What is Fever of Unknown Origin?
Fever of Unknown Origin (FUO) was classically defined by Petersdorf and Beeson (1961) as a fever exceeding 38.3°C on at least three occasions, lasting more than 3 weeks, and remaining undiagnosed after 1 week of in-patient investigation. Modern definitions have evolved, but the essence remains: a prolonged, documented, unexplained fever that resists standard diagnostic efforts.
FUO is a clinical challenge of the highest order — it requires a physician with broad medical knowledge, a systematic investigative approach, and the intellectual discipline to resist anchoring on an early hypothesis. Approximately 25-50% of FUO cases, even after exhaustive investigation, ultimately resolve spontaneously or remain undiagnosed. However, identifying the treatable cause — whether infectious, autoimmune, or neoplastic — can be lifesaving.
Systematic fever documentation — documenting the pattern, duration, height of fever, and associated symptoms — is the first step in FUO evaluation.
The Four Major Categories of FUO
The causes of FUO fall into four broad categories, with their relative frequency varying by patient age and geography:
1. Infectious Diseases (30-40% of cases)
The most common cause globally. Important causes in the Indian context include: Tuberculosis (extrapulmonary TB — the great mimic of FUO), Brucellosis (from unpasteurised milk), Infective Endocarditis, Enteric Fever (Typhoid), Visceral Leishmaniasis (Kala-Azar, particularly in endemic regions), HIV, and occult abscesses.
2. Autoimmune/Rheumatological Diseases (20-30% of cases)
Rheumatic causes are the Rheumatologist's domain. Key diagnoses include: Adult-onset Still's Disease (AOSD — characterised by quotidian fever, evanescent salmon-coloured rash, serositis, and markedly elevated ferritin), Systemic Vasculitis (Temporal Arteritis in the elderly, Takayasu's in young women), SLE, Haemophagocytic Lymphohistiocytosis (HLH), and Periodic Fever Syndromes (FMF, TRAPS, CAPS).
3. Malignancy (20% of cases)
Lymphoma (particularly Hodgkin's lymphoma — the classic cause of "B-symptoms": fever, night sweats, weight loss), Renal Cell Carcinoma ("the internist's tumour" classically presenting with FUO), hepatocellular carcinoma, and atrial myxoma (a benign cardiac tumour causing constitutional symptoms).
4. Miscellaneous and Drug Fevers (10-15%)
Drug-induced fever (from antibiotics, antiepileptics, allopurinol), pulmonary embolism, factitious fever, and rare conditions like Sweet's Syndrome.
FDG-PET/CT — the most powerful imaging tool for investigating FUO, identifying occult infections, inflammatory foci, and malignancies with a single whole-body scan.
🌡️ Adult-Onset Still's Disease: A Rheumatologist's FUO
Adult-onset Still's Disease (AOSD) is a systemic inflammatory disease presenting with daily (quotidian) spiking fevers (>39°C), an evanescent salmon-coloured skin rash appearing during fever spikes, arthritis, and serositis. The ferritin level in active AOSD is characteristically extremely high — often >5,000 or even >50,000 ng/mL (normal <300). This hyperferritinaemia is both diagnostically useful and, in the most severe form (Macrophage Activation Syndrome), life-threatening.
The Systematic Investigative Approach
Dr. Prateek Deo's workup for FUO follows a structured, evidence-guided protocol:
- Detailed history and physical examination: Including travel history, animal exposure, medication history, family history of periodic fevers, and a meticulous physical examination (lymph nodes, skin, heart sounds for murmurs, fundoscopy).
- Baseline Tier: CBC with differentials, peripheral blood smear, comprehensive metabolic panel, LFTs, ESR, CRP, ferritin (extremely high suggests AOSD or HLH), blood cultures (×3 sets), urine cultures, chest X-ray.
- Infectious Tier: TB (Quantiferon-Gold, sputum AFB, CBNAAT), Brucella serology, Widal, HIV, EBV, CMV, serology for Kala-Azar (rK39 antigen), ANA, ANCA, LDH.
- Advanced Imaging: CT chest/abdomen/pelvis (for lymphadenopathy, organomegaly, occult abscesses, masses). FDG-PET/CT — the single most powerful investigation for identifying occult inflammatory/neoplastic foci.
- Bone Marrow Examination: Mandatory in any FUO workup — can identify TB, lymphoma, Kala-Azar, HLH, and haematological malignancies.
Dr. Prateek's Approach to FUO
FUO evaluation is where a physician's breadth of knowledge and systematic thinking are most critical. Dr. Prateek Deo's training in both Rheumatology and Immunology — having managed hundreds of complex FUO cases at PGIMER — gives him the expertise to systematically work through the differential diagnosis without costly, unnecessary tests, and to identify the rare but treatable autoimmune causes of prolonged fever that many generalists miss.
Autoimmune FUO Expertise
We are particularly skilled at identifying rheumatological causes of FUO — Adult-onset Still's Disease, vasculitis, HLH, and periodic fever syndromes — that are frequently missed by non-rheumatologists.
FDG-PET/CT Coordination
We arrange and interpret FDG-PET/CT scans — the highest-yield investigation for FUO — which can identify occult lymphoma, vasculitis, and deep infections with a single whole-body scan.
Avoiding Empirical Steroids
We strongly advocate against empirical corticosteroid treatment of FUO without a diagnosis — steroids can mask infections and promote TB reactivation. We establish a diagnosis first.
Multi-Disciplinary Approach
FUO requires coordinated input from Rheumatology, Infectious Diseases, Haematology, and Oncology. We orchestrate this multi-disciplinary evaluation, serving as the integrating physician for your care.