How should you approach fever without an obvious source using a differential diagnosis framework?

• A. Start with broad categories (infectious, inflammatory, autoimmune, neoplastic, drug fever, heat illness) and refine with history, exam, targeted tests, and course.
• B. Immediately prescribe antibiotics without evaluation.
• C. Focus only on infectious causes.
• D. Assume fever is non-serious.

Answer: (A)

A structured, broad-to-narrow differential diagnosis approach is the best way to handle fever without an obvious source. Start by framing the possibilities in broad categories: infectious, inflammatory or autoimmune, neoplastic (cancer-related), drug fever, and heat-related illness, with awareness that fever can sometimes be deceptive or multifactorial. This wide net prevents premature narrowing and helps you consider conditions you might otherwise miss.

Then gather data to refine the differential. Take a thorough history focused on recent travel, exposures (animals, food, water, tick or insect bites), sick contacts, medications and new drugs, immune status (immunosuppression, HIV, steroids), and device-related risks. Do a careful exam looking for clues such as a rash, lymphadenopathy, organomegaly, meningismus, or focal findings that point toward a particular system. Use targeted tests guided by the most likely categories: basic labs (complete blood count, metabolic panel, inflammatory markers), blood cultures and possibly urine cultures, chest imaging if there are respiratory symptoms, and specific serologies or autoimmune panels if autoimmune processes are suspected. If initial tests don’t reveal a source, pursue appropriate imaging or specialist consultation and consider repeated assessments over time to track the fever’s course and response to therapy.

This approach balances timely management with the possibility of non-infectious etiologies and helps avoid unnecessary or inappropriate treatments.

Pushing antibiotics without evaluation can obscure non-infectious causes and contribute to resistance. Narrowing the scope to infectious etiologies alone risks missing autoimmune, inflammatory, or malignant processes. Assuming the fever is non-serious can delay critical diagnoses.