The FUO medical abbreviation means fever of unknown origin, a fever that keeps coming back over several weeks and has no clear cause even after a standard medical evaluation. It is a working label, not a final diagnosis, and it tells the team that a more careful search is under way.
Patients usually see FUO on an admission order, a referral to an infectious disease clinic or a problem list during a long hospital stay. British and Commonwealth hospitals tend to call the same idea PUO, pyrexia of unknown origin. Here you will learn the usual definition, the groups of causes doctors consider and how the workup appears in a chart.
At a glance
Stands for
Fever of unknown originAlso called
PUO, pyrexia of unknown originUsual fever level
Above about 38.3 C (101 F) on several occasionsUsual duration
At least 3 weeksSpecialty
Infectious disease, internal medicine, rheumatologyDo not confuse with
A short fever that has not yet been evaluated
How doctors define an FUO
A fever alone does not earn the label. The classic definition, still widely taught, has three parts: a temperature above about 38.3 degrees Celsius (101 degrees Fahrenheit) on several occasions, an illness lasting at least three weeks, and no diagnosis despite a reasonable evaluation. Older versions required a week in the hospital. Newer versions accept a set of outpatient visits or about three days of inpatient testing, since much of the work can now be done in clinic.
The idea is to separate truly puzzling fevers from ordinary viral illnesses, which usually settle on their own well before three weeks.
Four groups of FUO
| Category | Who it applies to |
|---|---|
| Classic FUO | Otherwise healthy people meeting the standard definition |
| Health care associated (nosocomial) | Hospital patients who develop fever not present on admission |
| Neutropenic FUO | People with very low neutrophil counts, often during chemotherapy |
| HIV-associated FUO | People living with HIV who have prolonged fever |
Each group has its own likely causes and time frames, so the category often appears in the note right next to the abbreviation.
Causes behind a fuo medical abbreviation
Most cases eventually fall into a few broad groups. Infections, such as abscesses, heart valve infection, tuberculosis and bone infection, are a major group. Cancers, especially lymphoma and some leukemias, are another. Noninfectious inflammatory conditions, such as lupus, giant cell arteritis and other forms of vasculitis, make up a third group. A final group includes drug fever, blood clots and thyroid inflammation, among others.
In a noticeable share of patients no cause is found even after full testing. Many of those fevers fade over time.
What an FUO workup looks like on paper
Admission note: "52 yo F with fevers to 38.9 C for 4 wks, night sweats, 4 kg wt loss. Outpt CBC, CMP, UA, blood cx x2 neg. Plan: FUO workup, repeat cultures, ESR/CRP, HIV, TB testing, TTE, CT chest/abd/pelvis. Hold empiric abx unless unstable." Cx means cultures, abx means antibiotics, and empiric means started before the cause is known.
The plan section often lists the next tests in order. You may see ESR and CRP for inflammation, a heart ultrasound written TTE, and imaging such as CT or a PET scan. Teams often hold antibiotics in a stable patient so they do not hide the cause.
Terms that sit near FUO
- Tmax is the highest temperature recorded in a period, usually 24 hours.
- Febrile means having a fever; afebrile means no fever.
- PUO is the same concept under another name.
- DDx, differential diagnosis, is the list of possible causes being considered.
- SIRS names a whole-body inflammatory reaction to illness and differs from as a fever without a source.
Helpful questions during a long fever workup
A prolonged evaluation can feel slow. Some questions help: which causes are most likely in my case, which tests are next and why, should I keep a temperature log at home, and what new symptoms should prompt a call. Listing every medicine, travel history, animal contact and recent dental work helps the team narrow the search.
The broader set of condition shorthand is in the diagnosis hub.
This describes the abbreviation for general understanding. Your own fever, test plan and any treatment should be discussed with the clinician or infectious disease specialist caring for you.
Frequently asked questions
What does FUO stand for?
FUO stands for fever of unknown origin. It labels a fever lasting at least about three weeks without a cause found after standard testing.
Is FUO the same as PUO?
Yes. PUO, pyrexia of unknown origin, is the wording favored by many British clinicians. Pyrexia is another word for fever.
What temperature counts as an FUO?
The usual definition is a temperature above about 38.3 degrees Celsius, or 101 degrees Fahrenheit, on several occasions. Hospitals and specialists may use slightly different working definitions.
Does FUO mean the fever will never be explained?
No. FUO is a starting label. Many patients receive a specific diagnosis after further testing, and in those without one, the fever often goes away on its own.
Why might doctors wait before giving antibiotics for FUO?
In a stable patient, antibiotics can mask an infection and make cultures negative, which hides the cause. Teams usually collect samples first, but they treat right away if a person is very unwell.