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Pyrexia of unknown origin has been defined as fever for more than 3 weeks where no cause is found despite seven days of basic investigations in hospital.

However, in a hospital with the full availability of sophisticated diagnostic techniques, a reduced period of two weeks with unexplained oral temperatures of 38 degrees centigrade or more has been considered to meet the definition (1).

In clinical practice, the acronym PUO is used widely.

Other classifications of PUO have been developed (2)

  • nosocomial PUO
    • defined as PUO in hospital patients
      • with fever of 38.3°C on several occasions
        • caused by a process not present or incubating on admission
        • where initial cultures are negative and diagnosis unknown after three days of investigation

  • neutropenic PUO
    • with fever fever of 38.3°C on several occasions
    • where the origin of the fever is unknown
    • with <1 x 10^9 neutrophils
    • with initial negative cultures and diagnosis uncertain after three days (3)

  • HIV-associated PUO
    • HIV-positive patients with fever fever of 38.3°C on several occasions
      • for four weeks as outpatients or three days as inpatients
        • where the origin of the fever is unknown
        • with an uncertain diagnosis after three days of investigation
        • where at least two days have been allowed for cultures to incubate (4)


  • Smith, C. (1992). Pyrexia of Unknown Origin. Medicine International. 104, 4364-4367.
  • Hayakawa K, Ramasamy B, Chandrasekar PH; Fever of unknown origin: an evidence-based review. Am J Med Sci. 2012 Oct;344(4):307-16.
  • Durack DT, Street AC; Fever of unknown origin--reexamined and redefined. Curr Clin Top Infect Dis. 1991;11:35-51.
  • Roth AR, Basello GM; Approach to the adult patient with fever of unknown origin. Am Fam Physician. 2003 Dec 1;68(11):2223-8.

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