What is the most appropriate next step in evaluating a 3-month-old girl with fever, non-bloody and non-bilious emesis, normal feeding, and no other symptoms, presenting with hyperthermia, tachycardia, and normal physical examination?

From the Guidelines

The most appropriate next step in evaluation for this 3-month-old girl with fever is catheterized urine collection for culture (option C). In febrile infants under 3 months of age, urinary tract infection (UTI) is a common source of fever without an obvious source on physical examination, as noted in the clinical practice guideline for the diagnosis and management of the initial UTI in febrile infants and children 2 to 24 months 1. This patient has fever without clear focus, as her examination shows no obvious source of infection. Young infants, particularly females, may have UTIs without specific urinary symptoms. Catheterized urine collection is preferred over bag specimens in this age group because it minimizes contamination, allowing for accurate diagnosis, with a sensitivity of 95% and a specificity of 99% 1. Blood cultures would also be reasonable but are less likely to yield a diagnosis than urine studies in this well-appearing infant with no specific symptoms pointing to bacteremia. The patient's normal physical exam, including normal fontanelle and good capillary refill, suggests she is not severely ill, but the fever in this age group requires evaluation for occult bacterial infection, with UTI being the most common. According to the guideline, if the clinician determines that the degree of illness does not require immediate antimicrobial therapy, then the likelihood of UTI should be assessed, and obtaining a urine specimen through catheterization or SPA for culture and urinalysis is a recommended option 1.

Some key points to consider in this case include:

• The importance of accurate diagnosis of UTI to prevent renal scarring and overtreatment
• The high sensitivity and specificity of catheterized urine collection for culture
• The need to assess the likelihood of UTI in febrile infants without an obvious source of infection
• The preference for catheterized urine collection over bag specimens in this age group to minimize contamination.

Given the strong recommendation for catheterized urine collection for culture in febrile infants with no apparent source of fever, as stated in the clinical practice guideline 1, this approach is the most appropriate next step in evaluation for this patient.

From the Research

Evaluation of the Patient

The patient is a 3-month-old girl presenting with a 2-day history of fever and two episodes of nonbloody, nonbilious emesis. Given her age and symptoms, it is crucial to consider the potential causes of her condition, including urinary tract infections (UTIs), which are common in infants.

Consideration of Diagnostic Tests

When evaluating the patient, the following diagnostic tests are considered:

• Abdominal ultrasonography: This test may be useful in evaluating the patient's abdominal organs, but it is not the most appropriate next step in this case.
• Blood culture: According to the study 2, blood cultures may be useful in selected patients with UTIs, particularly those with high fever or complicated infections.
• Catheterized urine collection for culture: This test is essential in diagnosing UTIs, especially in infants. The study 3 suggests that urine culture susceptibility results can accurately predict blood culture results when the same organism is isolated from both cultures.
• Liver function tests: These tests are not directly relevant to the patient's symptoms.
• Serum electrolyte concentrations: While these tests may be useful in evaluating the patient's overall health, they are not the most appropriate next step in this case.
• Stool culture: This test may be useful if the patient's symptoms suggest a gastrointestinal infection, but it is not the most appropriate next step in this case.
• Voiding cystourethrography: This test is used to evaluate the patient's urinary tract, but it is not the most appropriate next step in this case.

Most Appropriate Next Step

Based on the patient's symptoms and the studies 2 and 3, the most appropriate next step in evaluation is to perform a catheterized urine collection for culture. This test will help diagnose a potential UTI and guide antibiotic therapy. Additionally, considering the patient's age and symptoms, a blood culture may also be useful in selected patients, as suggested by the study 2. However, the most direct and relevant initial step is to assess for a UTI through urine culture.