What are the signs of infection?

Signs of Infection

Infection is diagnosed clinically by the presence of at least two of the following local inflammatory signs: erythema (redness), warmth, swelling (induration), tenderness or pain, and purulent discharge. 1

Local Signs of Inflammation

The classic cardinal signs of infection, first described by Cornelius Celsus in the first century A.D., remain the foundation for clinical diagnosis 2, 3:

• Rubor (redness/erythema): Erythema extending >0.5 cm from the wound margin indicates infection 1
• Calor (warmth): Local increased warmth results from vasodilation at the infection site 1, 3
• Tumor (swelling): Local swelling or induration from extravasation of plasma and inflammatory cells 1, 3
• Dolor (pain/tenderness): Local tenderness or pain, particularly pain disproportionate to physical findings which suggests deeper infection 1
• Functio laesa (loss of function): Impaired function of the affected area 3
• Purulent discharge: Presence of pus is a definitive sign of infection 1

Systemic Signs of Infection

When infection progresses beyond local tissues, systemic manifestations develop 1:

Systemic Inflammatory Response Syndrome (SIRS) Criteria

At least two of the following indicate severe infection 1:

• Temperature: >38°C (fever) or <36°C (hypothermia) 1
• Heart rate: >90 beats/min (tachycardia) 1
• Respiratory rate: >20 breaths/min or PaCO₂ <4.3 kPa (32 mmHg) 1
• White blood cell count: >12,000/mm³ or <4,000/mm³, or >10% immature (band) forms 1
• Hypotension: Systolic blood pressure <90 mm Hg or >20 mm Hg below baseline 1

Additional Systemic Symptoms

• Fever and chills: Early systemic symptoms, particularly with erysipelas 1, 4
• Night sweats: Common with chronic infections 1
• Weight loss: Indicates prolonged infection 1
• Extreme fatigue: Often the most striking systemic symptom, interfering with normal activities 1
• Altered mental status: A warning sign of severe sepsis 1, 5
• Dyspnea: Predicts severe sepsis (OR = 2.92) 5
• Gastrointestinal symptoms: Including nausea, vomiting, or diarrhea predict severe sepsis (OR = 2.31) 5
• Muscle weakness: Associated with severe infection (OR = 2.24) 5
• Malaise and lethargy: Non-specific symptoms of systemic inflammation 6
• Loss of appetite: Part of "sickness behavior" from systemic inflammatory mediators 6

Secondary Features Suggestive of Infection

When classic signs are equivocal, these findings support the diagnosis 1:

• Necrotic tissue: Presence of tissue death 1
• Friable or discolored granulation tissue: Abnormal wound healing 1
• Non-purulent secretions: Serous or serosanguinous drainage 1
• Foul odor: Particularly with anaerobic infections 1
• Failure of properly treated wound to heal: Suggests underlying infection 1

Signs of Severe Deep Soft-Tissue Infection

These findings indicate potentially life-threatening necrotizing infections requiring emergent surgical evaluation 1:

• Pain disproportionate to physical findings: Early warning sign 1
• Violaceous bullae: Hemorrhagic blisters 1
• Cutaneous hemorrhage: Bleeding into skin 1
• Skin sloughing: Tissue separation 1
• Skin anesthesia: Loss of sensation from nerve destruction 1
• Rapid progression: Worsening over hours 1
• Gas in tissue: Crepitus on palpation or visible on imaging 1

Laboratory Markers

While infection is diagnosed clinically, laboratory tests support the diagnosis and assess severity 1, 7:

• Elevated C-reactive protein (CRP): Particularly >13 mg/L suggests infection 1, 7
• Elevated erythrocyte sedimentation rate (ESR): Markedly elevated ESR suggests osteomyelitis 7
• Elevated creatine phosphokinase: 2-3 times upper limit of normal indicates severe infection 1
• Low serum bicarbonate: Suggests metabolic acidosis from sepsis 1
• Marked left shift: Increased immature white blood cells 1

Critical Pitfalls to Avoid

• Do not assume all neck masses in adults are infectious: Most are neoplastic, not infectious 1
• Do not rely solely on laboratory values: Systemic inflammatory markers are absent in up to one-half of patients even with severe infection 7
• Do not assume cystic masses are benign: Continue evaluation until diagnosis is obtained in patients at risk for malignancy 1
• Do not delay surgical consultation: When signs of deep infection or necrotizing fasciitis are present, emergent surgical evaluation is paramount 1, 7
• Recognize that signs may appear late: In necrotizing infections, classic signs often appear later in the course 1