Residency · Residency · Emergency Medicine
The Febrile Infant: Risk Stratification and Workup
Overview
Definitions
A febrile infant is defined as one with a rectal temperature of 38.0 degrees Celsius (100.4 degrees Fahrenheit) or higher who is 90 days old or younger. Rectal temperature is the gold standard in this age group because tympanic, axillary, and temporal measurements are unreliable in neonates. Fever in this age group represents serious bacterial infection (SBI) until proven otherwise. SBI includes urinary tract infection (the most common), bacteremia, and bacterial meningitis.
Epidemiology
The prevalence of SBI in febrile infants 90 days and younger is 8 to 12 percent. Urinary tract infection accounts for 5 to 8 percent and is the most common SBI. Bacteremia occurs in 1 to 3 percent and bacterial meningitis in 0.5 to 1 percent overall, with higher rates in neonates under 28 days. Herpes simplex virus infection occurs in approximately 0.3 percent of neonates and is devastating if missed. While viral illness accounts for the majority of fevers in this age group, SBI and viral infection can coexist, which means a positive viral test does not rule out bacterial infection.
Age-Based Risk Stratification
| Age Group | SBI Risk | LP Required? | Antibiotics | Disposition |
|---|---|---|---|---|
| 0–28 days | Highest | Always | Ampicillin + gentamicin ± acyclovir | Admit (no exceptions) |
| 29–60 days | Moderate | Yes (may defer if all low-risk criteria met) | If ill or high-risk labs | Admit or close outpatient follow-up |
| 61–90 days | Lower | If ill-appearing, abnormal labs, or no follow-up | If indicated | Outpatient if low-risk with reliable follow-up |
0-28 Days (Neonates)
This is the highest-risk group because of the immature immune system and exposure to maternal flora. All febrile neonates require a full sepsis workup consisting of a CBC with differential, blood culture, catheterized urinalysis and urine culture, lumbar puncture (with CSF cell count, protein, glucose, culture, and consideration of HSV PCR), and a chest X-ray if respiratory symptoms are present. Empiric antibiotics should include ampicillin plus gentamicin (or cefotaxime where available). Ampicillin is essential because it covers Listeria and Enterococcus, which cephalosporins miss. Acyclovir should be added if there is any concern for HSV, such as vesicles, seizures, elevated liver function tests, CSF pleocytosis, an ill appearance, or relevant maternal history. All febrile neonates require admission with no exceptions.
29-60 Days
This age group can benefit from risk stratification tools to identify low-risk infants who may avoid lumbar puncture or be candidates for outpatient management. The traditional approach involves a full sepsis workup with admission, but the modern approach applies validated risk stratification criteria.
61-90 Days
Infants in this age group are at lower risk than younger infants, but SBI still occurs. Risk stratification criteria are better validated in this age group, and well-appearing infants with negative screening labs may be managed with close outpatient follow-up, provided the caregivers are reliable.
Risk Stratification Tools
PECARN Febrile Infant Rule (2019)
This rule was validated in febrile infants 29 to 60 days old and identifies those at low risk for SBI who may not need a lumbar puncture. All of the following criteria must be met to classify an infant as low-risk: the infant is well-appearing (by clinician gestalt), the absolute neutrophil count is below 4090 per microliter, the urinalysis is negative (no leukocyte esterase, no nitrites, fewer than 5 WBC per high-power field), and procalcitonin is below 0.5 ng/mL. When all criteria are met, the risk of SBI is 0.4 percent, lumbar puncture may be deferred, and close follow-up should be arranged. The sensitivity for SBI is 97.7 percent.
Step-by-Step Approach (2016)
This tool was validated in febrile infants 29 to 90 days old and uses a sequential evaluation. If the infant is ill-appearing, a full workup and admission are indicated. For well-appearing infants aged 29 to 90 days, a urinalysis, procalcitonin, and CRP are obtained. A positive urinalysis leads to UTI treatment, with lumbar puncture if the infant is younger than 60 days. A procalcitonin of 0.5 ng/mL or higher indicates higher risk and warrants a full workup with admission. A CRP above 20 mg/L indicates intermediate risk. An ANC above 10,000 is also intermediate risk. If all markers are negative, the infant is low-risk and may be observed without lumbar puncture or antibiotics.
Rochester Criteria (Historical)
The Rochester Criteria, published in 1985, were among the earliest low-risk criteria. Low-risk infants were defined as well-appearing, previously healthy, term, without a focal infection, with a WBC between 5,000 and 15,000, band count below 1,500, normal urinalysis, and fewer than 5 WBC per high-power field on stool examination if diarrhea was present. These criteria are limited by the absence of procalcitonin and are less sensitive than newer tools.
Philadelphia Protocol and Boston Criteria (Historical)
These are older algorithms with higher sensitivity but lower specificity. They have largely been superseded by the PECARN rule and the Step-by-Step approach.
Key Laboratory Studies
Procalcitonin
Procalcitonin is the most accurate single biomarker for SBI in febrile infants. It rises within 4 to 6 hours of bacterial infection and peaks at 12 to 24 hours. It is more specific for bacterial infection than either CRP or WBC. The cut-off is 0.5 ng/mL, with values below this indicating low risk. There is a physiologic elevation in the first 48 to 72 hours of life, which makes it less reliable in neonates under 3 days old.
Urinalysis and Urine Culture
Specimens must be obtained by catheterization or suprapubic aspiration because bag specimens have unacceptable contamination rates for culture. Urinalysis findings suggesting UTI include positive leukocyte esterase, positive nitrites, more than 5 WBC per high-power field, and bacteria on Gram stain. The urine culture is the gold standard, with results available in 24 to 48 hours. Because UTI is the most common SBI, a urinalysis should always be obtained.
Blood Culture
Blood culture is the standard of care in all febrile infants under 60 days and should be obtained before antibiotics are administered. Many bacteremic infants appear well.
CSF Analysis
In neonates, more than 15 to 20 WBC per microliter in the CSF is abnormal, while in older infants the threshold is more than 8 WBC per microliter. Protein above 150 mg/dL in neonates or above 45 mg/dL in infants older than 2 months is abnormal. CSF glucose below 50 percent of serum glucose is abnormal. The Gram stain has low sensitivity (50 to 80 percent) but high specificity. Culture is the gold standard. HSV PCR should be obtained in neonates with any risk factors. Enteroviral PCR is helpful during enterovirus season, and a positive result with normal CSF indices is reassuring.
Inflammatory Markers
The WBC count is neither sensitive nor specific alone, though extreme values (below 5,000 or above 15,000) are more concerning. The absolute neutrophil count above 4,000 to 10,000 is associated with higher SBI risk. CRP rises 6 to 12 hours after infection and is less specific than procalcitonin in early presentations. No single marker is sufficient on its own, which is why validated tools combine multiple parameters.
Which Febrile Infants Can Safely Avoid LP?
Lumbar puncture is invasive, painful, and often technically difficult in infants. Traumatic taps occur in 20 to 30 percent of infant LPs, complicating interpretation. The risk of bacterial meningitis in well-appearing infants aged 29 to 60 days with normal screening labs is very low, below 0.5 percent. The PECARN and Step-by-Step tools suggest that LP may be safely deferred in select low-risk infants in this age group with close follow-up. However, many experts still recommend routine LP for all febrile infants under 60 days because of the catastrophic consequences of missed meningitis. Shared decision-making with families is increasingly advocated.
In current practice, LP is universal and without exceptions for infants under 28 days. For infants aged 29 to 60 days, LP is recommended by most guidelines but may be deferred if all low-risk criteria are met and reliable follow-up within 24 hours is available. For infants aged 61 to 90 days, LP is indicated if the infant is ill-appearing, has abnormal labs, or lacks reliable follow-up.
HSV Considerations
When to Consider HSV
HSV should be considered in any neonate under 21 days with fever (especially under 14 days), any ill-appearing neonate at any age, and any neonate with a maternal history of genital herpes. However, 60 to 80 percent of neonatal HSV occurs without known maternal history. Clinical clues include vesicular skin lesions, seizures, elevated transaminases (AST or ALT more than 2 to 3 times normal), CSF pleocytosis, thrombocytopenia, and coagulopathy.
HSV Workup
The workup includes CSF HSV PCR, surface cultures (from the mouth, nasopharynx, conjunctivae, rectum, and any vesicles), serum HSV PCR, liver function tests (for the hepatitis component), and a comprehensive metabolic panel for renal function.
Treatment
Acyclovir at 20 mg/kg IV every 8 hours should be started empirically while awaiting results if there is any suspicion. The duration is 14 days for skin, eye, and mouth (SEM) disease and 21 days for CNS or disseminated disease. Delays in starting acyclovir are associated with increased mortality and morbidity.
<image>A flowchart for the evaluation of the febrile infant aged 0-90 days. The algorithm begins with "Rectal temperature ≥ 38.0°C in infant ≤ 90 days" and splits into three age groups. For 0-28 days: full sepsis workup (CBC, blood culture, UA/UCx, LP with HSV PCR, consider CXR), empiric ampicillin + gentamicin ± acyclovir, admit. For 29-60 days: assess appearance; if ill-appearing → full workup + admit + empiric antibiotics. If well-appearing → obtain UA, procalcitonin, ANC, blood culture. If all low-risk (PECARN criteria: negative UA, PCT < 0.5, ANC < 4090, well-appearing) → may defer LP, consider outpatient management with 24-hour follow-up. If any high-risk feature → full workup + admit. For 61-90 days: similar but more latitude for outpatient management if low-risk by validated criteria.</image>
<image>A comparison table of febrile infant risk stratification tools. Four columns: Rochester Criteria, Philadelphia Protocol, Step-by-Step, and PECARN Rule. Each column lists the applicable age range, parameters evaluated (WBC, bands, UA, CRP, procalcitonin, ANC), low-risk definition, sensitivity for SBI, and recommendations for LP and antibiotics. The table highlights that PECARN and Step-by-Step include procalcitonin and have the best performance characteristics for identifying low-risk infants who may safely avoid LP.</image>
<image>A clinical photograph-style illustration showing the technique of infant lumbar puncture. The infant is shown in the lateral decubitus position with the spine flexed (assistant holding the infant with knees to chest and chin to chest while maintaining airway patency). An inset shows the anatomical landmarks: the intercristal line (iliac crests) identifying the L4 vertebral body, with the needle insertion point at the L3-L4 or L4-L5 interspace. A second inset shows proper needle advancement with the stylet in place, angled slightly cephalad. A note indicates to always monitor pulse oximetry during the procedure and to avoid excessive neck flexion which can compromise the airway.</image>
Clinical Pearls
All febrile neonates under 28 days get a full sepsis workup, empiric antibiotics, and admission with no exceptions. Procalcitonin is the single best biomarker for SBI in febrile infants, outperforming WBC, CRP, and ANC alone. UTI is the most common SBI, and the urine specimen must always be obtained by catheterization because bag specimens are inadequate for culture. A well-appearing infant can still have SBI, so clinical appearance alone is insufficient for risk stratification. HSV should be considered in every febrile neonate, especially those under 21 days, those who are ill-appearing, and those with vesicles, seizures, or elevated transaminases, and acyclovir should be started empirically. The absence of maternal HSV history does not exclude neonatal HSV, as most cases occur without known maternal infection. The PECARN febrile infant rule and Step-by-Step approach are the best-validated tools for identifying low-risk infants aged 29 to 60 days who may safely defer LP. A traumatic LP does not exclude meningitis, and if the clinical picture warrants treatment, treatment should be given.
References
- Kuppermann N, et al. A clinical prediction rule to identify febrile infants 60 days and younger at low risk for serious bacterial infections (PECARN). JAMA Pediatr. 2019;173:342-351.
- Gomez B, et al. Validation of the "step-by-step" approach in the management of young febrile infants. Pediatrics. 2016;138:e20154381.
- Jaskiewicz JA, et al. Febrile infants at low risk for serious bacterial infection (Rochester criteria). Pediatrics. 1994;94:390-396.
- Kimberlin DW, et al. Natural history of neonatal herpes simplex virus infections in the acyclovir era. Pediatrics. 2001;108:223-229.
- Pantell RH, et al. Clinical practice guideline: evaluation and management of well-appearing febrile infants 8-60 days old (AAP). Pediatrics. 2021;148:e2021052228.
