Performance of blood biomarkers to rule out invasive bacterial infection in febrile infants under 21 days old.

OBJECTIVES: To determine the performance of procalcitonin (PCT), C reactive protein (CRP) and absolute neutrophil count (ANC) in identifying invasive bacterial infection (IBI) among well-appearing infants ≤21 days old with fever without source and no leukocyturia. To compare this performance with that in those 22-90 days old.

DESIGN: Substudy of a prospective single-centre registry performed between September 2008 and August 2017.

SETTING: Paediatric emergency department of a tertiary teaching hospital.

PATIENTS: 196 infants ≤21 days old and 1331 infants 22-90 days old.

MAIN OUTCOME MEASURES: Sensitivity and negative likelihood ratio of blood tests for ruling out IBI (positive blood or cerebrospinal fluid culture). Abnormal blood test results: PCT ≥0.5 ng/mL, CRP >20 mg/L and ANC >10 000/µL.

RESULTS: Prevalence of IBI in infants ≤21 days old with normal or any abnormal blood test result was 3.6% and 6.8%, respectively (OR 0.52 (95% CI 0.13 to 2.01)), compared with 0.2% and 4.5% in older infants (OR 0.03 (95% CI 0 to 0.17)). Sensitivity and negative likelihood ratio of the blood tests for ruling out IBI in infants ≤21 days were 44.4% (95% CI 18.9% to 73.3%) and 0.79 (95% CI 0.43 to 1.44), respectively (vs 84.6% (95% CI 57.8% to 95.7%)%) and 0.19 (95% CI 0.05 to 0.67) in older infants). The values improved in infants with fever ≥6 hours aged 22-90 days, but not in those ≤21 days.

CONCLUSIONS: PCT, CRP and ANC are not useful for ruling out IBI in febrile infants ≤21 days old. It is still recommended that these patients are admitted and given empirical antibiotic therapy, regardless of their general appearance or blood test results.