Worldwide, infection is the leading cause of FUO, with prevalence varying by country and geographic region.
Thromboembolic disease (i.e. pulmonary embolism, deep venous thrombosis) occasionally shows fever.
[4] Endemic mycoses such as histoplasmosis, blastomycosis, coccidioidomycosis, and paracoccidioidomycosis can cause a fever of unknown origin in immunocompromised as well as immunocompetent people.
[4] Invasive opportunistic mycoses may also occur in immunocompromised people; these include aspergillosis, mucormycosis, Cryptococcus neoformans.
[4] Approximately 2 weeks after initial HIV infection, with viral loads being high, an acute retroviral syndrome can present with fevers, rash and mono-like symptoms.
The newly active immune system often has an exaggerated response against opportunistic pathogens leading to a fever and other inflammatory symptoms.
[7] A comprehensive and meticulous history (i.e. illness of family members, recent visit to the tropics, medication), repeated physical examination (i.e. skin rash, eschar, lymphadenopathy, heart murmur) and myriad laboratory tests (serological, blood culture, immunological) are the cornerstone of finding the cause.
Invasive techniques (biopsy and laparotomy for pathological and bacteriological examination) may be required before a definite diagnosis is possible.
[1][3] Positron emission tomography using radioactively labelled fluorodeoxyglucose (FDG) has been reported to have a sensitivity of 84% and a specificity of 86% for localizing the source of fever of unknown origin.
[4] In 1961 Petersdorf and Beeson suggested the following criteria:[1][2] A new definition which includes the outpatient setting (which reflects current medical practice) is broader, stipulating: Presently FUO cases are codified in four subclasses.
Studies show there are five categories of conditions:[citation needed] Nosocomial FUO refers to pyrexia in patients that have been admitted to hospital for at least 24 hours.
[1][2][3] Other conditions that should be considered are deep-vein thrombophlebitis, pulmonary embolism, transfusion reactions, acalculous cholecystitis, thyroiditis, alcohol/drug withdrawal, adrenal insufficiency, and pancreatitis.
[1][2][3] HIV-infected people with pyrexia and hypoxia will be started on medication for possible Pneumocystis jirovecii infection.