For a patient with a low‑grade fever (<38 °C), what initial work‑up is recommended and why are blood cultures performed only when the temperature reaches ≥38 °C or there are signs of systemic infection?

Workup for Low-Grade Fevers and the 38°C Blood Culture Threshold

Blood cultures are reserved for temperatures ≥38°C (100.4°F) because this threshold represents the point where fever becomes highly specific (90%) for serious bacterial infection, whereas lower temperatures lack sufficient specificity to justify the cost, false-positive risk, and resource utilization of blood cultures. 1, 2

Why the 38°C Threshold Exists

The 38°C cutoff is not arbitrary—it represents an evidence-based balance between sensitivity and specificity for detecting bacteremia:

• A temperature ≥38.3°C triggers clinical assessment but not necessarily laboratory or radiologic evaluation, as recommended by the Society of Critical Care Medicine and Infectious Diseases Society of America 1
• Temperatures below 38°C have poor positive predictive value for bacteremia, meaning most blood cultures drawn at these temperatures will be negative or represent contaminants rather than true pathogens 1, 2
• The specificity of fever for serious infection increases dramatically at ≥38°C, particularly in vulnerable populations like the elderly where it reaches 90% specificity 2

Initial Workup for Low-Grade Fever (<38°C)

For patients with temperatures below 38°C, the workup should be clinically directed rather than protocol-driven:

Clinical Assessment Priority

• Focus on localizing signs and symptoms rather than reflexive laboratory testing 1, 2
• Look for subtle manifestations including functional status changes, mental status alterations (especially in elderly), respiratory rate changes, and examination findings 2
• Assess for the three most common bacterial sources: respiratory tract (cough, tachypnea, rales, hypoxia), urinary tract (dysuria, frequency, CVA tenderness), and skin/soft tissue infections (cellulitis, wounds, pressure ulcers) 2

When to Escalate Testing Despite Low-Grade Fever

Blood cultures and aggressive workup are indicated regardless of temperature if any of the following are present:

• Hemodynamic instability or signs of sepsis (hypotension, altered mental status, organ dysfunction) 2
• Neutropenia or severe immunosuppression (including CAR T-cell therapy patients where even low-grade fever mandates blood cultures) 1
• Identified infectious source on examination requiring source control 2
• Clinical deterioration during observation 2

Appropriate Initial Studies for Low-Grade Fever

• Chest radiograph if any respiratory symptoms or signs are present (cough, tachypnea >30 breaths/min, rales, hypoxia) 1
• Urinalysis with culture if urinary symptoms present, but avoid treating asymptomatic bacteriuria even with delirium or falls (causes harm without benefit per IDSA) 2
• Directed imaging based on examination findings (e.g., abdominal ultrasound for RUQ tenderness, not routine screening) 1

Common Pitfalls to Avoid

• Do not obtain blood cultures reflexively for temperatures <38°C in stable patients without localizing signs—this increases false positives, contaminant rates, and unnecessary antibiotic exposure 1, 2
• Do not dismiss low-grade fever in immunocompromised patients—the threshold for aggressive workup including blood cultures should be lower (often any fever ≥38°C warrants cultures and empiric antibiotics) 1
• Do not overlook non-temperature indicators of serious infection, particularly in elderly patients who may present with isolated functional decline, confusion, or hypothermia (<36°C) rather than fever 1, 2
• Avoid empiric antibiotics for undifferentiated low-grade fever in stable, immunocompetent patients—this has not been shown effective and promotes resistance 3

Temperature Measurement Considerations

• Use oral or rectal temperatures over less reliable methods (axillary, tympanic, temporal artery) for clinical decision-making 1
• Central temperature monitoring (bladder catheter thermistor, esophageal probe, pulmonary artery catheter) is most accurate when these devices are already in place for critically ill patients 1
• Document the measurement site with every temperature recording 1

Special Population: Pediatrics

In children younger than 3 years, the approach differs significantly:

• Fever is defined as ≥38°C (100.4°F) and age-specific protocols guide workup intensity 1
• Infants <3 months with fever ≥38°C require aggressive evaluation including blood cultures, urinalysis/culture, and consideration of lumbar puncture due to immature immune systems 1
• Response to antipyretics does not predict absence of serious bacterial infection—this should never guide decision-making 1