Fever of unknown origin or FUO is a chronic fever for which no cause can be conclusively identified. There are three criteria that define FUO and this includes :
- A persistent or recurrent body temperature of 38.3C (101F) or more,
- A fever continuing for more than 3 weeks,
- No conclusive diagnosis of the cause of the fever despite 3 days of inpatient care or 2 outpatient visits.
Pyrexia with No or Few Symptoms
Non-specific Fever Tests
A diagnosis of a specific medical condition is made upon the clinical presentation, which are the signs evident to the doctor and symptoms reported by the patient, along with a medical history. Investigations confirm the diagnosis and allows a doctor to assess the severity of the condition.
The presence of a fever with no other symptoms or vague and transient symptoms is referred to as a non-specific fever. In this instance, a diagnosis is difficult to make without further investigation. Some of the investigations that would be considered in the event of a non-specific fever include:
- White blood cell (WBC) count and/or complete blood count (CBC)
- Sputum test
- Throat culture
- Stool test
- Lumbar puncture
- Chest x-ray
Other tests may be considered based on the patient’s medical, travel, occupational, sexual and family history. Recreational habits, drug usage (prescription/narcotic), lifestyle and nutrition as well as vector exposure (animal/insect contact, bites and stings). If no conclusive diagnosis can be reached after the clinical evaluation and investigations, the fever may be referred to as a fever of unknown origin (FUO). A factitious fever is where a patient manipulates the body temperature readings to record higher levels than is actually present and this has to be excluded as a possible cause of a fever of unknown origin.
Causes of a Fever of Unknown Origin
Infectious Causes of a Fever of Unknown Origin
An infection is caused by pathogenic microorganisms. These are the more common cause of a fever of unknown origin.
- Especially abdominal, pelvis and dental abscesses.
- Consider if there is a history of trauma, gynecological procedures, peritonitis, diverticulosis.
- Cat-scratch disease
- Consider if there is a history of exposure to cats, especially if been licked or scratched.
- Refer to Fever After Bites, Stings and Animal, Insect Contact
- Cytomegalovirus (CMV)
- Consider if there is a history of blood transfusion or if HIV-positive.
- Epstein-Barr virus (EBV)
- Human Immunodeficiency Virus (HIV)
- Consider if there is a history of risky behavior like multiple sexual partners, unprotected sex, IV drug use.
- Refer to Stages of HIV/AIDS.
- Inflammation of the inner lining of the heart and heart valves. In an FUO, this is usually due to an infection and is referred to as infective endocarditis.
- Travel-Related Fevers
- Consider if there is a history of recent travel.
- Refer to Traveler’s Fever for a list of causes of a non-specific fever after travel.
- Inflammation of the bone due to an infection.
- Inflammation of the paransal sinuses due to an infection.
- Tuberculosis (TB) – especially extrapulmonary TB.
- Fungal infections
- Parasitic infections
Rheumatological / Autoimmune Diseases
This include inflammatory conditions caused by the body’s immune activity against its own tissues.
- Adult Still’s disease
- Giant cell arteritis (temporal arteritis)
- Inflammatory bowel disease
- Polyarteritis nodosa
- Polymyalgia rheumatica
- Reactive arthritis
- Rheumatoid arthritis
- Systemic lupus erythematosus (SLE)
Neoplastic / Malignancy
This includes malignant tumors.
- Colon cancer
- Liver cancer (hepatoma)
- Leukemia and myeloma
- Renal cell carcinoma
- Stomach cancer
- Cancer of the pancreas
- Metastatic cancer
Miscellaneous (Other Causes)
- Alcoholic cirrhosis
- Drug fever
- Factitious fever – high body temperature due to manipulation by the patient.
- Deep vein thrombosis (DVT)
- Pancreatitis (inflamed pancreas)
- Inflammatory bowel disease
- Familial Mediterranean Fever (refer to periodic fever syndrome in Chronic Fever)
- Thyrotoxicosis (hyperthyroidism) and thyroiditis