Approach to Fever
A board-focused framework for the febrile patient: distinguish fever from hyperthermia, triage the can't-miss emergencies (febrile neutropenia, meningitis, sepsis, endocarditis, drug-induced hyperthermia), then work up prolonged fever as FUO — all anchored to the reflex that fever plus neutropenia demands empiric antibiotics within one hour.
Fever vs. hyperthermia — and the board approach
Fever = core temperature ≥38.0°C (100.4°F). It reflects a hypothalamic set-point raised by pyrogens (IL-1, IL-6, TNF, PGE2). Contrast with hyperthermia: the set-point is normal but heat generation/dissipation fails (heat stroke, NMS, serotonin syndrome, malignant hyperthermia) — antipyretics don't work; rapid active cooling is key.
On Step 2, fever is rarely the whole answer — the exam rewards a systematic approach: (1) stabilize and screen for the can't-miss emergencies (febrile neutropenia, meningitis, sepsis, endocarditis, drug-induced hyperthermia); (2) localize with history/exam (travel, sick contacts, new drugs, indwelling devices, immunosuppression); then (3) target the workup. Tempo matters: acute fever with a source is treated empirically, while prolonged undiagnosed fever becomes a fever of unknown origin (FUO) evaluation.
The single most-tested reflex: fever + neutropenia = empiric antibiotics within 1 hour, before cultures or imaging return.
- Fever: core temp ≥38.0°C (100.4°F). FUO: ≥38.3°C (101°F) on several occasions, >3 weeks, undiagnosed after appropriate workup.
- Neutropenic fever: a single temp ≥38.3°C, or ≥38.0°C sustained ≥1 hr, with ANC <500/µL (or expected to fall below 500) → antibiotics within 1 hr.
- Fever + hypotension / lactate ↑ → sepsis: cultures, then broad-spectrum antibiotics + fluids within 1 hr.
- Fever + headache / neck stiffness / AMS → meningitis: blood cultures + LP; start antibiotics + dexamethasone early (dexamethasone with/just before the first dose). CT head before LP only if focal deficit, papilledema, new seizure, marked ↓consciousness, or immunocompromised.
- Fever + new murmur / IVDU → endocarditis: 3 sets of blood cultures + echocardiography.
- Fever + rigidity / AMS on antipsychotics or serotonergics → drug-induced hyperthermia (see table).
- Treat before a source is found — mortality rises with each hour of antibiotic delay; the patient often looks deceptively well (no pus, minimal signs, because they can't mount inflammation).
- Workup (do NOT delay antibiotics): CBC, blood cultures from each catheter lumen + a peripheral stick, urinalysis/culture, CXR, inspect catheter and perianal area — no DRE / suppositories / rectal temps (bacteremia risk).
- Empiric monotherapy = antipseudomonal β-lactam: cefepime, piperacillin-tazobactam, meropenem, or imipenem within 1 hour.
- Add vancomycin only for: catheter/skin-soft-tissue infection, pneumonia, hemodynamic instability, or known MRSA.
- Add antifungal (echinocandin or voriconazole) if fever persists >4–7 days despite broad antibacterials.
- Do not mask the picture with acetaminophen before cultures are drawn.
Vignette: A 54-year-old woman 7 days after induction chemotherapy for AML has a single oral temperature of 38.6°C and mild chills. BP 118/74, HR 96, and she looks well; her tunneled catheter site is clean. Labs: ANC 220/µL.
Diagnosis: Febrile neutropenia — an emergency despite the reassuring exam and absent localizing source.
Next best step: Draw blood cultures (each catheter lumen + peripheral) and start an empiric antipseudomonal β-lactam (e.g., cefepime) within 1 hour. Do not wait for culture results or imaging, and do not add vancomycin reflexively — reserve it for instability or a documented line/skin source.
Drug-induced hyperthermia syndromes
| Syndrome | Trigger | Onset | Neuromuscular clue | Treatment |
|---|---|---|---|---|
| NMS | Dopamine antagonists (antipsychotics); or stopping Parkinson meds | Days–weeks (slow) | Lead-pipe rigidity, ↓ reflexes, ↑↑ CK | Stop drug, cool, dantrolene ± bromocriptine |
| Serotonin syndrome | Serotonergics (SSRI+MAOI, tramadol, linezolid) | <24 hr (fast) | Clonus, hyperreflexia, myoclonus (legs>arms) | Stop drug, benzodiazepines, cyproheptadine |
| Malignant hyperthermia | Volatile anesthetics + succinylcholine | Minutes, intraop | Masseter/whole-body rigidity, ↑ end-tidal CO2 | Stop agent, dantrolene |
| Anticholinergic toxicity | Antihistamines (diphenhydramine), atropine, jimsonweed; TCAs | Hours | No rigidity; dry, flushed skin, mydriasis, urinary retention | Cooling/supportive; physostigmine for pure anticholinergic delirium — avoid in TCA overdose (use NaHCO₃) |
Vignette: A 28-year-old man who returned from Nigeria 10 days ago has recurrent, spiking fevers, chills, headache, and myalgias. Temp 39.4°C. Labs: thrombocytopenia, mild anemia, indirect hyperbilirubinemia (hemolysis); no rash or eschar.
Diagnosis: Malaria (worry most about Plasmodium falciparum) until proven otherwise — fever in a traveler from sub-Saharan Africa is malaria until excluded.
Next best step: Giemsa-stained thick and thin blood smears (thick = sensitive detection; thin = speciation + % parasitemia); repeat ×3 over 48 hr if initially negative, or send a rapid antigen test. Falciparum is treated urgently with artemisinin-based combination therapy, and IV artesunate for severe disease/high parasitemia.
Timed causes of post-op fever:
- Wind (POD 1–2): atelectasis / early pneumonia → incentive spirometry, ambulation
- Water (POD 3–5): UTI (Foley catheter) → urinalysis/culture
- Walking (POD 4–6): DVT / PE → duplex ultrasound, consider CT-PA
- Wound (POD 5–7): surgical-site infection → inspect the incision
- Wonder drugs / Wonky lines (any time): drug fever, C. difficile, IV-catheter infection
Fever within <24 hr / intraoperative is different — think malignant hyperthermia, transfusion reaction, a pre-existing infection, or necrotizing fasciitis (Strep/Clostridium) if pain is out of proportion and the wound looks dusky.
- Categories (shift with age/region): infection, malignancy, autoimmune/connective-tissue, miscellaneous, and undiagnosed (~20–30%).
- Infection: TB (esp. extrapulmonary), occult abscess, endocarditis, osteomyelitis — the leading cause in the developing world.
- Malignancy: lymphoma (classic), leukemia, renal cell carcinoma.
- Autoimmune: adult-onset Still's disease (young: quotidian fever, salmon-pink rash, arthritis, very high ferritin); giant cell arteritis (elderly: headache, jaw claudication, ↑ ESR — the classic elderly FUO).
- Miscellaneous: drug fever (relative bradycardia, eosinophilia, well-appearing; resolves off the drug), DVT/PE, factitious fever.
- Workup: repeat H&P guide testing; blood cultures, ESR/CRP, HIV, TB testing, and CT chest/abdomen/pelvis; PET/CT can localize occult disease.
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