Diagnostic Tests for Sepsis
Blood cultures should be obtained immediately as the primary diagnostic test for sepsis, followed by complete blood count, lactate measurement, and organ function tests including renal and liver panels. 1
Core Diagnostic Tests for Sepsis
First-Line Tests (Obtain Immediately)
- Blood cultures: At least two sets (aerobic and anaerobic bottles), with one drawn percutaneously and one through each vascular access device 1
- Complete blood count (CBC): Look for leukocytosis (>12,000/μL), leukopenia (<4,000/μL), or >10% immature forms (bands) 1, 2
- Lactate measurement: Critical for risk stratification; levels ≥4 mmol/L indicate severe sepsis 1
- Basic metabolic panel: To assess organ dysfunction and electrolyte abnormalities 1
- Procalcitonin (PCT): Levels >0.5 ng/mL suggest bacterial infection; >2.0 ng/mL highly indicative of sepsis 3
Additional Tests Based on Clinical Presentation
- Urinalysis and urine culture: Especially if urinary symptoms or no obvious source 1
- Cultures from suspected sites of infection: Including respiratory secretions, wounds, cerebrospinal fluid as clinically indicated 1
- Liver function tests: To assess for hepatic dysfunction (bilirubin >4 mg/dL indicates organ dysfunction) 1
- Coagulation studies: INR >1.5 or PTT >60 seconds indicates coagulopathy 1
- C-reactive protein (CRP): Less specific than PCT but useful when combined with other tests; levels >50 mg/L have 98.5% sensitivity for sepsis 1, 3
Clinical Signs to Evaluate for Sepsis
Vital Sign Abnormalities
- Temperature >38.3°C (fever) or <36°C (hypothermia)
- Heart rate >90 beats/minute
- Respiratory rate >20 breaths/minute
- Systolic blood pressure <90 mmHg or MAP <70 mmHg 1
Organ Dysfunction Indicators
- Respiratory: SpO₂ <90% or PaO₂/FiO₂ <300 1
- Renal: Oliguria (<0.5 mL/kg/hr) or creatinine increase >0.5 mg/dL 1
- Hepatic: Jaundice or hyperbilirubinemia >4 mg/dL 1
- Hematologic: Thrombocytopenia (<100,000/μL) 1
- Neurologic: Altered mental status 1
- Tissue perfusion: Decreased capillary refill or skin mottling 1
Imaging Studies When Source Unclear
If the source of infection isn't immediately apparent after initial laboratory evaluation:
- CT chest with IV contrast: High diagnostic yield (76.5%) for detecting pulmonary sources of sepsis 1
- CT abdomen/pelvis with IV contrast: Identifies abdominal (22%) and pelvic/genitourinary (20.5%) sources of infection 1
- Ultrasound: First-line for suspected urosepsis or gynecological sources 1
Diagnostic Algorithm
- Recognize sepsis triggers: Fever/hypothermia, tachycardia, tachypnea, hypotension, altered mental status
- Obtain immediate labs: Blood cultures, CBC, lactate, basic metabolic panel, procalcitonin
- Assess for organ dysfunction: Using laboratory values and clinical examination
- Identify source of infection:
- If respiratory symptoms: Chest imaging
- If abdominal symptoms: Abdominal/pelvic imaging
- If urinary symptoms: Urinalysis, urine culture, renal ultrasound
- If no clear source: Consider CT chest/abdomen/pelvis
- Monitor response: Serial lactate measurements and vital signs
Important Considerations and Pitfalls
- Don't delay antibiotics for cultures: Obtain cultures first but administer antibiotics within 1 hour of sepsis recognition 1
- Serial measurements are superior: Trending of biomarkers (especially lactate and procalcitonin) provides more value than single measurements 3
- Beware of sepsis mimics: Conditions like anaphylaxis, pulmonary embolism, adrenal crisis, and drug toxicity can present similarly to sepsis 4
- Procalcitonin limitations: Can be elevated in non-infectious conditions like trauma, surgery, and renal failure 3
- Negative cultures don't rule out sepsis: Clinical sepsis can be diagnosed and treated despite negative cultures 1
By systematically applying these diagnostic tests while simultaneously initiating resuscitation measures, you can rapidly identify sepsis, determine its source, and guide appropriate antimicrobial therapy to improve patient outcomes.