Lab Tests for Infection & Inflammation
When infection is suspected, blood tests help confirm the presence and severity of inflammation, distinguish bacterial from viral causes, identify the source, and monitor response to treatment — all within hours of a blood draw.
Clinical overview
No single blood test diagnoses infection. Instead, a pattern of raised WBC (particularly neutrophils), elevated CRP and ESR, and cultures taken from blood or urine work together. CRP rises and falls fastest — making it the most useful marker to track treatment response.
Primary tests ordered for Infection & Inflammation
These are the tests most commonly ordered first when infection & inflammation is suspected or being monitored.
WBC Count (White Blood Cell Count)
Your WBC is elevated — or low — and you're not sure what that means. White blood cells…
Neutrophil Count
Neutrophils are the soldiers of your immune system — the first white cells to arrive at any bacterial…
ESR & CRP (Inflammation Markers)
ESR and CRP are both markers of inflammation — but they rise and fall at different speeds, and…
What to expect
- 🩸 Blood cultures must be taken before antibiotics — always.
- ⏰ CRP peaks at 48–72 hours after infection onset — a single negative early result can be misleading.
- 🌡️ WBC may initially be normal or low in overwhelming sepsis — this is a bad sign, not reassurance.
- 📊 Serial CRP (daily) is more informative than a single measurement.
- 💊 Procalcitonin-guided therapy can reduce antibiotic duration and side effects.
Testing frequency
Also commonly ordered
These tests are frequently added to the primary panel based on initial results or specific clinical circumstances.
Blood Culture (× 2)
Two sets from different sites before antibiotics — identifies the causative organism and sensitivities.
Procalcitonin (PCT)
More specific than CRP for bacterial sepsis — helps guide antibiotic stewardship.
Lactate
Elevated blood lactate (>2 mmol/L) indicates tissue hypoperfusion in sepsis.
D-Dimer
Elevated in sepsis-associated coagulopathy (DIC). Also used to exclude PE and DVT.
Urine MC&S
Microscopy, culture and sensitivity — the gold standard for UTI diagnosis.
LFTs + Bilirubin
Sepsis commonly causes hepatocellular dysfunction. LFTs monitor multi-organ involvement.
Urgent result thresholds
These result patterns require prompt clinical attention — always discuss with your doctor rather than interpreting alone.
Severe bacterial infection or sepsis — immediate antibiotic therapy and source control.
Septic shock — resuscitation and ICU care required immediately.
Neutropenic sepsis — life-threatening. Broad-spectrum IV antibiotics within 1 hour.
Treatment failure — source not controlled, wrong antibiotic, or alternative diagnosis.
Medical Disclaimer: This guide is for educational reference only. It does not constitute medical advice and should not replace consultation with a qualified healthcare professional. Always discuss your test results with your doctor. Full disclaimer →