Evaluating C-Reactive Protein and Procalcitonin Discordance Among Febrile Infants at Risk of Serious Bacterial Infections

Aim: We aimed to investigate the prevalence and factors associated with C-reactive protein (CRP) and procalcitonin (PCT) discordance in febrile infants with serious bacterial infections (SBIs). Methods: We performed a retrospective review of febrile infants <= 90 days old presenting to the emergency department between December 2018 and June 2023. We compared conservative and pragmatic thresholds for PCT (< 0.5 ng/mL and < 1.7 ng/mL) and CRP (< 10 mg/L and < 20 mg/L). Discordance was defined as normal CRP with abnormal PCT and vice versa. Performance was presented using sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Results: Among 3459 infants, 426 infants (12.3%) had SBIs, among whom 355 (83.3%) had both CRP and PCT performed. Overall, a conservative CRP threshold had the highest sensitivity (74.1%, 95% CI 69.2%-78.6%) and NPV (95.6%, 95% CI 94.6%-96.4%). Among those with SBIs, 148/355 (41.7%) had a normal PCT (< 1.7 ng/mL) and an abnormal CRP (>= 20 mg/L), while 16/355 (4.5%) had a normal CRP (< 20 mg/L) and an abnormal PCT (>= 1.7 ng/mL). An increased discordance, specifically abnormal CRP with normal PCT, was found in males, infants 29-90 days old, and those with urinary tract infections. Conclusion: SBI clinical decision rules should consider CRP-PCT discordance in specific patient populations.