CRP (C-reactive protein) is an acute phase protein made in the liver, driven mainly by IL-6. It rises rapidly in acute inflammation but is poorly reliable in certain diseases: SLE, Scleroderma, Ulcerative Colitis, Leukaemia. 👉 Remember SLUL mnemonic. In these, ESR or disease-specific markers are more informative.

🧾 Overview

CRP rises within 6–8 hours of a trigger, peaks by 48 hours, half-life ~19 hours.
Falls quickly once inflammation resolves (unlike ESR).
Acts as an opsonin, activates complement, and signals to immune cells.
Useful for infection, post-operative complications, and cardiovascular risk prediction (hs-CRP).

🔬 Why CRP is Unhelpful in Some Diseases

SLE 🦋: Type I interferon suppresses CRP production. ESR more reliable.
Scleroderma: Fibrotic process with minimal acute-phase response.
Ulcerative Colitis: CRP correlates poorly with flare severity (Crohn’s > UC for CRP).
Leukaemia: Cytokine signalling is disrupted → blunted CRP response.

⚠️ Causes of Very High CRP

Cause	Key Clinical Features	Investigations	Management
Bacterial Sepsis / Pneumonia 🦠	Fever, hypotension, tachycardia, rigors	Blood cultures, CXR, lactate, CRP, procalcitonin	IV antibiotics, fluids, source control
Severe Pancreatitis 🔥	Epigastric pain radiating to back, vomiting	Amylase/lipase, CT abdo, CRP >150 = severe	Supportive, fluids, ITU if severe
Autoimmune Disease (RA, Vasculitis) 🤲	Arthritis, rash, systemic symptoms	ANA, RF, ANCA, CRP	DMARDs, steroids, biologics
Severe Viral Infections 🦠	COVID-19, flu with hypoxia, myalgia	Viral PCR, CXR, CRP	Supportive ± antivirals, oxygen
Malignancy 🎗️	Weight loss, night sweats, lymphadenopathy	Imaging, biopsy, tumour markers	Chemo, radiotherapy, surgery
Major Trauma / Surgery 🩺	Pain, fever, wound changes	CRP trend post-op (should fall by day 5–7)	Wound care, antibiotics if infection
Burns 🔥	Large TBSA, fluid loss, infection risk	Electrolytes, wound cultures	Fluids, analgesia, infection control

📊 CRP vs ESR

Feature	CRP	ESR
Protein measured	Direct hepatic acute phase protein	Rate of RBC settling influenced by fibrinogen & Igs
Onset of rise	6–8 hrs, peaks 48 hrs	24–48 hrs, peaks later
Half-life	~19 hrs (rapid fall)	Slow to fall (can stay high for weeks)
Best for	Acute infection, post-op monitoring	Chronic inflammation, autoimmune disease
Affected by	Few external factors	Anaemia, pregnancy, ↑ Igs (myeloma)
Limitations	Not reliable in SLUL diseases	Non-specific; rises with age, pregnancy

🔑 Clinical Pearls

Normal CRP does not exclude flare in SLE, UC, scleroderma, leukaemia.
CRP rises/falls rapidly → best for monitoring infection & treatment response.
CRP >100 mg/L usually bacterial, <50 more often viral (but overlap exists).
ESR + CRP together: ESR for chronic autoimmunity, CRP for acute infection.

📚 References

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Causes of high C reactive protein (CRP)