What labs and imaging studies should be ordered for a patient presenting with sepsis?

Labs and Imaging for Sepsis

For patients presenting with sepsis, immediately obtain blood cultures (at least two sets), serum lactate, complete blood count, comprehensive metabolic panel, and coagulation studies within the first hour, followed by contrast-enhanced CT imaging of the chest and/or abdomen based on clinical localization of the suspected source. 1, 2

Essential Initial Laboratory Tests

Blood Cultures - First Priority

• Draw at least two sets of blood cultures before starting antibiotics, with one drawn percutaneously and one through each vascular access device 1, 2, 3
• Complete blood culture collection within 45 minutes to avoid delaying antibiotic administration beyond the critical 1-hour window 2, 3
• Obtain additional cultures from other potential infection sources based on clinical presentation (urine, sputum, wound, cerebrospinal fluid) 2, 3
• Never delay antibiotics beyond 1 hour if obtaining cultures is not immediately feasible 2

Serum Lactate - Critical for Risk Stratification

• Measure serum lactate immediately in all suspected sepsis cases 1, 2, 3
• Lactate >2 mmol/L indicates more severe disease requiring aggressive resuscitation 1, 2, 3
• Lactate ≥4 mmol/L defines sepsis-induced tissue hypoperfusion and triggers immediate fluid resuscitation with 30 mL/kg crystalloid 2
• Repeat lactate measurements every 2-4 hours until normalized to guide resuscitation and assess treatment response 1, 2, 3

Complete Blood Count

• Assess for leukocytosis, leukopenia, or bandemia 1, 2, 3
• Evaluate platelet count for sepsis-induced coagulopathy 2
• Consider neutrophil-to-lymphocyte ratio as a diagnostic and prognostic marker 2
• Important caveat: White blood cell count alone lacks sensitivity for sepsis diagnosis and should not be relied upon as a sole diagnostic criterion 2

Comprehensive Metabolic Panel

• Evaluate renal function (creatinine, BUN) to assess organ dysfunction 1, 2, 3
• Assess hepatic function (bilirubin, transaminases) for organ failure progression 2
• Measure electrolytes to identify abnormalities requiring correction 2
• Check glucose levels, as hyperglycemia is common in sepsis 2

Coagulation Studies

• Obtain PT/INR and PTT to assess for coagulopathy and sepsis-induced coagulopathy 1, 2, 3
• Coagulopathy indicates more severe disease; patients with severe sepsis and DIC have 20% lower survival 2
• Monitor coagulation parameters daily for development or progression of sepsis-induced coagulopathy 2

Adjunctive Biomarkers

Procalcitonin (PCT)

• PCT rises within 4 hours of bacterial exposure and peaks at 6-8 hours 2
• PCT ≥1.5 ng/mL shows 100% sensitivity and 72% specificity for identifying sepsis in ICU populations 2
• PCT has higher diagnostic accuracy than CRP (area under ROC curve of 0.85 vs. 0.73) 2
• Use low PCT levels to assist in discontinuing empiric antibiotics in patients who initially appeared septic but have no subsequent evidence of infection 2, 3
• Do not delay antibiotics in high-probability cases while waiting for PCT results 2

C-Reactive Protein (CRP)

• CRP rises 12-24 hours after inflammatory insult, reaching maximum at 48 hours 2
• CRP ≥50 mg/L demonstrates 98.5% sensitivity and 75% specificity for probable sepsis 2, 3
• CRP is less specific than PCT for bacterial infections but remains widely available and cost-effective 2

Special Considerations

• Use 1,3-β-D-glucan assay when invasive candidiasis is in the differential diagnosis 2

Imaging Studies

Chest Imaging

For patients with cough, dyspnea, or chest pain:

• CT chest with IV contrast is the preferred initial imaging modality, with a positive predictive value of 81.82% for identifying septic foci 4
• CT chest identifies pneumonia in 38.6% of septic patients and leads to management changes in 45% of cases 4
• CT chest has particularly high diagnostic yield (76.5%) in extremely ill patients requiring ICU admission 4
• Chest radiography has only 58% sensitivity for pneumonia diagnosis but is useful as an initial screening tool when CT is not immediately available 4
• CT chest without IV contrast has similar diagnostic yield to contrast-enhanced CT for pulmonary disease, but contrast is preferred for comprehensive evaluation 4

Abdominal/Pelvic Imaging

For suspected intraabdominal sepsis:

• CT abdomen and pelvis with IV contrast is the imaging modality of choice 4
• Abdominal/pelvic ultrasound is often chosen as initial imaging in patients of childbearing age 4
• Abdominal radiography rarely provides definitive diagnosis and should not delay CT imaging 4

Advanced Imaging - Limited Role

• FDG-PET/CT should only be considered after source localization with CT has failed 4
• FDG-PET/CT identified infection source in 70% of patients with culture-proven bloodstream infection after negative conventional workup, with 90.9% sensitivity and 87.5% specificity 4
• MRI has no role as initial diagnostic imaging in sepsis due to length of procedure and patient instability 4

Practical Algorithm for Laboratory and Imaging Workup

Within First Hour:

1. Draw at least two sets of blood cultures 1, 2, 3
2. Measure serum lactate immediately 1, 2, 3
3. Obtain CBC with differential 1, 2, 3
4. Order comprehensive metabolic panel 1, 2, 3
5. Perform coagulation studies 1, 2, 3
6. Consider PCT or CRP for low-to-intermediate probability cases 2
7. Order CT imaging with IV contrast based on suspected source (chest for respiratory symptoms, abdomen/pelvis for abdominal symptoms) 4

Ongoing Monitoring (Every 2-4 Hours):

• Repeat lactate until normalized 1, 2, 3
• Reassess CBC and metabolic panel daily 2
• Repeat PCT/CRP at 24-48 hours to assess treatment response and guide antibiotic de-escalation 2
• Obtain follow-up blood cultures to document clearance of bacteremia 2, 3
• Monitor coagulation parameters for development of DIC 2

Critical Pitfalls to Avoid

• Never delay antibiotic administration beyond 1 hour while waiting for laboratory results or imaging 1, 2
• Do not rely solely on white blood cell count for sepsis diagnosis 2
• Negative cultures do not exclude sepsis, especially in patients with prior antibiotic exposure 2, 3
• Interpret biomarkers within clinical context; no single laboratory test accurately diagnoses sepsis 2, 5
• Do not order CT without IV contrast when contrast-enhanced CT is feasible, as it provides superior diagnostic information 4