In 1961 Petersdorf and Beeson suggested the following criteria:
* Fever higher than 38.3°C (101°F) on several occasions
* Persisting without diagnosis for at least 3 weeks
* At least 1 week's investigation in hospital
A new definition which includes the outpatient setting (which reflects current medical practice) is broader, stipulating:
* 3 outpatient visits or
* 3 days in the hospital without elucidation of a cause or
* 1 week of "intelligent and invasive" ambulatory investigation.
Presently PUO cases are codified in four subclasses.
This refers to the original classification by Petersdorf and Beeson.
* infections (e.g. abscesses, endocarditis, tuberculosis, and complicated urinary tract infections),
* neoplasms (e.g. lymphomas, leukaemias),
* connective tissue diseases (e.g. temporal arteritis and polymyalgia rheumatica, Still's disease, systemic lupus erythematosus, and rheumatoid arthritis),
* miscellaneous disorders (e.g. alcoholic hepatitis, granulomatous conditions),
Nosocomial PUO refers to pyrexia in patients that have been admitted to hospital for at least 24 hours. This is commonly related to hospital associated factors such as, surgery, use of urinary catheter, intravascular devices , drugs (antibiotics induced Clostridium difficile colitis, and drug fever), immobilization (decubitus ulcers). Sinusitis in the intensive care unit is associated with nasogastric and orotracheal tubes.Other conditions that should be considered are deep-vein thrombophlebitis, and pulmonary embolism, transfusion reactions, acalculous cholecystitis, thyroiditis, alcohol/drug withdrawal, adrenal insufficiency, pancreatitis.
Immunodeficiency can be seen in patients receiving chemotherapy or in hematologic malignancies. Fever is concommittent with neutropenia (neutrophil less than 500/uL) or impaired cell-mediated immunity. The lack of immune response masks a potentially dangerous course. Infection is the most common cause.
Human immunodeficiency virus (HIV)-associated
HIV-infected patients are a subgroup of the immunodeficient PUO, and frequently have fever. The primary phase shows fever since it has a mononucleosis-like illness. In advanced stages of infection fever mostly is the result of a superimposed infections.