in infants.
¥ Inflammatory marker threshold values demonstrating high sensitivity for IBI in febrile infants include:
  • Procalcitonin >0.5 ng/mL
  • C-reactive protein ≥20 mg/L
  • ANC >4,000/microL (mm3) or 5,200/microL (mm3)
Reference: Pantell RH, Roberts KB, Adams WG, et al. Evaluation and management of well-appearing febrile infants 8 to 60 days old. Pediatrics 2021; 148.
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