REASONING GP · Clinical algorithm · Quick reference

Fever in adults — triage and diagnostic approach

Daytime GP and OOH/111, face-to-face or remote. Assess physiology first, then hunt the source, then decide whether the absence of a source is safe. Ongoing management of each diagnosis is in the full Steps pathway.
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AdultsGP + OOHPhysiology firstv1.0 · Sep 2026
Core rule. Severity is judged from physiology and risk group, not from the thermometer. Use NEWS2 and the NG51 high-risk criteria in section 0 — they outrank any temperature. Absence of fever does not exclude sepsis — older, frail and steroid-treated patients may be normothermic or hypothermic. Neutropenia, asplenia and recent travel to a malarial area convert an ordinary-looking fever into an emergency.
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Before any history: is this an emergency now? Stop the consultation and call 999 if any of these is present
Systolic <90 mmHg (or >40 below baseline) · pulse >130 · respiratory rate ≥25 · SpO₂ <92% on air · new altered mental state, or not responding normally to voice · mottled, ashen or cyanosed skin · non-blanching rash · no urine for 18 hours · NEWS2 ≥5 · fever within 6 weeks of chemotherapy. ABCDE, full observations, oxygen if hypoxic. Suspected meningococcal disease with a non-blanching rash: benzylpenicillin 1.2 g IM/IV before transfer (section 7). Do not delay transfer for bloods, cultures, urine or oral antibiotics.
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Define the fever in four lines Enough to place the patient in the table below

Time

<5 days (usually viral) · 5–14 days (look harder) · >3 weeks undiagnosed = PUO. Pattern, rigors, swinging fever. Antipyretics taken and when — a recorded normal temperature may be masked.

Character

Highest recorded temperature and how measured. Rigors suggest bacteraemia. Hyperthermia pictures — heatstroke, neuroleptic malignant or serotonin syndrome, thyroid storm — in which antipyretics do not work.

Context

Travel (where, when, prophylaxis, freshwater, bites) · immune status (chemotherapy ≤6 weeks, biologics, DMARDs, steroids, transplant, asplenia, sickle cell, HIV, diabetes) · devices (catheter, line, prosthesis, pacemaker, recent surgery, dental work) · injecting drug use · new drug · pregnancy · contacts, occupation.

Associated

Sweep: cough, sputum · dysuria, frequency, flank pain · sore throat, earache, sinus or dental pain · headache, neck stiffness · abdominal pain, diarrhoea · skin breaks, bites, rash · joint swelling · back pain · vaginal or urethral discharge · night sweats, weight loss, bruising, lymph nodes.
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Red-flag screen: trigger → concern → destination → what to do before handover One row is enough; do not add up reassuring features
Trigger / red flagLeading concern · destinationDo now · do not delay
NEWS2 ≥5, or any NG51 high-risk criterion: systolic <90, pulse >130, RR ≥25, SpO₂ <92%, new altered mental state, mottled or ashen skin, no urine for 18 h. Sepsis
999 now		 ABCDE, oxygen to target, IV access if it does not delay transfer. Record the full observation set including respiratory rate and hand it over. No oral antibiotics and review; do not wait for a CRP or urine result.
Non-blanching purpuric rash (glass test), or fever with meningism: headache, neck stiffness, photophobia, vomiting, altered mental state, seizure. Meningococcal disease · bacterial meningitis · encephalitis
999 + pre-alert		 Benzylpenicillin 1.2 g IM/IV (or ceftriaxone) as soon as possible where meningococcal disease is strongly suspected, unless it delays transfer or there is anaphylaxis. Examine the whole skin undressed — a few petechiae are enough. Notify UKHSA on suspicion, not confirmation.
Immunocompromised, or a device in situ. Fever within 6 weeks of chemotherapy, known neutropenia, asplenia or sickle cell. Or biologics, DMARDs, high-dose steroids, transplant, advanced HIV, poor diabetes control; prosthetic joint or valve, line, catheter, pacemaker, recent surgery or instrumentation. Neutropenic sepsis · encapsulated-organism sepsis · occult deep or device infection
999 / same-hour if chemotherapy ≤6 weeks, neutropenia, asplenia, sickle cell Otherwise same-day acute assessment		 CG151: immediate admission for broad-spectrum antibiotics within 1 hour — send directly; no review first if it causes delay, no FBC-and-wait, no oral antibiotic. Check the alert card and standby plan. Otherwise the signs are blunted: escalate on a lower threshold and on physiology, not appearance; inspect every line, catheter and wound. Long-term steroids plus vomiting means replacement is not absorbed — give parenteral hydrocortisone or admit.
Fever within 1 year of travel to a malarial area — especially within 6 weeks — with or without a localising source, whatever prophylaxis was taken. Falciparum malaria · typhoid · dengue · viral haemorrhagic fever
Same-day admission		 Malaria until three thick and thin films are negative — a hospital test, not an outpatient one. Early falciparum fever is usually continuous or irregular, not cyclical. If travel to a VHF risk area within 21 days: telephone the local infection service and UKHSA before face-to-face assessment.
Fever with back pain, focal spinal tenderness, or any new limb weakness, sensory level or bladder disturbance. Discitis · spinal epidural abscess
999 for urgent MRI		 Percuss the whole spine — seconds, and the only bedside test that raises the possibility. Do not attribute fever plus back pain to a coincidental musculoskeletal problem; once neurology appears the deficit may be permanent.
Deep-seated infection. New murmur, or injecting drug use, with fever — often weeks of malaise, sweats and weight loss on previous antibiotics. Or a hot, swollen, painful joint or prosthesis, with fever or upset. Infective endocarditis · septic arthritis · prosthetic joint infection
Same-day medicine / orthopaedics — cultures and echo, or aspiration		 Cultures or aspiration must precede any further antibiotic — prior antibiotics render cultures negative and delay endocarditis by months; an oral antibiotic with review obscures the joint culture and delays washout. Never treat blind. Splinter haemorrhages, Osler's nodes, Janeway lesions, Roth spots. Gout and pseudogout mimic septic arthritis; still aspirate.
Age ≥50, fever or malaise with new headache, jaw claudication, scalp tenderness or any visual symptom. Giant cell arteritis
Same-day rheum / acute med Emergency eye if visual		 Start prednisolone now: 40–60 mg daily (minimum 0.75 mg/kg); 60 mg immediately plus same-day ophthalmology with any visual symptom. Do not wait for ESR or CRP — a normal ESR does not exclude GCA. Sight loss is irreversible.
Unexplained fever with night sweats and weight loss, bruising, bleeding, bone pain, or unexplained lymphadenopathy or splenomegaly. Leukaemia · lymphoma · TB
Very urgent FBC within 48 h · suspected lymphoma referral · NICE NG12		 FBC on the very urgent route, acted on the same day it returns. Children and young people with fever plus unexplained petechiae, hepatosplenomegaly or generalised lymphadenopathy need immediate specialist assessment (NG12). Cough >3 weeks with sweats and weight loss: CXR, consider TB — notifiable.
Safety rule. A fever that "breaks" with paracetamol proves nothing, and a normal CRP in the first day does not exclude serious bacterial infection. In older adults a positive urine dipstick is not a source without urinary symptoms — asymptomatic bacteriuria is common, and labelling it as the cause is a recognised route to a missed pneumonia, endocarditis or spinal infection. A genuinely sourceless fever in an unwell adult is a reason to escalate, not to reassure.
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Focused examination and investigation Only after the emergency question is answered; remote assessment that cannot deliver these is an escalation criterion

Observations

Temperature, pulse, BP, respiratory rate, SpO₂, conscious level, capillary refill, blood glucose, NEWS2. Respiratory rate is the most sensitive early marker and the most often omitted.

System examination

Chest and heart sounds (listen for a new murmur) · abdomen including renal angles, liver, spleen · throat, teeth, ears, sinuses · all lymph node groups · calves. Pelvic or PR where indicated.

Targeted signs

Whole skin undressed for non-blanching rash, cellulitis, ulcers, bites, injection sites, embolic signs — include back, buttocks, between the toes. Percuss the whole spine. Examine every joint, line, catheter, wound and pacemaker site. Fundoscopy.

Investigations

None if well, <5 days, with a clear source. Otherwise question-led: FBC with differential, CRP, U&Es, LFTs, LDH, ESR, HIV test, blood cultures before antibiotics, urine culture, CXR (highest yield). Not in primary care: malaria films, echo, CT, MRI — via the acute or specialist route.
Colour is semantic: red = emergency now · amber = same-day acute assessment · blue = define/assess · green = benign diagnosis established · purple = uncertain / specialist route. Continued on page 2: classification, cause classifier, routine referral, primary-care treatment, endpoint.
REASONING GP · Fever in adults — triage and diagnostic approach
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Classify: three exits, not two "No source found" is a conclusion that needs a plan, not a discharge

Serious infection or serious cause likely

Any red flag, deranged physiology, a high-risk group, or a source needing acute care. Output: name the leading concern, use the destination above, and hand over the full observation set with drugs given and times.

Self-limiting illness established

Well, <5 days, normal physiology, a coherent viral or minor localised source, no risk group, and reliable support and access. Output: record observations, the expected course, the written safety-net, and a named review point.

Unclassified / uncertain

No source after a full sweep, unreliable or remote assessment, a frail or comorbid patient, or a fever that has already lasted >5 days. Output: do not label "viral". State what is outstanding; arrange bloods, a chest X-ray and a defined review, or admit.
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Where adult fever comes from Use only once the emergency and red-flag screens are complete
Respiratory and ENT (commonest)Viral URTI, influenza, COVID-19, bronchitis, community-acquired pneumonia (use CRB-65), tonsillitis (FeverPAIN or Centor), sinusitis, otitis media, dental abscess, quinsy. Stridor or drooling = epiglottitis → 999.
Urinary, skin and abdominalPyelonephritis (fever, rigors, flank pain), prostatitis, catheter-associated infection; cellulitis, abscess, infected ulcer or bite; gastroenteritis, diverticulitis, appendicitis, cholecystitis and cholangitis, C. difficile, PID.
Deep and device-relatedEndocarditis, septic arthritis, prosthetic joint infection, osteomyelitis, discitis, spinal epidural or psoas abscess, line infection, intra-abdominal collection. Suspect where the fever has no surface source.
ImportedMalaria (most important), dengue, typhoid, rickettsial infection, leptospirosis, amoebic liver abscess, hepatitis A/E, schistosomiasis, viral haemorrhagic fever. Several are notifiable; discuss with the infection service.
Non-infective inflammatory and malignantGiant cell arteritis and PMR, adult-onset Still's disease (very high ferritin), SLE and vasculitis, IBD flare, sarcoidosis, gout; lymphoma and leukaemia, renal cell and hepatocellular carcinoma; pulmonary embolism causes low-grade fever and is missed once an infective label is applied.
Do not label by default"Viral", "UTI" on a dipstick alone, and "probably chest" without signs all need positive evidence. Drug fever appears 7–10 days after a new drug and settles within 72 hours of stopping it. Hyperthermia is not fever: heatstroke, neuroleptic malignant syndrome, serotonin syndrome and thyroid storm need active cooling and treatment of the cause — antipyretics do not work.
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Refer routinely from this consultation Not same-day, not 999 — the cases that leave by letter
PUO — fever >3 weeks with no diagnosis after first-line investigation: refer to general medicine or infectious diseases; the differential is roughly one-third infection, one-third malignancy or inflammatory disease, one-third undiagnosed. Routine rheumatology: suspected adult-onset Still's disease, vasculitis or connective tissue disease once GCA has been excluded or already treated. Routine infectious diseases: a returning traveller with a resolved or low-grade undiagnosed fever, or suspected chronic imported infection. Interim advice: keep a temperature and symptom diary, avoid further empirical antibiotics, and give the written safety-net below. Not routine: unexplained fever with night sweats and weight loss (very urgent FBC within 48 hours, NG12), suspected TB (TB service plus UKHSA notification), and every page-1 red flag.
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The only treatment decisions that belong in this consultation Everything else is in the management pathway
Suspected meningococcal disease (non-blanching rash)Benzylpenicillin 1.2 g IM/IV (adult), or ceftriaxone, as soon as possible before transfer — unless it would delay the ambulance or there is anaphylaxis to either. Call 999 first or simultaneously. Record drug, dose, route and time. Notify UKHSA on suspicion; contact chemoprophylaxis is arranged by UKHSA.
Suspected giant cell arteritisPrednisolone 40–60 mg daily (minimum 0.75 mg/kg), not enteric-coated, started immediately; 60 mg with any visual symptom plus same-day ophthalmology. Do not wait for ESR, CRP, ultrasound or biopsy. Add gastroprotection and bone protection per local pathway; note diabetes.
Sepsis, neutropenic sepsis, malaria, endocarditis, septic arthritis, spinal infectionTransfer is the treatment. Oxygen to target, IV access if it does not delay transfer. Do not give an oral antibiotic and review — in neutropenic sepsis it delays the 1-hour IV target, and in endocarditis and septic arthritis it destroys the cultures or the aspirate. No CT, MRI or malaria film requested from primary care.
Adrenal insufficiency or long-term steroidsDouble the oral replacement dose during febrile illness. If vomiting or unable to absorb: hydrocortisone 100 mg IM and admit. Give the sick-day plan in writing and check the emergency injection kit and steroid card.
Identified bacterial source, physiologically stableTargeted antibiotic per local formulary and NICE antimicrobial guidance, shortest effective course — amoxicillin or doxycycline for CAP guided by CRB-65; nitrofurantoin or trimethoprim for lower UTI; flucloxacillin for cellulitis; phenoxymethylpenicillin for streptococcal tonsillitis. Take cultures first where feasible; review results at 48–72 hours and de-escalate or stop.
Self-limiting viral illnessNo antibiotic is the correct prescription. Paracetamol 1 g up to four times daily for comfort only — it does not change the course and the response carries no diagnostic information. Do not alternate antipyretics routinely; avoid ibuprofen in dehydration, AKI risk, varicella or possible streptococcal soft-tissue infection. Fluids, expected timeline, written safety-net, and review sick-day rules for diabetes, steroids, ACE inhibitors, diuretics and SGLT2 inhibitors.
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Endpoint and documentation Finish with one of four conclusions and a named owner

1 · Time-critical

999 or same-hour route taken; suspected diagnosis, full observation set with respiratory rate and NEWS2, GCS, immune and travel status, devices, any drug given with dose, route and time, and the notification made.

2 · Serious cause possible

Same-day route or targeted test arranged; state the question being excluded, who was spoken to, and the deadline by which the patient must be seen. Record that cultures precede antibiotics.

3 · Self-limiting illness established

Observations, the systems examined and found normal, the expected duration given, sick-day rules where relevant, written safety-net, and the review owner and date.

4 · Unclassified

No source found: uncertainty stated, investigations arranged and chased by name, the review point agreed, and explicit escalation criteria. Not recorded as "viral".
Why a benign conclusion is safe today, in one line: normal physiology and NEWS2 including respiratory rate · a systematic sweep documented as negative, with the whole skin and spine examined · no high-risk group (neutropenia, asplenia, immunosuppression, travel, device, pregnancy) · <5 days and improving · written safety-net given and a review agreed. Safety-net wording: call 999 for a rash that does not fade when pressed, drowsiness or difficulty waking, a severe headache with a stiff neck, breathlessness, cold mottled hands and feet, a fit, or passing no urine; contact us the same day if you feel much worse, the fever lasts beyond 5 days, you cannot keep fluids down, or you develop new pain anywhere — including your back.
OOH / remote limitation. Convert to face-to-face or emergency assessment if a full observation set including respiratory rate cannot be obtained; the skin cannot be examined for a non-blanching rash; the patient is neutropenic, asplenic, immunosuppressed, pregnant, has a device or has travelled; mental state is altered or changing during the call; the caller is not the patient; or transport, support at home or timely review cannot be guaranteed. Under time pressure the default is the safer destination.
Clinical decision support only; follow local emergency-transfer, emergency-medicines and referral policies and check doses against the current BNF. Source hierarchy: NICE guidelines first, then BNF/MHRA for doses and safety, then national specialty guidance. Sources: NICE NG51 Sepsis: recognition, diagnosis and early management · CG151 Neutropenic sepsis · NG240 Meningitis (bacterial) and meningococcal disease · NG12 Suspected cancer: recognition and referral · NG33 Tuberculosis · NG143 Fever in under 5s (paediatric comparison) · NICE guidance — Feverish illness / Giant cell arteritis · BNF · Royal College of Physicians NEWS2 · UKHSA malaria and imported fever guidance and notifiable diseases list. Review after any guidance or medicines-safety update, otherwise every 6 months. © Reasoning GP · gpreasoning.uk