C-reactive protein (latex and quantitative)

Principle

• CRP is a pentraxin acute-phase protein made by the liver in response to IL-6. It binds phosphocholine (e.g. C-polysaccharide of pneumococcus) in the presence of calcium.
• Rises within about 6–8 hours of an inflammatory stimulus, peaks at about 48 hours and has a half-life of about 19 hours, so it falls quickly when inflammation resolves.
• Latex slide test: latex coated with anti-human CRP antibody agglutinates when CRP exceeds the kit sensitivity (commonly about 6 mg/L).
• Quantitative: particle-enhanced immunoturbidimetry or nephelometry; high-sensitivity (hs-CRP) assays measure low levels below 1 mg/L.

Specimen

• Serum or heparin plasma.

Procedure

• Latex: one drop of serum + one drop of latex, mix, rotate about 2 minutes, read. Semi-quantitative: test serial dilutions; CRP ≈ highest positive dilution factor × kit sensitivity.
• Automated: follow analyser protocol with calibrators and controls.

Results

• Typical healthy adults: below about 5 mg/L (some labs use <10 mg/L). Local reference ranges apply.
• Bacterial infections often produce values above 100 mg/L; viral infections usually cause smaller rises.
• hs-CRP cardiovascular risk: <1 mg/L low, 1–3 mg/L average, >3 mg/L high; values >10 mg/L suggest acute inflammation and should be repeated.

Quality control and pitfalls

• Prozone in latex tests with very high CRP; test a diluted sample.
• Read latex tests at the exact time; late reading gives false positives.
• Run positive and negative controls.
• CRP is non-specific; it is often normal or only mildly raised in active SLE unless serositis or infection is present.

Clinical use

• Detection and monitoring of infection, sepsis, inflammatory disorders and post-operative complications; cardiovascular risk (hs-CRP).