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SOP

C-Reactive Protein (CRP) Testing

SOP for serum CRP measurement - standard and high-sensitivity - for inflammation monitoring and cardiovascular risk stratification.

Last verified 1 month ago 3 min read Clinical Chemistry Immunology #inflammation #crp #c-reactive protein #acute phase #hs-crp
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Document Control
Version
1.0
Effective
Aug 30, 2026
Reviewed
Aug 30, 2026
Department
Clinical Chemistry
Principle

CRP is an acute-phase protein synthesized by hepatocytes under the control of IL-6 and other cytokines; plasma levels rise within hours and can increase a thousand-fold in major inflammation. Immunoturbidimetry measures the turbidity of antibody-antigen complexes. High-sensitivity assays extend the analytical range downward to distinguish low from moderate cardiovascular risk in apparently healthy persons.

Standard Operating Procedure: C-Reactive Protein (CRP) Testing

1. Purpose and Scope

To measure serum C-reactive protein (CRP) for the detection and monitoring of inflammation, infection, and tissue injury, and to quantify high-sensitivity CRP (hs-CRP) for cardiovascular risk stratification in otherwise healthy adults.

2. Specimen Requirements

  • Type: Serum (plain tube or SST) or plasma validated for the method.
  • Volume: Minimum 0.5 mL.
  • Timing: No fasting required. CRP rises within 6-12 hours of an inflammatory stimulus and peaks around 24-48 hours, so serial measurement spaced 8-12 hours apart captures the peak.
  • Interference: Marked lipemia may interfere with turbidimetric methods; borderline samples should be cleared or re-collected.
  • Stability: Separated serum is stable 3-7 days at 2-8°C and longer when frozen.
  • Rejection: Grossly lipemic, icteric, or grossly hemolyzed samples with unacceptable indices are repeated from a fresh specimen.

3. Safety and Precautions

  • Standard PPE (gloves, lab coat, eye protection). All blood samples are potentially infectious.
  • Dispose of sharps and biohazard waste per laboratory policy.
  • Do not pipette by mouth. Use mechanical pipetting devices.

4. Equipment and Reagents

  • Automated chemistry analyzer with immunoturbidimetric or nephelometric capability.
  • Method: Immunoturbidimetric assay (anti-CRP antibody coupled particles); high-sensitivity (hs-CRP) variant validated at low concentrations for CV risk.
  • Calibration: Multipoint calibration traced to an international reference preparation (ERM-DA474).
  • Two levels of quality control material, including a low control near the hs-CRP decision range.

5. Step-by-Step Procedure

  1. Centrifuge the clotted specimen at 3000-3500 rpm for 10 minutes.
  2. Load serum onto the analyzer per instrument instructions with the CRP and hs-CRP protocols.
  3. Run two levels of quality control (including low) before patient samples.
  4. Measure CRP by immunoturbidimetry; verify calibration linearity up to the upper analytical limit.
  5. Dilute and re-run values above the linear range (common in severe infection/sepsis).
  6. Report hs-CRP results for risk stratification using the validated low-range protocol only.
  7. Release results after QC acceptance.

6. Quality Control

  • Run two levels (including a low clinical-decision level) daily, with every calibration, each reagent lot, and each shipment.
  • Enforce Levey-Jennings rules; participate in an external quality assessment for CRP.
  • Verify control values fall within acceptable target ranges before reporting.

7. Decision limits

The interpretive thresholds below are clinical decision limits; the canonical adult upper reference is displayed in the Reference Ranges panel.

  • Cardiovascular risk (hs-CRP mg/L): below 1.0 low risk; 1.0-3.0 moderate risk; above 3.0 mg/L high cardiovascular risk. Test on two occasions (2 weeks apart, preferably fasted) and average for risk classification.
  • Inflammation/infection: CRP is an acute-phase protein; infection and inflammation typically produce markedly elevated values. A single normal value does not exclude sepsis, and a fall, not absolute value, is used to monitor treatment response.
  • Non-specificity: CRP elevation occurs in tissue injury, surgery, malignancy, autoimmune disease, and chronic inflammation; the value is never diagnostic alone and CRP does not identify the cause.
  • hs-CRP caveats: any ongoing inflammatory condition invalidates the value as a purely cardiovascular estimate; re-test when the patient is well.

Reference Ranges

Browse all ranges

3 mg/L

Frequently Asked Questions

Both measure the same protein with the same method; hs-CRP is simply optimized and validated for very low concentrations so that differences below 3 mg/L are meaningful, which is what cardiovascular risk stratification requires.
CRP varies with minor intercurrent inflammation, so two measurements taken about 2 weeks apart, ideally after any acute illness has resolved, are averaged to classify cardiovascular risk reliably.
Not alone. CRP is a nonspecific acute-phase marker; a normal value does not exclude bacterial infection or sepsis, especially early in the course, and other markers and culture results are needed.