Fever (R50.9 – fever unspecified, NOS, FUO, fever with chills, with rigors, hyperpyrexia NOS, persistent fever, and pyrexia NOS), which is also known as high fever or a high temperature, is in itself not an illness, but a symptom of some underlying condition, most often an infection. However, there are many non-infectious causes of fever. Fever is number 29 in the top 30 list of diagnosis codes identified in the emergency medicine diagnosis coding performed at web2pro.

“Fever can be referred to as FUO or Fever of Unknown Origin and this definition was formulated to compare retrospective and prospective clinical studies and should not be regarded as an absolute finding. It has been suggested that the definitions be modified to two weeks of fever and either three days of hospital investigation or three outpatient visits without discovering the source of the fever”.¹

Fever can also be called hyperthermia (ICD-10 instructs you to also see Hyperpyrexia – R50.9), which is the condition of having the body temperature greatly above normal - an unusually high fever. The most severe form of hyperthermia is heat stroke (T67.0 – "use additional codes to identify any associated complications”). "Fever is the abnormal elevation of temperature. The normal temperature taken orally ranges from about 97.6 to 99.6 degrees Fahrenheit, although there is some individual variation. Rectal temperature is 0.5 to 1.0 degrees higher than oral temperature. Infections, drugs, tumors, breakdown of necrotic tissue, Central Nervous System damage, and collagen diseases are underlying causes of fevers. Despite common beliefs, fever is not harmful except in patients who cannot tolerate its hypermetabolic effects, some elderly patients in whom it can cause delirium and children with a history of febrile seizure.²

Types of Fever
There are many types of fever – literally hundreds identified in the ICD-10 code book, including some of those listed below:

  • Typhoid Fever (ICD-10 – A01.00 – Typhoid fever - unspecified): A systemic disease caused by S. typhi and characterized by fever, prostration (absolute exhaustion), abdominal pain, and a rose colored rash.
  • Yellow Fever (ICD-10 – A95.9 – Yellow Fever – unspecified): an acute Flavivirus (viruses that cause yellow fever, certain types of encephalitis, and dengue are species of this genus) infection of variable severity, characterized by sudden onset, fever, a relatively slow pulse, and a headache.
  • Continuous (not identified in ICD-10 as continuous, but in ICD-9 it is classified as 780.60 which cross-walks to R50.9): a sustained fever as in scarlet fever, typhus or pneumonia.
  • Drug (ICD-10 – R50.2 – Drug Induced fever – use additional code for drug): elevated body temperature caused by administration of a drug.
  • Factitious(not found in ICD9 or ICD-10 code books): fever produced artificially by a patient.
  • Induced (not found in ICD9 or ICD-10 code books): fever produced artificially to treat certain diseases such as central nervous system syphilis.
  • Intermittent (Bilious – see also Malaria – of unknown origin – ICD-10 - R50.9): fever in which symptoms disappear completely between paroxysms (A periodic attach or recurrence of symptoms of a disease).

Drug treatment may include Acetaminophen (Tylenol® and others), Ibuprofen (Motrin®, Advil®), and aspirin but generally, if the fever does not cause discomfort, the fever itself may not need to be treated.

Medications for fever are not a “one size fits all” – there are limitations on what should be given and to whom. Other treatment can include: comfort, no over-dressing, cool water baths to bring fever down, staying hydrated and alcohol avoidance.

Diagnosis Coding
ICD-10 contains many other types and reasons for fever including, but not limited to, what is shown above. It is imperative that the provider document medical history, the examination and the diagnostic studies’ findings, if any, in order to determine the exact course of action for the condition presented or defined after study. This will also allow the coding staff to select the most appropriate ICD-10 diagnosis code to help ensure proper reimbursement.

¹ The Merck Manual Seventh Edition, Centennial Edition, Section 13, Chapter 150, Page 1093-1094, Editor – Keryn A. G. Lane, Publisher – Gary Zelko, June 1999
² Taber’s Cyclopedic Medical Dictionary, Edition 19, Page 756, Editor – Donald Venes, M.D., M.S.J., Publisher – F. A. Davis Company 2001


  • ICD-10-CM, 2016 Complete Official Code Set, Chapter 18, Page 883, Signs, Symptoms, and Abnormal Clinical and Laboratory Findings, Not Elsewhere Classified (R00-R99), AAPC, Optum360, 2015
  • ICD-10-CM, 2016 Complete Official Code Set, Chapter 19, Page 1129, Injury, Poisoning, and Certain other Consequences of External Causes, Not Elsewhere Classified (R00-R99), AAPC, Optum360, 2015
  • Merck Manual, Section 19 – Pediatrics, Chapter 265 – Childhood Infections, Page 2359-2360, Editor – Keryn A. G. Lane, Publisher – Gary Zelko, June 1999
  • First Aid and Emergencies, Fever Facts. WebMD.
Author - Male

About the author

Robert Bunting, CPC, CPC-H, CHC, CEDC, CEMC, CAC is Compliance Director – Emergency Medicine, web2pro Business Performance Services.