ASO Test

Introduction

1. The Anti-Streptolysin O (ASO) test is a vital diagnostic tool for measuring antibodies produced against Streptolysin O (SLO), a cytotoxic enzyme secreted by Streptococcus pyogenes.
2. These antibodies indicate exposure to or infection by Group A Streptococcus.
3. Elevated ASO titers are associated with post-streptococcal complications such as:

• Rheumatic fever (affecting the heart, joints, and nervous system).
• Post-streptococcal glomerulonephritis (inflammation of the kidneys).

Key Points:

• ASO titers begin to rise 1–3 weeks post-infection, peak around 3–6 weeks, and decline over months.
• ASO levels are expressed in Todd units (International Units, IU/mL).

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Principle

The ASO test relies on the ability of anti-SLO antibodies in the patient’s serum to neutralize the hemolytic activity of the SLO enzyme:

1. Neutralization of Hemolysis:
   • SLO can lyse red blood cells (RBCs).
   • When a serum containing anti-SLO antibodies is mixed with SLO, the antibodies neutralize the enzyme, preventing RBC lysis.
2. Qualitative Test: Determines the presence of anti-SLO antibodies.
3. Quantitative Test: Determines the highest serum dilution (titer) that inhibits hemolysis completely.

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Requirements

1. Sample

• • Collect venous blood in a plain (clot) tube.
  • Allow clotting at room temperature and centrifuge at 2,500–3,000 rpm for 10 minutes to separate serum.
  • Avoid hemolysis or contamination.
  • Use fresh serum or store at 2–8°C (up to 7 days). For longer storage, freeze at −20°C.

2. Reagents

1. 1. Streptolysin O (SLO):
      • A lyophilized preparation reconstituted per manufacturer instructions.
      • Concentration must be standardized (e.g., causing complete hemolysis of a defined RBC volume).
   2. Sensitized RBCs:
      • Sheep red blood cells (1%–2%) washed and suspended in isotonic saline.
   3. Diluent:
      • Normal saline or phosphate-buffered saline (PBS).
   4. Controls:
      • Positive Control: Serum with known anti-SLO concentration.
      • Negative Control: Normal saline or serum without ASO antibodies.

3. Equipment

• • Test tubes or microtiter plates for serial dilution.
  • Micropipettes and disposable tips.
  • The incubator or water bath is maintained at 37°C.
  • Timer or stopwatch.

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Procedure

Qualitative ASO Test (Screening)

The qualitative ASO test is a rapid screening method used to detect the presence of Antistreptolysin O (ASO) antibodies in serum by observing visible agglutination.

Materials Required

• Patient serum sample
• ASO latex reagent
• Positive control
• Negative control
• Reaction slide (glass or disposable)
• Disposable mixing sticks
• Mechanical rotator (optional)

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Procedure

Step 1: Prepare the Reagents

• Bring the serum sample and ASO latex reagent to room temperature (20–30°C).
• Mix the latex reagent gently before use. Do not shake vigorously.

Step 2: Label the Reaction Circle

• Label separate circles on the reaction slide for the patient sample, positive control, and negative control.

Step 3: Add the Serum Sample

• Place 50 µL (one drop) of the patient’s serum into the designated reaction circle.

Step 4: Add the Latex Reagent

• Add one drop of ASO latex reagent to the serum sample.

Step 5: Mix the Reagents

• Using a clean disposable applicator stick, mix the serum and reagent evenly over the entire reaction circle.

Step 6: Rotate the Slide

• Gently rotate the slide manually or place it on a mechanical rotator at approximately 80–100 rpm for 2 minutes.

Step 7: Observe the Reaction

• Examine the reaction immediately under adequate lighting for visible agglutination.

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Interpretation of Results

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Quantitative ASO Test (Titration)

Materials Required

• Patient serum (qualitative ASO positive)
• ASO latex reagent
• Isotonic saline (0.9% NaCl)
• Reaction slide with multiple circles
• Micropipette or disposable droppers
• Mixing sticks
• Mechanical rotator (optional)

Procedure

Step 1: Prepare Serial Dilutions

• Label six reaction circles or tubes as 1:2, 1:4, 1:8, 1:16, 1:32, and 1:64.
• Prepare serial two-fold dilutions of the patient’s serum using normal saline.

Step 2: Add Diluted Serum

• Place 50 µL (one drop) of each diluted serum sample into the corresponding reaction circle.

Step 3: Add ASO Latex Reagent

• Add one drop of ASO latex reagent to each diluted serum sample.

Step 4: Mix the Reagents

• Mix each reaction thoroughly using a separate disposable mixing stick.

Step 5: Rotate the Slide

• Rotate the slide manually or on a mechanical rotator at 80–100 rpm for 2 minutes.

Step 6: Observe for Agglutination

• Examine each dilution for visible agglutination under good lighting.

Step 7: Determine the End-Point Titer

• The highest serum dilution showing visible aggl

  utination is considered the ASO antibody titer.

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Calculation of ASO Titer

The ASO concentration is calculated using the formula:

ASO Titer (IU/mL) = Sensitivity of the reagent × Highe

st positive dilution

Example:

• Kit sensitivity = 200 IU/mL
• Highest positive dilution = 1:8

ASO Titer = 200 × 8 = 1600 IU/mL

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Interpretation of Results

 

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Results

Normal ASO Ranges

• • Adults: Up to 200 IU/mL.
  • Children (5–12 years): Up to 250 IU/mL.

Significant Titers

• • A single elevated ASO titer is not conclusive of active disease. Instead:
    • Rising Titers: A fourfold increase in ASO titer over 2–3 weeks is significant.

Reporting

• • Results are expressed in Todd units or IU/mL.
    • 1 Todd unit = the amount of antibody that neutralizes 0.01 mL of SLO.

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Clinical Significance

1. Diagnosis of Rheumatic Fever

• Elevated ASO levels support the diagnosis of acute rheumatic fever in patients with a history of recent streptococcal throat infection.

2. Diagnosis of Post-Streptococcal Glomerulonephritis (PSGN)

• Helps confirm a recent streptococcal infection in patients presenting with hematuria, edema, and hypertension.

3. Evidence of Recent Streptococcal Infection

• Indicates a recent infection with Group A Streptococcus, especially when throat culture is negative or antibiotics have already been started.

4. Monitoring Antibody Response

• Serial ASO titers can help assess the rise or fall of antibody levels, indicating recent infection or recovery.

5. Differentiating Post-Infectious Complications

• Helps distinguish complications caused by a previous streptococcal infection from those due to other diseases.

6. Supportive Diagnostic Test

• The ASO test should be interpreted along with clinical findings and other laboratory tests such as Anti-DNase B, CRP, ESR, and throat culture for accurate diagnosis.

7. Guiding Patient Management

• Early identification of post-streptococcal complications allows timely treatment and helps prevent long-term complications, particularly rheumatic heart disease.

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Limitations

1. False Negatives:
   • Not all streptococcal infections produce significant ASO levels (e.g., skin infections like impetigo).
2. False Positives:
   • Elevated ASO titers may persist after the resolution of infection.
3. Non-Specificity:
   • ASO cannot distinguish between recent and remote infections.
   • Results must be interpreted alongside clinical findings and other investigations.

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Precautions

1. Reagents and Controls:
   • Always use fresh or properly stored reagents. Validate test accuracy with controls.
2. Temperature Control:
   • Maintain consistent incubation temperatures (37°C) for reliable results.
3. Interpretation:
   • Always interpret ASO titers in the clinical context; elevated titers alone are not diagnostic of disease.