Interpreting laboratory tests in infection
Reading the FBC, CRP, procalcitonin, lactate and liver tests in a febrile patient, the white cell and platelet patterns that point to particular infections, how microbiology, serology and virology answer different questions, and worked examples of results that change management.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Laboratory tests in infection answer four separate questions: is there inflammation (CRP, procalcitonin), how sick is the host (lactate, creatinine, bilirubin, platelets), what pattern fits (white cell differential, liver tests, sodium), and which organism (microbiology, serology, virology).
- No single blood test separates bacterial from viral infection. Use the trend and the clinical picture.
- The film and the differential count often give the diagnosis faster than the microbiology.
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Sources
- RCPA Manual (pathology test interpretation)AU/NZRoyal College of Pathologists of Australasia, online edition, accessed November 2026
- Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2026Society of Critical Care Medicine and European Society of Intensive Care Medicine, 2026 (replaces 2021)
- Therapeutic Guidelines: AntibioticAU/NZTherapeutic Guidelines Limited (eTG complete), 2025 to 2026 staged update (topic versions not verified)
- Miller JM et al. A guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by IDSA and ASM. Clin Infect Dis 2024Infectious Diseases Society of America and American Society for Microbiology, 2024
- Bouadma L et al. Use of procalcitonin to reduce patients' exposure to antibiotics in intensive care units (PRORATA). Lancet 2010;375:463-74The Lancet, 2010
- de Jong E et al. Efficacy and safety of procalcitonin guidance in reducing the duration of antibiotic treatment in critically ill patients (SAPS). Lancet Infect Dis 2016;16:819-27The Lancet Infectious Diseases, 2016
- Dark P et al. Biomarker-guided antibiotic duration for hospitalized patients with suspected sepsis: the ADAPT-Sepsis randomized clinical trial. JAMA 2025;333:682-93JAMA, 2025
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy