Which initial steps are essential to differentiate likely infection from non-infectious causes in fever without source?

• A. Focused history and exam; obtain blood cultures before antibiotics when possible; urinalysis; chest radiograph if respiratory symptoms; CBC; pregnancy test in women of childbearing age.
• B. Broad-spectrum antibiotics should be started immediately before diagnostic testing.
• C. Rely only on fever measurement and imaging studies; laboratory tests are unnecessary.
• D. Delay evaluation until hospital admission.

Answer: (C)

The key idea is to approach fever without a source with a structured, patient-centered assessment that goes beyond just measuring temperature or relying on imaging. Start with a focused history and physical exam to pick up clues about possible infection sources, exposures, recent procedures, medications, and red flags that suggest sepsis or alternative diagnoses. This informs what tests to pursue next.

Then use targeted diagnostic steps before initiating treatment whenever possible. A urinalysis helps detect a urinary source, and a pregnancy test in women of childbearing age can reveal pregnancy-related fever or eliminate pregnancy-related conditions. A complete blood count can show leukocytosis or other clues, and imaging like a chest radiograph is important if there are respiratory symptoms or signs suggesting pneumonia. Blood cultures are useful when there is concern for bacteremia, and they should be collected before starting antibiotics if feasible to maximize yield.

Relying only on fever measurement and imaging misses important possibilities. Fever is nonspecific and many noninfectious conditions—autoimmune disorders, drug fevers, malignancies, or localized inflammatory processes—can mimic infection. Imaging alone may not reveal early infection or differentiate infectious from noninfectious causes. Laboratory tests provide crucial information to refine the differential, guide management, avoid unnecessary antibiotics, and determine the urgency of treatment, especially if sepsis could be developing.