Fever of Unknown Origin (FUO) - Causes. Unexplained Fever, Low grade fever

If fever lasts more then three weeks and we can not identify underlying cause despite appropriate investigation, it is considered as Fever of unknown origin (FUO).
Fever is defined as a temperature higher than 38.3 degree Celsius, or 100.9 Fahrenheit.
4 main categories of fever of unknown origin exists.
1. Classic - it is Fever in previously healthy persons. Which lasts more then 3 weeks.
The most common causes of classic FUO are infection, malignancy, and collagen vascular disease.
2. Nosocomial - if fever is associated the time after patient has been hospitalized.
Most Common causes of nosocomial fever are infections.
3. Immune-deficient FUO, also known as neutropenic FUO, is defined as recurrent fever in a patient whose neutrophil count is 500 cubic millimeter. In most of these cases, the fever is caused by opportunistic bacterial infections.
4. HIV-related FUO. At this case, fever lasts more then 3 weeks and patient has HIV infection.
In patients with HIV infection, opportunistic infections (eg, tuberculosis; infection by atypical mycobacteria, disseminated fungi, or cytomegalovirus) should be sought.
The of differential diagnosis FUO generally is broken into four major subgroups: infections, malignancies, autoimmune conditions, and miscellaneous.
Most cases are infections around 50 %
Connective tissue disorders can be up to 20 % of cases.
Neoplasms around 5 to 35 %
And miscellaneous around 20 %
Infections agents are most common: Tuberculosis (especially extrapulmonary) and Abdominal abscesses are most common among infections.
Pelvic abscesses, Dental abscesses, Endocarditis, Osteomyelitis, Sinusitis, Cytomegalovirus, Epstein-Barr virus, Human immunodeficiency virus, Lyme disease, Prostatitis, Sinusitis.
Rheumatoid arthritis and rheumatic fever are inflammatory diseases that used to be commonly associated with autoimmune FUO, but with advances in serologic testing, these conditions usually are diagnosed more promptly.
Common connective tissue disorders include systemic lupus erythematosus, rheumatoid arthritis, giant cell arteritis, vasculitis, and juvenile rheumatoid arthritis of adults (adult Still disease).
The most common neoplastic causes are lymphoma, leukemia, renal cell carcinoma, hepatocellular carcinoma, and metastatic carcinomas. However, the incidence of neoplastic causes of FUO has been decreasing, probably because they are being detected earlier, by ultrasonography and CT during initial evaluation.
Important miscellaneous causes include drug reactions, deep venous thrombosis, recurrent pulmonary emboli, sarcoidosis, inflammatory bowel disease, and factitious fever.
No cause of FUO is identified in about 10% of adults.
In patients who have had contact with pets or other animals, diseases common to animal handlers must be suspected.
The family history should be carefully scrutinized for hereditary causes of fever, such as familial Mediterranean fever.
Diagnosis of fever of unknown origin:
First Step is Complete history and physical assessment. if we can define cause we can follow our findings.
If We cannot identify cause, then do tests:
Complete blood count, liver function, Blood Culture, Erythrocyte sedimentation rate, Urine analysis, Urine culture, Mantoux test for tuberculosis, and Chest Radiograph.
If we still can not identified underlying cause then,
CT scan of abdomen and pelvis, with contrast.
Then Think accordingly.
If malignancies is suspected then, biopsy,
If autoimmune disease is suspected, define rheumatoid factor, ANA.
PPD or Mantoux test should be done in all patients with fever of unknown origin. Its inexpensive and less invasive test. Which has high informative value.
By Blausen Medical - BruceBlaus. When using this image in external sources it can be cited as:Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436., CC BY-SA 4.0, commons.wikimedia.org/w/index...