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What is pyrexia of unknown origin (puo) ?

It is also known as fever of unknown origin (PUO).
Definition of PUO (by Petersdorf and Beeson. 1961) 
  • Fever higher than 101°F on several occasions.
  • A duration of more than 3 weeks
  • Failure to reach a diagnosis by 1 week of inpatient investigation or 3 out patient visit
Classification of PUO by Durack and Street 1991
  • Classical PUO
  • Nosocomial PUO (hospital-acquired).
  • Neutropenic PUO (when neutrophil count is < 500/mnr1)
  • HIV-associated (eg. tuberculosis, NHL,drug fever).
What are the diseases which may initially present as PUO 
Tuberculosis, SBE, subphrenic abscess, liver abscess, brucellosis,AIDS and fungal infections
2.Immune causes
Collagen vascular diseases,SLE, rheumatoid arthritis, polyarteritis
3.Factitious fever, drug fever, habitual hyperthermia
Lymphomas, leukaemias, multiple myeloma and carcinoma (specially of lungs liver,colon and kidney)