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Fever is defined as "an abnormal elevation of body temperature, usually as a result of a pathologic process."[1]

In adults aged 40 years of less, the upper limits of normal temperature are:[2]

  • 7.2°C (98.9°F) in the early morning
  • 37.7°C (99.9°F) overall


Acute febrile illness in a healthy adult

Assuming that local inflammatory processes such as an abscess or cellulitis are not present, several clinical findings can suggest the underlying cause of an acute fever.

  • Influenza-like illness is defined as "fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat in the absence of a known cause other than influenza."[3] Possible causes include respiratory syncytial virus, rhinovirus, adenovirus, parainfluenza viruses, coronaviruses, and metapneumovirus.[4]

Acute febrile illness in an adult with neutropenia

For more information, see: neutropenia.

Clinical practice guidelines define febrile neutropenia as "a single oral temperature of >=38.3°C (101°F) or a temperature of >=38.0°C (100.4°F) for >= 1 h. Neutropenia is defined as a neutrophil count of <500 cells/mm3, or a count of <1000 cells/mm3 with a predicted decrease to <500 cells/mm3"[8]

Fever of unknown origin

Sometimes a biopsy of the bone marrow may help conditions such a hematological malignancies, especially if thrombocytopenia or anemia are present.[9]

Fever due to mediations

Hypersensitivity reaction

Medications may cause fever via hypersensitivity reactions.[7]

Toxin-induced hyperthermia

Drug toxicity from medications may cause hyperthermia by interfering with thermal regulation.[10]

The distinction between serotonin syndrome, neuroleptic malignant syndrome, malignant hyperthermia, and toxicity from cholinergic agents has been reviewed (see chart).[11] The most difficult distinction is between serotonin syndrome and neuroleptic malignant syndrome as patients may be on drugs that could cause either disorder. Serotonin syndrome shows hyperkinesia, hyperreflexia, and hyperactive bowel sounds, while neuroleptic malignant syndrome shows bradykinesia, bradyreflexia and normal or diminished bowel sounds. A helpful guide is that "dopamine antagonists [such as used to sedate a psychosis] produce bradykinesia, whereas serotonin agonists [such as used to activate a depression] produce hyperkinesia".[11] Lastly, neuroleptic malignant syndrome may develop over several days while serotonin syndrome develops faster.

Malignant hyperthermia

For more information, see: Malignant hyperthermia.

Neuroleptic malignant syndrome

For more information, see: Neuroleptic malignant syndrome.

Serotonin syndrome

For more information, see: serotonin syndrome.


Mother's touch can diagnose fever among children according to a systematic review.[12] The sensitivity was 89% and specificity was 50%.


For reducing fever, although acetaminophen combined with ibuprofen may be better than either drug alone according to a randomized controlled trial[13], a more recent systematic review concluded there is no benefit in combination therapy[14].


  1. National Library of Medicine. Fever. Retrieved on 2007-11-16.
  2. Mackowiak PA, Wasserman SS, Levine MM (1992 Sep 23-30). "A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich.". JAMA 268 (12): 1578-80. DOI:10.1001/jama.1992.03490120092034. PMID 1302471. Research Blogging.
  3. Centers for Disease Control and Prevention (2007). CDC - Influenza (Flu) - Flu Activity. Retrieved on 2007-11-19.
  4. Kelly H, Birch C (2004). "The causes and diagnosis of influenza-like illness". Australian family physician 33 (5): 305–9. PMID 15227858[e]
  5. Hurt C, Tammaro D (2007). "Diagnostic evaluation of mononucleosis-like illnesses". Am. J. Med. 120 (10): 911.e1–8. DOI:10.1016/j.amjmed.2006.12.011. PMID 17904463. Research Blogging.
  6. 6.0 6.1 Babyatsky MW, Keroack MD, Blake MA, Rosenberg ES, Mino-Kenudson M (2007). "Case 35-2007 -- A 30-Year-Old Man with Inflammatory Bowel Disease and Recent Onset of Fever and Bloody Diarrhea" 357 (20): 2068–2076. DOI:10.1056/NEJMcpc079029. PMID 18003964. Research Blogging.
  7. 7.0 7.1 Mackowiak PA, LeMaistre CF (1987). "Drug fever: a critical appraisal of conventional concepts. An analysis of 51 episodes in two Dallas hospitals and 97 episodes reported in the English literature". Ann. Intern. Med. 106 (5): 728-33. PMID 3565971[e]
  8. Hughes WT, Armstrong D, Bodey GP, et al (2002). "2002 guidelines for the use of antimicrobial agents in neutropenic patients with cancer". Clin. Infect. Dis. 34 (6): 730–51. PMID 11850858[e]
  9. Hot A, Jaisson I, Girard C, French M, Durand DV, Rousset H et al. (2009). "Yield of bone marrow examination in diagnosing the source of fever of unknown origin.". Arch Intern Med 169 (21): 2018-23. DOI:10.1001/archinternmed.2009.401. PMID 19933965. Research Blogging.
  10. Rusyniak DE, Sprague JE (2005). "Toxin-induced hyperthermic syndromes". Med. Clin. North Am. 89 (6): 1277-96. DOI:10.1016/j.mcna.2005.06.002. PMID 16227063. Research Blogging.
  11. 11.0 11.1 Boyer EW, Shannon M (2005). "The serotonin syndrome.". N Engl J Med 352 (11): 1112-20. DOI:10.1056/NEJMra041867. PMID 15784664. Research Blogging.
  12. Teng CL, Ng CJ, Nik-Sherina H, Zailinawati AH, Tong SF (2008). "The accuracy of mother's touch to detect fever in children: a systematic review". J. Trop. Pediatr. 54 (1): 70–3. DOI:10.1093/tropej/fmm077. PMID 18039678. Research Blogging.
  13. Hay AD, Costelloe C, Redmond NM, et al (2008). "Paracetamol plus ibuprofen for the treatment of fever in children (PITCH): randomised controlled trial". BMJ 337: a1302. PMID 18765450. PMC 2528896[e]
  14. Purssell E (2011). "Systematic review of studies comparing combined treatment with paracetamol and ibuprofen, with either drug alone.". Arch Dis Child 96 (12): 1175-9. DOI:10.1136/archdischild-2011-300424. PMID 21868405. Research Blogging.