Pyrexia of unknown origin

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Pyrexia of unknown origin is a problem-solving state rather than a single disease. Contemporary practice often begins before a rigid duration threshold is met, but the essential features are documented recurrent fever, no diagnosis after an appropriate initial evaluation and sufficient clinical stability to investigate rationally. The diagnostic strategy depends on host factors and local epidemiology rather than a universal test panel.

The highest-yield tool is repeated synthesis. Construct a timeline linking fever, procedures, travel, medicines and evolving laboratory abnormalities. Re-examine the skin, mouth, fundi where appropriate, lymph nodes, heart, abdomen, joints and spine. Potentially diagnostic clues should be verified because a false-positive autoimmune marker or incidental imaging lesion can drive an expensive and harmful detour.

Empirical therapy can obscure endocarditis, tuberculosis, lymphoma and inflammatory disease. In a stable patient, protect culture and tissue yield and involve infectious diseases, microbiology, rheumatology or haematology according to the phenotype. In an unstable patient, collect critical samples rapidly and treat the dangerous syndrome while continuing the diagnostic process.

Key points

Confirm that fever is objectively present, sustained or recurrent, and not explained by measurement error, normal diurnal variation or transient self-limited infection.
Repeat the history because diagnostic clues emerge over time: travel, animals, occupation, dental work, implants, medicines, immune suppression, sexual exposure and family origin all matter.
Examine repeatedly for lymph nodes, rash, murmurs, temporal tenderness, synovitis, hepatosplenomegaly, spinal tenderness, focal neurological signs and device or surgical-site changes.
Build four parallel categories—infection, malignancy, inflammatory disease and miscellaneous causes—then let positive findings determine tests rather than ordering every available panel.
Blood cultures, urine testing, HIV testing with consent, chest imaging and a core haematological, renal, hepatic and inflammatory screen commonly form the first tier.
Avoid empirical antibiotics in a stable undifferentiated patient because partial treatment reduces microbiological yield; treat immediately when instability, neutropenia or a defined dangerous syndrome changes the balance.
A negative scan or marker does not close the case. Review the fever curve, medicines, repeated examination, new localising symptoms and whether the original definition remains valid.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Occult infection

Endocarditis, tuberculosis, deep abscess, osteomyelitis, viral infection and zoonoses can produce persistent fever before localising symptoms become obvious.

Inflammatory disease

Large-vessel vasculitis, connective-tissue disease, adult-onset Still disease and other immune syndromes cause fever through sustained cytokine activation.

Malignancy

Lymphoma, leukaemia, renal malignancy and other cancers can generate fever through tumour cytokines, necrosis, obstruction or secondary infection.

Medicines and miscellaneous causes

Drug fever, thrombosis, endocrine crisis and factitious temperature disturbance remain diagnoses requiring positive contextual evidence rather than exclusion alone.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Pyrogen signalling
Microbial products or sterile inflammatory signals induce cytokines that raise the hypothalamic temperature set point through prostaglandin-mediated pathways.
2
Heat generation and conservation
Vasoconstriction, behavioural warming and shivering raise core temperature toward the new set point, producing chills despite an objectively rising temperature.
3
Systemic metabolic demand
Sustained fever increases heart rate, oxygen consumption, insensible losses and catabolism, which can destabilise people with limited cardiopulmonary reserve.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Endovascular patternRed flag

Persistent bacteraemia, a new murmur, embolic features, prosthetic valve, cardiac device or injection exposure raises infective endocarditis and requires multiple cultures and echocardiographic planning.

Granulomatous or travel pattern

Weight loss, night sweats, lymphadenopathy, hepatosplenomegaly, pulmonary change or relevant country exposure suggests tuberculosis, fungal, parasitic or other chronic infection.

Inflammatory patternRed flag

Headache with temporal tenderness, synovitis, rash, myalgia, serositis, renal sediment or recurrent mucosal inflammation supports vasculitic, connective-tissue or autoinflammatory disease.

Malignant pattern

Progressive lymphadenopathy, cytopenias, bone pain, pruritus, organomegaly or disproportionate lactate dehydrogenase elevation warrants tissue-directed haematological or oncological assessment.

Drug-fever pattern

Fever begins after a new medicine and persists without a convincing focus, sometimes with rash, eosinophilia or liver injury; improvement after supervised withdrawal supports causality.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Verified temperature chart and repeated examinationFirst step
Why
Confirm the phenotype and capture new localising clues over time.
Interpretation and limitations
Record method, timing, rigors and associated symptoms. Apparent patterns are rarely diagnostic alone, but absence of documented fever should prompt reconsideration before invasive testing.
02
Core blood and urine assessment
Why
Identify cytopenia, organ involvement, inflammation, haematuria and a common occult urinary source.
Interpretation and limitations
Use FBC and film, U&E, liver tests, CRP or ESR and urinalysis with culture when indicated. Trends and combinations are more informative than isolated mild abnormalities.
03
Three appropriately timed blood-culture sets when endovascular infection is plausible
Why
Increase detection of continuous bacteraemia before antimicrobial exposure.
Interpretation and limitations
Collect from separate peripheral venepunctures rather than a device alone. Typical organisms in multiple sets support true infection; prior antibiotics reduce sensitivity and may require specialist culture strategy.
04
Targeted infection testing
Why
Investigate HIV, tuberculosis, hepatitis, malaria or zoonoses when epidemiology and phenotype justify testing.
Interpretation and limitations
Request tests with correct specimen and timing after discussing unusual pathogens with microbiology. Broad serological panels can yield cross-reactions and evidence of past rather than active infection.
05
Cross-sectional or functional imaging followed by tissue
Why
Locate occult abscess, malignancy, vasculitis or metabolically active tissue suitable for diagnosis.
Interpretation and limitations
CT chest, abdomen and pelvis is often guided by initial clues. PET-CT may identify a biopsy target after specialist review; tissue should be sent for histology and planned microbiology.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Unmeasured or spurious fever

Subjective warmth, technique error and environmental variation can mimic persistent pyrexia; supervised consistent measurement establishes whether the phenotype exists.

Infection

Exposure, immune status, focal findings, direct microbiology and tissue determine whether occult microbial disease explains the inflammatory response.

Inflammation or malignancy

Vascular, joint, skin, haematological and imaging patterns direct tissue or syndrome-specific testing after infection risk is considered.

Drug-related fever

A compatible medicine timeline, absence of another source and resolution after safe withdrawal support the diagnosis, although rash or eosinophilia may be absent.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Stable unexplained feverProtect diagnostic yieldFirst stepDocumented recurrent fever with no high-risk physiology and no established diagnosis after initial assessment.

1Reconstruct the timeline and exposures, repeat a complete examination, verify fever and stop non-essential medicines safely when drug fever is plausible.
2Obtain a focused first-tier screen and cultures before antimicrobials, then select serology, imaging and specialty referral from genuine potential diagnostic clues.
3Review all results together at a defined interval, pursue tissue from the safest highest-yield lesion and resist repeating low-yield tests without a changed hypothesis.

02Unstable or immunocompromisedTreat the dangerous syndrome while samplingShock, organ dysfunction, neutropenia, profound immune suppression or a time-critical focal syndrome accompanies unexplained fever.

1Perform ABCDE, isolate when transmission risk is possible, obtain cultures and targeted samples promptly and involve senior infection and organ-support teams.
2Start empirical therapy appropriate to the immune defect, probable source and resistance history without waiting for the full PUO work-up.
3Continue urgent imaging and source control, then narrow therapy and revisit non-infectious causes as physiology and microbiology evolve.

03Persistent negative evaluationRe-check definition and hidden cluesFirst lineFever continues despite negative first-line cultures, imaging and targeted serology in a clinically stable patient.

1Audit temperature documentation, medicine exposure, travel and prior antimicrobial history, and repeat examination for lymph nodes, vascular, ocular, cardiac, spinal and skin signs.
2Review pathology and radiology with relevant specialists, consider echocardiography or PET-CT only when it can locate a diagnostic target, and plan biopsy specimens before the procedure.
3Avoid diagnostic corticosteroids unless an organ-threatening inflammatory syndrome justifies them; document surveillance and safety-netting if no cause is found.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Delayed dangerous diagnosis

Non-localising fever can precede endocarditis, tuberculosis, vasculitis or malignancy, allowing embolic, structural or organ damage before recognition.

Iatrogenic diagnostic harm

Untargeted tests generate incidental findings, while empirical antibiotics or corticosteroids can cause toxicity and obscure microbiology or tissue histology.

Dehydration and deconditioning

Persistent heat, poor intake and catabolism can cause renal injury, weight loss, weakness and functional decline during prolonged investigation.

Transmission risk

Unrecognised airborne or contact-transmitted infection may expose household members, staff and vulnerable patients before isolation or public-health action begins.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Maintain a dated fever and symptom chart linked to medicines, cultures and procedures so temporal associations remain interpretable.
Trend full blood count, renal and hepatic function, inflammatory markers and urinalysis according to emerging abnormalities rather than repeating them mechanically every day.
Track weight, lymph-node size, organomegaly, new murmurs, skin lesions, neurological findings and spinal or joint pain through scheduled repeated examination.
Create a result log for cultures, serology, histology and imaging, including pending tests and the clinician responsible for acting on results after discharge.
Escalate new hypotension, confusion, hypoxaemia, visual symptoms, focal neurological deficit, severe headache, rash or rapidly progressive organ dysfunction immediately.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Repeated examination is an investigation

Endocarditis signs, lymph nodes, rashes and focal tenderness may emerge after the first assessment, making a documented serial examination higher yield than indiscriminate panels.

Serology shows context

Antibody positivity may indicate remote exposure, vaccination or cross-reaction. Active disease requires compatible timing, phenotype and where possible direct microbiological evidence.

Tissue needs advance planning

A biopsy placed only in formalin cannot provide bacterial, fungal or mycobacterial culture; agree specimen allocation with pathology and microbiology before collection.

Therapeutic trials can mislead

Fever may fluctuate spontaneously and anti-inflammatory treatment suppresses many diseases, so response rarely proves a specific diagnosis and can reduce later yield.

11Common pitfallsFrequent interpretation and management errors.

01
Calling subjective warmth or one unverified reading persistent pyrexia.
02
Ordering broad autoimmune and infection panels without a pre-test hypothesis.
03
Giving repeated antibiotics to a stable patient before adequate blood cultures.
04
Treating a positive antibody result as proof of active disease.
05
Biopsying tissue without arranging the microbiological specimens needed for the differential.