"CRP Test: What is C-Reactive Protein, Normal Range, and Lab Procedure Explained"

 What is CRP Test?

CRP Test stands for C-Reactive Protein Test.

• CRP is a protein made by the liver and released into the blood when there is inflammation (swelling, infection, or tissue damage) in the body.

• A high CRP level means your body is fighting infections, inflammation, or other health problems.

• Doctors use this test to check for inflammation, monitor infections, or detect diseases such as arthritis, heart disease, or autoimmune conditions.

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Why is CRP Test Done?

Doctors recommend a CRP test when a person has:

• Fever or infection symptoms

• Joint pain or swelling

• Suspected heart problems

• Autoimmune diseases (like rheumatoid arthritis, lupus)

• To monitor treatment progress in infections or inflammatory conditions

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Types of CRP Test

1. Standard CRP Test → Detects high inflammation (e.g., infections, arthritis).

2. High-sensitivity CRP (hs-CRP) Test → Detects tiny increases in CRP, mainly used for heart disease risk check.

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How to Test CRP in a Laboratory?

1. Sample Collection

• Type of sample: Venous blood (taken from arm vein).

• Container: Plain red-top tube or serum-separating tube (SST).

• Preparation: Usually no fasting is required.

Steps:

1. Clean patient’s arm with antiseptic.

2. Insert sterile needle into vein.

3. Collect 2–3 mL blood in a test tube.

4. Label sample correctly with patient details.

5. Send to lab for analysis.

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2. Testing Methods in Lab

There are several laboratory methods to measure CRP:

1. Immunoturbidimetric Assay (most common)

   • Patient’s serum is mixed with CRP antibodies.

   • Antigen-antibody reaction forms a cloudy (turbid) solution.

   • Machine (biochemistry analyzer) measures the turbidity → CRP level.

2. Nephelometry

   • Similar to above, but light scattering is measured instead of turbidity.

3. ELISA (Enzyme-Linked Immunosorbent Assay)

   • Antibodies fixed on a plate capture CRP from the sample.

   • An enzyme-linked antibody produces a color reaction → CRP is measured.

4. High-sensitivity CRP (hs-CRP)

   • Specially designed immunoassays with very high sensitivity.

   • Detects very low CRP levels (useful in predicting heart disease).

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3. CRP Normal Values

• Standard CRP:

  • Normal: < 10 mg/L

  • Mild elevation: 10–40 mg/L → Mild infection or inflammation

  • Moderate elevation: 40–200 mg/L → Serious infection, autoimmune disease

  • Very high: >200 mg/L → Severe bacterial infection, sepsis

• hs-CRP (for heart risk):

  • Low risk: < 1 mg/L

  • Average risk: 1–3 mg/L

  • High risk: > 3 mg/L

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Conclusion

• The CRP test is a simple blood test used to detect inflammation or infection.

• Sample: Blood from vein.

• Lab method: Immunoturbidimetric, Nephelometry, or ELISA.

• Results: Help doctors diagnose infections, inflammation, and heart disease risks.

here’s a clear, lab-manual style SOP you can use right away.

(I’ve written it for the immunoturbidimetric CRP method, which is the most common on routine chemistry analyzers, and added a short ELISA addendum + hs-CRP notes.)

CRP (C-Reactive Protein) – Laboratory SOP

1) Test purpose

Quantitative measurement of CRP in human serum or plasma to aid in the detection and monitoring of inflammation/infection.

(Use hs-CRP for cardiovascular risk assessment; see Section 12.)

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2) Principle (Immunoturbidimetry)

CRP in the patient sample reacts with anti-CRP antibodies in the reagent to form immune complexes. These increase turbidity proportional to CRP concentration. The analyzer measures the change in absorbance (typically at 340–700 nm depending on kit). Concentration is calculated from a multi-point calibration curve.

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3) Specimen

• Type: Serum (preferred) or plasma (lithium heparin or EDTA; follow kit instructions).

• Volume: 100–200 µL per test (analyzer dependent).

• Stability:

  • 2–8 °C: 72 hours

  • −20 °C: 3 months (avoid repeated freeze–thaw).

• Rejection: Hemolyzed, lipemic, or icteric samples beyond kit interference limits; unlabeled or leaking tubes.

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4) Reagents & materials (typical)

• R1/R2 CRP reagent (anti-CRP antibodies, buffers; ready-to-use or 2-reagent format per kit).

• Calibrators: At least 5 levels spanning 0–200 mg/L (or kit range).

• Controls: Two levels (normal & high).

• Saline or sample diluent (for on-board dilutions of high results).

• Cuvettes (if not reusable).

• PPE: Lab coat, gloves, eye protection.

Always follow your specific manufacturer’s IFU for storage (commonly 2–8 °C) and on-board stability.

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5) Equipment

• Automated clinical chemistry analyzer supporting immunoturbidimetry
  (or a semi-auto photometer with fixed wavelength compatible with the kit)

• Micropipettes & tips, vortex mixer, timer, refrigerator (2–8 °C), −20 °C freezer.

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6) Calibration

• Perform a full multi-point calibration when installing a new lot, after major maintenance, or when QC trends shift.

• Accept calibration only if the analyzer reports:

  • Correlation coefficient within kit limits (e.g., r ≥ 0.995),

  • Back-calculated calibrator recoveries within ±10%.

• Store and track the calibration curve per LIS/analyzer policy.

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7) Quality control

• Run two QC levels at the start of each shift, after calibration, and after any maintenance or reagent change.

• Accept run only if both controls fall within manufacturer or lab-established ranges (e.g., Westgard rules).

• If QC fails: do not report patient results. Investigate (reagent integrity, bubbles, cuvette cleanliness, drift, pipetting).

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8) Step-by-step procedure (Immunoturbidimetric)

1. Preparation

   • Bring reagents, calibrators, controls to room temperature (per IFU).

   • Mix gently—do not foam.

   • Check analyzer cuvette path & cleanliness.

2. Load analyzer

   • Scan/enter reagent lot, calibrator & control IDs.

   • Select the CRP method with correct parameters (sample/reagent volumes, wavelength, reaction time, calibration model, hook-effect check if available).

3. Calibration

   • Run the assigned calibrator set.

   • Review curve fit and recoveries; approve if within limits.

4. QC

   • Run Normal and High controls; verify both are in range.

5. Patient samples

   • Mix samples; avoid bubbles.

   • Load racks; start batch/run.

   • If the result exceeds the assay’s upper limit, the analyzer will flag for automatic dilution (e.g., 1:10 or 1:20). Re-run and multiply by the dilution factor.

6. Review & release

   • Confirm no analyzer flags (e.g., HIL interferences, prozone/hook).

   • Verify delta checks against prior results if available.

Typical analyzer parameters (example—adjust to your kit):

• Wavelength: 540–600 nm (kit-specific)

• Reaction: endpoint or rate at 2–10 minutes

• Sample volume: 3–10 µL; Reagent total: 200–400 µL

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9) Calculations & reporting

• The analyzer computes mg/L from the stored calibration.

• Report to one decimal (e.g., 12.4 mg/L) for standard CRP.

• If diluted: Reported CRP = analyzer result × dilution factor.

• Critical values: define locally (e.g., >200 mg/L may require physician notification).

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10) Reference intervals & medical decision points

(Adopt kit- or population-validated ranges; examples below)

• Standard CRP (inflammation):

  • Normal: <10 mg/L

  • Mild: 10–40 mg/L

  • Moderate: 40–200 mg/L

  • Very high: >200 mg/L (consider severe bacterial infection/sepsis context)

• hs-CRP (cardiovascular risk; not interchangeable with standard CRP):

  • Low risk: <1.0 mg/L

  • Average risk: 1.0–3.0 mg/L

  • High risk: >3.0 mg/L (repeat on a separate day if used for risk stratification)

Always interpret with clinical findings and other labs (CBC, cultures, ESR, procalcitonin, etc.).

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11) Interferences & limitations

• Lipemia, icterus, hemolysis: may cause positive/negative bias—follow kit HIL indices.

• Rheumatoid factor, heterophile antibodies: rare interference in immunoassays.

• High-dose hook/prozone: extremely high CRP can give falsely low results—use analyzers with prozone detection or auto-dilution.

• Matrix effects: Serum vs plasma differences—use permitted anticoagulants only.

• CRP is non-specific: It indicates inflammation; it does not identify the cause.

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12) hs-CRP notes (if your lab offers it)

• Uses high-sensitivity immunoassay with a validated low measuring range (e.g., 0.1–10 mg/L).

• Requires stricter pre-analytics: avoid intercurrent infections; repeat if >10 mg/L (likely acute inflammation, not baseline CV risk).

• Calibrate and QC with hs-CRP-specific materials.

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13) ELISA addendum (manual method, if no analyzer)

Principle: Solid-phase sandwich ELISA using anti-CRP capture antibody; enzyme-conjugate detection; colorimetric read at 450 nm.

Reagents: Pre-coated 96-well plate, standards, conjugate, wash buffer (1×), substrate (TMB), stop solution (acid), controls, sample diluent.

Steps (summary):

1. Bring all components to room temp.

2. Add standards, controls, and diluted samples (e.g., 1:100 if instructed) to wells.

3. Incubate (e.g., 30–60 min).

4. Wash 3–5× thoroughly.

5. Add conjugate; incubate.

6. Wash 3–5×.

7. Add TMB; incubate protected from light.

8. Add stop; read A450 within 10–15 min.

9. Generate a standard curve (4-PL/5-PL), calculate concentrations, apply dilutions.

10. Run two QC levels; accept only if within range.

Critical points: Precise pipetting, consistent timing, complete washing, plate-layout map, duplicate wells.

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14) Troubleshooting quick guide

• QC out of range → Re-mix reagents, check temperatures, recalibrate, inspect cuvettes, verify pipettors.

• Unexpected low results in very sick patient → Check for prozone; force a 1:20 manual dilution and re-run.

• Drifting results → Re-calibrate; check reagent on-board stability dates.

• High blank/absorbance → Cuvette contamination or bubbles; prime lines; replace cuvettes.

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15) Safety & waste

• Treat all human samples as potentially infectious (BSL-2).

• Wear PPE; avoid sharps injuries.

• Dispose of biohazard waste and chemical reagents per local regulations.

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16) Result wording for reports (examples)

• “CRP = 8.6 mg/L (Reference: <10 mg/L). Within normal limits.”

• “CRP = 126 mg/L. Markedly elevated; consistent with significant inflammation/infection. Correlate clinically.”

• hs-CRP: “2.4 mg/L – Average cardiovascular risk category. Repeat in absence of acute illness if used for risk assessment.”

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17) Documentation

• Record reagent lot/expiry, calibration ID, QC results, instrument flags, dilutions, and release time in the LIS or bench log.