Differential Diagnosis for Fever of Unknown Origin in Adults
The differential diagnosis for fever of unknown origin (FUO) falls into four major categories: infectious diseases (37.8% of cases), noninfectious inflammatory disorders (20.9%), malignancies (11.6%), and miscellaneous causes, with approximately 23% remaining undiagnosed despite thorough evaluation. 1
Primary Diagnostic Categories
Infectious Diseases (Most Common Overall)
Infectious etiologies represent the leading cause of FUO, particularly in lower-income countries and Asian populations 1:
- Subacute bacterial endocarditis - one of the six most common specific diagnoses 2
- Cytomegalovirus infection - frequently identified cause 2
- Tuberculosis - especially important in endemic regions 3
- Epstein-Barr virus 3
- Occult abscesses - intra-abdominal or deep-seated 4
- Complicated urinary tract infections 4
Noninfectious Inflammatory Diseases (Rising in Frequency)
This category has significantly increased over time and is more common in women and elderly patients 1, 2:
- Adult-onset Still's disease - one of the six most common specific diagnoses 2
- Rheumatic polymyalgia with or without temporal arteritis - particularly in elderly patients 2
- Systemic lupus erythematosus 4
- Vasculitides 3
- Inflammatory bowel disease 4
Malignancies
Neoplastic causes account for approximately 11.6% of FUO cases 1:
- Lymphomas (Hodgkin and non-Hodgkin) 4, 3
- Leukemias 3
- Renal cell carcinoma 4
- Hepatocellular carcinoma 3
- Metastatic solid tumors 4
Miscellaneous Causes
- Subacute thyroiditis - identified as one of the six most common specific diagnoses 2
- Venous thromboembolism 4
- Drug fever 3
- Factitious fever 5
- Granulomatous diseases (sarcoidosis) 3
Critical Clinical Context
Most FUO cases in adults represent uncommon presentations of common diseases rather than rare diseases 4. Up to 75% of cases may resolve spontaneously without reaching a definitive diagnosis 4.
Geographic and Demographic Factors
- Income level matters: Infectious diseases increase as country income decreases, while undiagnosed FUO increases in higher-income countries 1
- Geographic variation: Asia has higher infectious disease prevalence; Europe has higher rates of undiagnosed FUO 1
- Age and gender: Rheumatic diseases are more frequent in women and elderly patients 2
Common Pitfalls
In 44% of patients, the final diagnosis comprises only six clinical entities (subacute thyroiditis, subacute endocarditis, Still's disease, rheumatic polymyalgia with/without temporal arteritis, and cytomegalovirus infection) 2. Clinicians should maintain high suspicion for these specific diagnoses rather than pursuing exotic possibilities.
The diagnostic approach should always be directed toward the known frequency of diseases in your specific population and geographic setting 2. Empiric antimicrobial therapy should be avoided except in neutropenic, immunocompromised, or critically ill patients, as it has not been shown effective and may obscure the diagnosis 4.