(PUO) Pyrexia of Unknown Origin Causes Diagnosis and Treatment
A fever usually has a clear story. A sore throat, a chest infection, a urinary tract infection, or a short viral illness often explains it. Pyrexia of unknown origin is different. The temperature keeps returning or never fully settles, yet the first round of tests does not show why.
PUO, also called fever of unknown origin, can feel frustrating for patients and clinicians because it is not a diagnosis in itself. It is a clinical problem that needs a careful, step-by-step search for the cause.
This article explains what PUO means, the common causes, how doctors investigate it, and how treatment is chosen.
This content is for general information only. It does not replace medical advice, diagnosis, or treatment from a qualified clinician.
What pyrexia of unknown origin means
Pyrexia means fever. In many medical settings, a fever is a body temperature above 38°C, though the exact cut-off can vary with the method of measurement and the clinical situation.
Classic definitions of pyrexia of unknown origin describe:
Fever above 38.3°C on several occasions
Fever lasting for more than three weeks
No diagnosis after an initial medical assessment and appropriate investigations
In real practice, doctors may use the term more flexibly. A person with repeated high fevers, weight loss, night sweats, or abnormal blood tests may need a PUO-style assessment even before three weeks have passed.
PUO is not the same as a short fever during a cold or flu-like illness. It is used when the illness lasts long enough, or is concerning enough, to suggest a deeper cause may be present.
The main causes of PUO
The causes of PUO fall into several broad groups. Thinking in groups helps doctors avoid missing important possibilities.
Infections can hide in the body
Infections are one of the classic causes of prolonged fever. Some are obvious early. Others are harder to find because they sit deep in the body, produce subtle symptoms, or are partly treated before testing.
Possible infection-related causes include:
Tuberculosis
Abscesses in the abdomen, pelvis, liver, teeth, or spine
Endocarditis, which is infection of a heart valve
Bone or joint infection
HIV and other viral infections
Infections linked to travel, such as malaria
Brucellosis or other infections linked to animals, unpasteurised dairy, or farming exposure
The history often gives the clue. Travel, animal contact, dental procedures, intravenous drug use, recent surgery, implanted devices, and immune suppression all matter.
A person may not feel “infected” in the usual way. There may be no cough, no pain passing urine, and no clear rash. That is why blood cultures, imaging, and targeted tests become important.
Inflammatory and autoimmune diseases can drive fever
The immune system can cause fever even when there is no infection. This happens when inflammation becomes overactive or misdirected.
Common inflammatory causes considered in PUO include:
Giant cell arteritis, especially in older adults
Polymyalgia rheumatica
Rheumatoid arthritis
Adult-onset Still’s disease
Systemic lupus erythematosus
Vasculitis
Inflammatory bowel disease
Sarcoidosis
These conditions may cause joint pain, muscle aches, headaches, jaw pain while chewing, mouth ulcers, rashes, bowel symptoms, or eye inflammation. Sometimes fever comes first and the other signs appear later.
Blood markers such as C-reactive protein and erythrocyte sedimentation rate may show inflammation, but they do not prove the cause. They are useful clues, not final answers.
Cancer can present with unexplained fever
Some cancers can cause fever before a lump, pain, or organ-related symptom becomes clear. This is one reason persistent fever needs proper assessment.
Cancers sometimes linked to PUO include:
Lymphoma
Leukaemia
Kidney cancer
Liver cancer
Cancers that have spread to the liver or other organs
Warning features may include unexplained weight loss, drenching night sweats, persistent swollen glands, unusual bruising, severe fatigue, or ongoing loss of appetite.
Fever alone does not mean cancer. Many people with PUO do not have cancer. Still, doctors consider it because early recognition can change management.
Medicines can cause fever
Drug fever is easy to overlook. A person may develop fever because of a medicine, even if the medicine was prescribed correctly.
Medicines that can sometimes be linked with fever include certain:
Antibiotics
Anti-seizure medicines
Blood pressure medicines
Anti-inflammatory medicines
Treatments that affect the immune system
The timing is important. Fever may begin days or weeks after starting a medicine. A rash, raised eosinophil count, abnormal liver tests, or generalised itch can support the possibility, though they are not always present.
No one should stop prescribed medicine without medical advice, especially treatments for epilepsy, heart disease, blood clots, or immune conditions.
Other causes may be less obvious
Some cases come from conditions outside the usual categories. Examples include blood clots, thyroid inflammation, inherited fever syndromes, factitious fever, or fever related to tissue damage after surgery or trauma.
A proportion of PUO cases remain undiagnosed even after careful investigation. That can be unsettling, but it is not always a bad sign. Some undiagnosed fevers settle on their own, especially when repeated assessments show no progressive disease.
How doctors diagnose PUO
The best PUO assessment is usually methodical rather than rushed. Random testing can produce confusing results. A careful history and examination often point towards the right test.
The medical history sets the direction
A clinician will ask about the fever pattern, but the pattern alone rarely gives the answer. The wider story matters more.
Key questions often include:
When did the fever start?
Is it continuous or does it come and go?
How high does the temperature rise?
Are there night sweats, weight loss, fatigue, pain, rash, cough, diarrhoea, or urinary symptoms?
Has there been recent travel?
Has there been contact with animals, ticks, TB, or unwell people?
Are there new medicines or supplements?
Is there a history of cancer, autoimmune disease, HIV, transplant, or immune-suppressing treatment?
Have there been recent dental procedures, surgery, injections, or implanted devices?
A fever diary can help. Recording the time, temperature, symptoms, and any medicine taken gives a clearer picture than memory alone.
Examination looks for quiet clues
A full examination may include the skin, eyes, mouth, lymph nodes, chest, heart, abdomen, joints, nervous system, and temporal arteries in older adults.
Small findings matter. A heart murmur may suggest endocarditis. Tenderness over the spine may raise concern about bone infection. Enlarged lymph nodes may lead to further imaging or biopsy. A subtle rash may point towards inflammation, infection, or a drug reaction.
Initial tests are broad but purposeful
Common first-line investigations for PUO may include:
Test or assessment | What it may show |
Full blood count | Anaemia, high white cells, low platelets, blood disorders |
CRP and ESR | Level of inflammation |
Liver and kidney blood tests | Organ involvement or medicine effects |
Blood cultures | Bacteria in the bloodstream |
Urine test and urine culture | Urinary infection or kidney inflammation |
Chest X-ray | Pneumonia, TB clues, cancer clues, sarcoidosis |
HIV test and other infection tests | Hidden viral or immune-related causes |
Autoimmune screen | Clues for lupus, vasculitis, or related conditions |
These tests do not all diagnose PUO on their own. They guide the next decision.
Imaging and specialist tests come next
If initial tests do not explain the fever, doctors may use imaging. This can include ultrasound, CT, MRI, echocardiography, or nuclear medicine scans depending on the suspected cause.
For example:
An echocardiogram may help assess possible endocarditis.
CT imaging may detect abscesses, enlarged lymph nodes, or hidden tumours.
MRI may help when spinal infection or inflammation is suspected.
Ultrasound may assess the liver, gallbladder, kidneys, pelvis, or blood vessels.
Sometimes the most useful test is a biopsy. This may involve a lymph node, bone marrow, liver, temporal artery, skin lesion, or another affected site. A biopsy can identify cancer, infection, or inflammatory disease when blood tests cannot.
Repeating the assessment can be safer than guessing
PUO diagnosis often takes time. New symptoms may appear. Old results may need review. A test that was normal early may change later.
If the person is stable, doctors may avoid broad “just in case” treatment because it can hide the real cause. Antibiotics can make cultures negative. Steroids can reduce inflammation while allowing some infections to worsen. Painkillers and antipyretics can also mask the fever pattern.
The aim is not to delay care. The aim is to treat the right condition.
How PUO is treated
Treatment depends on the cause. There is no single treatment for PUO because the fever is a sign, not the disease.
Treating infection-related PUO
When an infection is confirmed or strongly suspected, treatment targets that infection. This may involve antibiotics, antiviral medicines, antiparasitic treatment, drainage of an abscess, removal of an infected device, or surgery in selected cases.
The choice depends on the organism, the site of infection, test results, allergies, kidney function, and local antimicrobial guidance.
Doctors try to collect cultures before starting antibiotics when it is safe to do so. This improves the chance of identifying the organism and choosing the right treatment.
Treating inflammatory causes
If an autoimmune or inflammatory condition explains the fever, treatment may include corticosteroids or other medicines that calm the immune system. The exact choice depends on the condition and severity.
For instance, suspected giant cell arteritis needs urgent treatment because of the risk of sight loss. By contrast, some inflammatory conditions need further confirmation before long courses of immune-suppressing medicine begin.
This is one of the hardest parts of PUO care. Steroids can be very effective, but they can also hide infection and change biopsy results. Timing matters.
Treating cancer-related fever
If cancer is the cause, treatment may involve haematology, oncology, surgery, radiotherapy, chemotherapy, immunotherapy, or targeted medicines depending on the diagnosis.
Supportive care is also important. Fever, fatigue, anaemia, pain, appetite loss, and infection risk all need attention.
Supportive care still matters
While the cause is being investigated, symptom control can help. A clinician may advise:
Drinking enough fluids
Resting during fever spikes
Using paracetamol if suitable
Avoiding excess alcohol
Monitoring temperature and symptoms
Seeking urgent help if red flags appear
Ibuprofen or other anti-inflammatory medicines may not be suitable for everyone, especially people with kidney disease, stomach ulcers, certain heart conditions, or those taking blood thinners.
When to seek urgent medical help
Persistent fever should be assessed, but some symptoms need urgent care.
Seek urgent medical advice or emergency help if fever occurs with:
Confusion, fainting, or severe drowsiness
A stiff neck or sensitivity to light
Shortness of breath or chest pain
A non-blanching rash
Severe abdominal pain
Signs of dehydration
A temperature that is very high or not coming down
Fever in pregnancy
Fever in someone with a weakened immune system
Fever after recent chemotherapy, transplant, or major surgery
Children, older adults, and people with long-term medical conditions may become unwell faster. A lower threshold for assessment is sensible in these groups.
Living with uncertainty during a PUO work-up
The waiting can be difficult. Tests may come back normal while the fever continues. This does not mean the symptoms are imagined. It means the cause has not yet declared itself or has not been captured by the tests done so far.
A practical approach can make the process easier:
Keep a fever and symptom diary.
Bring a full medicine list, including supplements.
Mention travel, animal exposure, dental work, and sexual health risks honestly.
Ask which symptoms should trigger urgent review.
Keep follow-up appointments, even if the fever improves.
Avoid starting leftover antibiotics without medical advice.
Good PUO care is often a partnership between primary care, hospital medicine, infectious diseases, rheumatology, haematology, oncology, radiology, and pathology. The right specialist depends on the clues that emerge.
The key takeaway
PUO needs careful investigation because the causes range from self-limiting illness to serious infection, inflammatory disease, or cancer. The safest path is usually a structured one: confirm the fever, build a detailed history, examine carefully, run appropriate tests, and treat the cause once it is found.
A persistent unexplained fever should not be ignored, but it should not be treated by guesswork either. Clear records, timely review, and targeted testing give clinicians the best chance of finding the answer.
Comments