What is the diagnostic approach for fever of unknown origin (FUO) in a pediatric patient under six months?

From the Guidelines

The evaluation of fever of unknown origin (FUO) in infants under six months requires a systematic approach due to their immature immune systems and higher risk of serious bacterial infections. For infants under 6 months with fever ≥38°C (100.4°F) without an obvious source, immediate medical evaluation is necessary. The algorithm begins with a complete history and physical examination, followed by laboratory tests including complete blood count with differential, C-reactive protein, blood culture, urinalysis and urine culture. For infants under 28 days, a full sepsis workup is mandatory, including lumbar puncture for CSF analysis and culture, regardless of appearance. For infants 29 days to 3 months, lumbar puncture is strongly recommended, while for those 3-6 months, it may be performed based on clinical judgment. Empiric antibiotic therapy should be initiated promptly in all febrile infants under 28 days (typically ampicillin plus gentamicin or a third-generation cephalosporin), and in toxic-appearing older infants. For well-appearing infants 29 days to 6 months, antibiotic administration may be deferred pending culture results if reliable follow-up is assured. Hospitalization is recommended for all febrile infants under 28 days and for toxic-appearing older infants. The aggressive approach in young infants is justified by their limited ability to localize infections and their higher susceptibility to rapid clinical deterioration from bacterial infections, as supported by studies such as 1 and 1.

Some key considerations in the management of febrile infants include:

• The risk of serious bacterial infections, including urinary tract infections, bacteremia, and bacterial meningitis, which can be as high as 13% in neonates and 7% in infants aged 90 days or younger 1
• The importance of a thorough history and physical examination, as well as laboratory tests, in identifying the source of the fever and guiding management
• The need for empiric antibiotic therapy in certain high-risk groups, such as febrile infants under 28 days and toxic-appearing older infants
• The potential for rapid clinical deterioration in young infants with bacterial infections, highlighting the need for prompt and aggressive management, as discussed in 1 and 1.

Overall, the approach to FUO in infants under six months should prioritize the identification and management of serious bacterial infections, while also considering the potential for viral and other non-bacterial causes of fever.

From the Research

Diagnostic Approach for Fever of Unknown Origin (FUO) in Pediatric Patients

The diagnostic approach for FUO in pediatric patients, particularly those under six months, involves a systematic and individualized evaluation.

• Age, climate, local epidemiology, and host factors are crucial in determining the choice of definitive tests 2.
• In pediatric patients, infections are a major cause of FUO, and serious bacterial infections, such as bacteraemia, meningitis, and urinary tract infections, occur in 3 to 20% of cases of acute FUO 2.
• A careful analysis of the history, physical findings, and laboratory tests is essential in the diagnostic approach to FUO 3.
• The diagnostic workup for FUO in pediatric patients may include screening and definitive laboratory tests to determine the need for empiric antibiotic therapy and hospitalization 2.

Considerations for Pediatric Patients Under Six Months

• The approach to acute FUO may be less applicable to neonates and infants younger than 90 days, particularly those aged 0 to 7 days 2.
• In pediatric patients under six months, the etiology of FUO may differ from that in older children, and a high index of suspicion for serious bacterial infections is necessary 2.
• The diagnostic approach for FUO in pediatric patients under six months should be tailored to the individual patient's clinical presentation and risk factors 2.

Laboratory Tests and Diagnostic Evaluation

• A range of laboratory tests, including blood cultures, complete blood counts, and inflammatory markers, may be used to evaluate pediatric patients with FUO 4, 5.
• Imaging studies, such as chest radiographs and abdominal ultrasonography, may also be useful in the diagnostic evaluation of FUO in pediatric patients 4, 5.
• The choice of laboratory tests and diagnostic evaluation should be guided by the patient's clinical presentation, age, and risk factors 2, 3.