Approach to Fever and Sepsis

Infectious causes by mechanism (majority of febrile presentations)

• Gram-negative bacilli: E. coli, Klebsiella, Pseudomonas — LPS is the archetypal TLR4 ligand; urinary, biliary, and intra-abdominal sources predominate
• Gram-positive cocci: S. aureus (including MRSA), streptococci, enterococci — lipoteichoic acid and superantigens (TSST-1) drive polyclonal T-cell activation and toxic shock physiology
• Encapsulated organisms: S. pneumoniae, N. meningitidis, H. influenzae type b — opsonization-dependent clearance, hence fulminant disease in asplenia
• Fungal and viral: Candida (central lines, prolonged broad-spectrum antibiotics, TPN), influenza/SARS-CoV-2; endemic mycoses and TB in the subacute febrile patient

Non-infectious fever — the differential examiners hide in the stem

• Malignancy: lymphoma, renal cell carcinoma, leukemia — tumor-derived IL-1/IL-6 and pyrogenic cytokines
• Connective tissue disease: adult-onset Still disease, giant cell arteritis, vasculitis
• Drug fever: beta-lactams, sulfonamides, anticonvulsants, allopurinol; typically well-appearing patient, eosinophilia, temporal link to drug start
• Thromboembolism, hematoma resorption, thyroid storm, adrenal crisis, neuroleptic malignant syndrome/serotonin syndrome (the latter are hyperthermia, not true set-point fever)
• Fever of unknown origin is classically stratified into classic, nosocomial, neutropenic, and HIV-associated categories, each with a distinct pathogen and non-infectious spectrum

Modifiable risk factors

• Indwelling hardware: central venous catheters, urinary catheters, prosthetic joints/valves — biofilm shields organisms from opsonins and antibiotics
• Iatrogenic immunosuppression: corticosteroids, TNF-α inhibitors (reactivation TB), chemotherapy-induced neutropenia, recent surgery
• Poorly controlled diabetes, alcohol use disorder, malnutrition, injection drug use, unvaccinated status (pneumococcal and meningococcal vaccination per CDC/ACIP is protective and modifiable)

Non-modifiable risk factors

• Age extremes: neonates lack adaptive memory; the elderly show blunted febrile response and may present hypothermic
• Asplenia/functional asplenia (sickle cell disease) and terminal complement deficiency (C5–C9) — the latter is the classic recurrent Neisseria association
• Cirrhosis, end-stage renal disease, hematologic malignancy, HIV with low CD4 count, pregnancy (Listeria, pyelonephritis)

• Pyrogenic cytokine cascade: Bacterial lipopolysaccharide (LPS) and other pathogen-associated molecular patterns (PAMPs) activate toll-like receptors (TLRs) on macrophages and dendritic cells, triggering release of IL-1, IL-6, and TNF-α, which act on the hypothalamic preoptic area to elevate the temperature set-point
• Endothelial dysfunction and capillary leak: Pro-inflammatory cytokines damage the glycocalyx and tight junctions, increasing vascular permeability and leading to interstitial edema, hypovolemia, and distributive shock; nitric oxide-mediated vasodilation causes inappropriate peripheral vasodilation
• Coagulation and fibrinolysis dysregulation: Tissue factor upregulation activates extrinsic coagulation pathway; simultaneous impairment of fibrinolysis (elevated PAI-1) and protein C depletion lead to disseminated intravascular coagulation (DIC), microvascular thrombosis, and tissue hypoperfusion
• Mitochondrial dysfunction and cellular hypoxia: Sepsis causes mitochondrial uncoupling, impaired ATP production, and inability to extract oxygen at the cellular level (cytopathic hypoxia), even in the setting of adequate systemic oxygen delivery
• Immune paralysis: Prolonged inflammatory response followed by immunosuppression with T-cell exhaustion (elevated PD-1/PD-L1 expression), regulatory T-cell expansion, and reduced HLA-DR expression on monocytes—explaining late-phase mortality and secondary infections

• Core fever characteristics: Temperature typically rises in a stepwise fashion over hours to days; fever curves may be continuous (sustained), remittent (fluctuating but not returning to baseline), intermittent (spiking with return to normal), or hectic (wild swings); defervescence occurs either by crisis (rapid drop with diaphoresis) or lysis (gradual decline)
• Systemic inflammatory response: Tachycardia (>90 bpm), tachypnea (>20/min or RR >30 in severe cases), altered mental status ranging from subtle confusion to delirium, and generalized malaise; hypotension and shock develop as sepsis progresses
• Sepsis-specific findings: Warm extremities and bounding pulses in early distributive shock (warm shock); cool extremities, mottled skin, and weak pulses in late/cardiogenic shock (cold shock); petechial or purpuric rash (suggests meningococcemia or gonococcemia); oliguria and acute kidney injury
• Organ-specific signs: Productive cough with infiltrate suggests pneumonia; costovertebral angle tenderness suggests pyelonephritis; focal neurologic deficits suggest meningitis; acute abdomen may indicate perforated viscus or necrotizing fasciitis
• Clinical pearl: The presence of fever + hypotension + altered mental status = septic triad requiring immediate intervention; however, elderly patients, immunocompromised hosts, and those on antipyretics may present without fever despite life-threatening infection

• qSOFA (quick SOFA) criteria for bedside assessment: Altered mentation, systolic BP ≤100 mmHg, or respiratory rate ≥22/min; score ≥2 suggests increased mortality risk and warrants ICU consideration; however, qSOFA is not sensitive enough to exclude sepsis
• Sepsis-3 diagnostic criteria (2016): Suspected or confirmed infection PLUS sequential organ failure assessment (SOFA) score ≥2 (derived from respiratory, cardiovascular, hepatic, renal, neurologic, and coagulation components); alternatively, lactate >2 mmol/L indicates tissue hypoperfusion and portends worse prognosis
• Blood cultures and microbiologic sampling: Obtain 2-3 sets of blood cultures (aerobic and anaerobic bottles) before antibiotics if possible, but do NOT delay antibiotics for culture acquisition; culture sites based on clinical suspicion (sputum, urine, cerebrospinal fluid, wound drainage, intra-abdominal fluid)
• Laboratory markers: Elevated procalcitonin (>0.5 ng/mL) and C-reactive protein support infection diagnosis; complete blood count may show leukocytosis, left shift, or leukopenia (poor prognostic sign); lactate level is single best predictor of mortality; coagulation studies may reveal prolonged PT/INR, elevated fibrin degradation products, and thrombocytopenia (DIC); liver and kidney function tests assess organ dysfunction
• Imaging: Chest X-ray, abdominal imaging (ultrasound or CT), lumbar puncture (if meningitis suspected), echocardiography (if endocarditis suspected); source control often requires imaging or procedural intervention
• Diagnostic pearl: Lactate is the single most important prognostic marker in sepsis; persistent lactate >4 mmol/L or clearance <10% over 6 hours predicts ICU mortality >40%; serum procalcitonin useful for de-escalation of antibiotics after clinical improvement

Immediate Management (First Hour—"Sepsis Bundle")

• Empiric broad-spectrum antibiotics within 1 hour (ideally within 60 minutes): Third or fourth-generation cephalosporin (ceftriaxone 1-2g IV q12h) OR piperacillin-tazobactam (4.5g IV q6-8h) OR carbapenems (meropenem 1g IV q8h) PLUS vancomycin (15-20 mg/kg IV q8-12h with AUC-guided monitoring, AUC/MIC 400-600 per the 2020 IDSA/ASHP consensus) if risk for resistant gram-positives; add fluoroquinolone or azithromycin for atypical coverage if respiratory source; add metronidazole or clindamycin for anaerobic coverage (intra-abdominal, gynecologic sources)
• Fluid resuscitation: 30 mL/kg crystalloid bolus (normal saline or balanced crystalloid such as Lactated Ringer's) for hypotension or lactate ≥4; titrate to mean arterial pressure ≥65 mmHg and urine output 0.5 mL/kg/hr; reassess volume status frequently as fluid overload increases mortality
• Vasopressor support: Initiate norepinephrine (0.05-0.2 mcg/kg/min titrated to MAP ≥65) as first-line vasopressor if hypotension persists after fluid resuscitation; add vasopressin (0.04 units/min fixed) or epinephrine if norepinephrine inadequate; use low-dose hydrocortisone (50 mg IV q6h × 7 days) only if refractory shock despite adequate fluids and high-dose vasopressors

Ongoing Management

• Source control (absolutely critical for survival): Drainage of abscess, debridement of necrotizing tissue, removal of infected devices, or source removal; often requires surgical or interventional radiology consultation within first few hours
• Reassessment and de-escalation: Repeat lactate at 6 hours and q6h thereafter; de-escalate antibiotics at 48-72 hours based on culture results and clinical improvement; continue appropriate monotherapy or dual therapy based on organism susceptibilities; typical course is 7-10 days for most infections (longer for bacteremia, endocarditis, or immunocompromised hosts)
• Supportive care: Maintain euglycemia (glucose 150-180 mg/dL in ICU), stress ulcer prophylaxis, DVT prophylaxis with mechanical measures ± pharmacologic prophylaxis, judicious use of sedation/analgesia, renal replacement therapy if acute kidney injury develops

Special Populations

• **Immun

Complications of sepsis itself

• Septic shock (emergency): vasoplegia plus myocardial depression; defined by vasopressor requirement to maintain MAP ≥65 mmHg plus lactate >2 mmol/L despite adequate fluids — mortality substantially exceeds sepsis without shock
• ARDS (emergency): cytokine-mediated alveolar-capillary leak; bilateral infiltrates with hypoxemia not explained by heart failure; manage with low tidal volume ventilation (ARDSNet strategy endorsed by ATS/ESICM/SCCM)
• Acute kidney injury: hypoperfusion plus inflammatory tubular injury → muddy brown granular casts of ATN, rising creatinine, oliguria; KDIGO staging guides renal replacement timing
• DIC (emergency): tissue factor–driven consumptive coagulopathy; thrombocytopenia, prolonged PT/aPTT, low fibrinogen, elevated D-dimer, with simultaneous bleeding and microthrombi; purpura fulminans signals Waterhouse–Friderichsen syndrome in meningococcemia
• Septic cardiomyopathy: cytokine-mediated reversible biventricular dysfunction; new global hypokinesis on echocardiography with troponin elevation absent coronary occlusion
• Ischemic hepatitis and cholestasis: transaminases in the thousands (shock liver) or a disproportionate conjugated hyperbilirubinemia
• Critical illness polyneuropathy/myopathy and post-sepsis immune paralysis: diffuse weakness with failure to wean; late secondary and opportunistic infections
• Necrotizing soft tissue infection (emergency): pain out of proportion, crepitus, rapid progression — surgical debridement, not imaging, is the priority

Complications of treatment

• Fluid overload: aggressive crystalloid beyond resuscitation targets causes pulmonary and interstitial edema and worsens mortality; Surviving Sepsis Campaign favors dynamic reassessment over fixed repeated boluses
• Vancomycin-associated AKI: risk rises when combined with piperacillin-tazobactam; monitor with AUC-guided dosing (AUC/MIC 400–600, 2020 IDSA/ASHP consensus). Vancomycin infusion reaction ("red man") is histamine-mediated, rate-related, and not an IgE allergy
• Clostridioides difficile colitis: broad-spectrum antibiotics ablate colonization resistance; new watery diarrhea plus leukocytosis — treat per IDSA/SHEA with oral vancomycin or fidaxomicin, and consider toxic megacolon an emergency
• Vasopressor digital/mesenteric ischemia and extravasation necrosis; hyperglycemia and hypernatremia from hydrocortisone; hyperchloremic metabolic acidosis from large-volume normal saline

• Antibiotics beat cultures: draw two sets of blood cultures from separate sites, but the Surviving Sepsis Campaign is explicit that antimicrobials should not be delayed — in shock, give them within one hour. If the stem asks the single best next step in a hypotensive febrile patient, it is fluids plus empiric antibiotics, not a CT scan
• Suspected bacterial meningitis with focal deficits, papilledema, or altered mentation: blood cultures → dexamethasone plus empiric antibiotics → CT head → LP. Never let imaging delay therapy; dexamethasone must be given before or with the first dose to blunt the inflammatory response to pneumococcal lysis
• Febrile neutropenia (ANC <500, single temp ≥38.3°C) is an emergency: IDSA recommends immediate empiric antipseudomonal beta-lactam monotherapy — cefepime, meropenem, or piperacillin-tazobactam. Vancomycin is not added routinely; reserve it for catheter infection, skin/soft tissue source, hemodynamic instability, or known MRSA colonization
• qSOFA is a mortality-risk flag, not a screening test: the Surviving Sepsis Campaign recommends against using qSOFA alone to screen for sepsis because sensitivity is poor. Absence of qSOFA points never excludes sepsis
• Lactate is the prognostic hinge: elevated or non-clearing lactate reflects tissue hypoperfusion and cytopathic hypoxia, and drives repeat measurement and escalation
• Norepinephrine is first-line vasopressor, not dopamine — dopamine causes more arrhythmias. Steroids are for shock refractory to fluids and vasopressors, not for every septic patient
• Classic associations: petechial/purpuric rash + hypotension → meningococcemia with adrenal hemorrhage; diffuse erythroderma with desquamation and tampon or packed nasal cavity → staphylococcal toxic shock (superantigen); asplenia → overwhelming encapsulated organism sepsis; recurrent Neisseria → terminal complement deficiency
• Common distractors: a well-appearing patient with fever, eosinophilia, and a new drug → drug fever, not treatment failure; hypothermia and leukopenia in the elderly are worse prognostically than fever and leukocytosis; and a reported penicillin allergy rarely precludes cephalosporins — cross-reactivity is roughly 1–3% and driven by shared R1 side chains