Cardiolipin Antibody (ACL) -IgG

Immunity

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The IgG isotype is the most specific and clinically relevant for diagnosing antiphospholipid syndrome (APS) — an autoimmune disorder characterized by: Blood clots (venous/arterial), Recurrent miscarriages, Thrombocytopenia

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Cardiolipin Antibody (ACL) -IgG Test Information Guide

Why is it done?
- Test Purpose: Detects IgG antibodies against cardiolipin, a phospholipid found on cell membranes. This test is used to identify antiphospholipid antibodies associated with antiphospholipid syndrome (APS) and other autoimmune conditions.
- Primary Indications: Evaluating recurrent venous or arterial thrombosis, recurrent pregnancy losses, thrombocytopenia (low platelet count), prolonged activated partial thromboplastin time (aPTT), suspected antiphospholipid syndrome, and monitoring systemic lupus erythematosus (SLE) patients.
- When Test is Performed: When patients present with unexplained thrombosis, recurrent miscarriages, autoimmune conditions, or as part of initial screening for APS. Also performed during disease monitoring and after anticoagulation therapy initiation.
Normal Range
- Reference Range: Less than 12 Antibody Units (U) per milliliter (mL) or negative. Some laboratories may report as less than 5-10 U/mL depending on their specific reference standards.
- Result Interpretation Categories:
  - Negative (< 12 U/mL): No detectable cardiolipin IgG antibodies; consistent with normal immune system.
  - Low-Positive (12-40 U/mL): Low levels of cardiolipin IgG antibodies present; may be transient or clinically significant depending on clinical context.
  - Medium-Positive (40-80 U/mL): Moderate levels of antibodies; typically indicates persistent APS and increased thrombotic risk.
  - High-Positive (> 80 U/mL): High levels of cardiolipin IgG antibodies; strong association with thrombotic complications and definitive for APS diagnosis.
- Units of Measurement: Antibody Units per milliliter (U/mL) or milliInternational Units per milliliter (mIU/mL).
- What Normal vs Abnormal Means: Normal (negative) results indicate absence of pathogenic cardiolipin antibodies and no evidence of APS. Abnormal (positive) results suggest antiphospholipid syndrome, increased thrombotic risk, or associated autoimmune disease.
Interpretation
- Clinical Significance of Positive Results: Positive cardiolipin IgG antibodies indicate potential antiphospholipid syndrome, particularly when confirmed on repeat testing (≥12 weeks apart). Strong association with venous and arterial thrombosis, recurrent pregnancy loss, and thrombocytopenia.
- APS Diagnosis Criteria: Diagnosis of APS requires both clinical and laboratory criteria. Positive cardiolipin IgG (medium or high) confirmed on repeat testing is one laboratory criterion. Must be paired with clinical features such as thrombosis or pregnancy morbidity.
- Transient vs Persistent Positivity: Transient low-positive results may occur with infections, medications, or vaccines and typically resolve within weeks. Persistent positivity (positive on ≥2 occasions ≥12 weeks apart) indicates true antiphospholipid antibodies and increased thrombotic risk.
- Titer Level Correlation: Higher antibody titers (medium to high) correlate with increased thrombotic risk compared to low titers. However, even low-positive results can be clinically significant in symptomatic patients.
- Factors Affecting Results:
  - Recent infections (viral, bacterial)
  - Autoimmune diseases (SLE, rheumatoid arthritis)
  - Certain medications (hydralazine, procainamide)
  - Recent vaccinations
  - Malignancy and lymphoproliferative disorders
  - HIV infection
- Triple Antiphospholipid Positivity: Patients positive for cardiolipin IgG, cardiolipin IgM, and lupus anticoagulant have highest thrombotic risk and are at increased risk of catastrophic antiphospholipid syndrome.
Associated Organs
- Primary Organ Systems Affected:
  - Vascular System: Primarily affected through thrombosis of veins and arteries.
  - Reproductive System: Placental thrombosis and infarction leading to recurrent pregnancy loss.
  - Hematologic System: Thrombocytopenia and hemolytic anemia.
  - Immune System: Underlying autoimmune dysfunction.
- Diseases Associated with Abnormal Results:
  - Primary Antiphospholipid Syndrome (PAPS): Occurs without underlying autoimmune disease.
  - Secondary APS: Associated with SLE, rheumatoid arthritis, and other connective tissue diseases.
  - Systemic Lupus Erythematosus (SLE): 30-40% of SLE patients have cardiolipin antibodies.
  - Recurrent Thromboembolism: Deep vein thrombosis (DVT), pulmonary embolism (PE), arterial thrombosis.
  - Recurrent Fetal Loss: Repeated miscarriages, particularly in second and third trimester.
  - Catastrophic Antiphospholipid Syndrome (CAPS): Rare but severe manifestation with multiple organ thromboses.
- Potential Complications and Risks:
  - Venous thrombosis (DVT, pulmonary embolism)
  - Arterial thrombosis (stroke, myocardial infarction)
  - Cerebral events and stroke
  - Fetal complications including intrauterine growth restriction and stillbirth
  - Thrombocytopenia with bleeding risk
  - Valve disease and endocarditis
  - Cognitive impairment and dementia
Follow-up Tests
- Confirmatory Tests if ACL IgG is Positive:
  - Repeat ACL IgG Testing: Repeat testing 12 weeks after initial positive result to confirm persistent positivity.
  - Cardiolipin IgM Testing: To determine if both IgG and IgM antibodies are present; double positivity increases thrombotic risk.
  - Lupus Anticoagulant (LA) Testing: Detects functional antiphospholipid antibodies; highest risk when all three antiphospholipid tests are positive.
  - Anti-Beta-2 Glycoprotein I Antibodies: Associated with increased thrombotic risk and APS clinical manifestations.
- Complementary Diagnostic Tests:
  - Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT): May show prolonged aPTT due to lupus anticoagulant presence.
  - Platelet Count: To evaluate for thrombocytopenia associated with APS.
  - Antinuclear Antibody (ANA): Screen for underlying SLE or other connective tissue disease.
  - Comprehensive Metabolic Panel (CMP): To assess baseline organ function before initiating anticoagulation therapy.
- Imaging Studies if Clinically Indicated:
  - Venous ultrasound (DVT assessment)
  - CT pulmonary angiography (PE assessment)
  - Brain MRI (for neurological manifestations)
  - Echocardiography (cardiac involvement)
- Monitoring Frequency: Initial repeat testing at 12 weeks; subsequently, annual monitoring or as clinically indicated. More frequent monitoring if positive result is equivocal or if clinical status changes.
Fasting Required?
- Fasting Status: No
- Fasting Not Required: Cardiolipin antibody testing does not require fasting. Blood can be drawn at any time of day without dietary restrictions.
- Medications to Avoid: No specific medications need to be withheld. However, if on anticoagulation therapy (warfarin, heparins, direct oral anticoagulants), continue as prescribed unless otherwise directed by physician.
- Timing Considerations: Blood draw should preferably be taken during morning hours when possible, though time of day does not significantly affect cardiolipin antibody levels.
- Patient Preparation:
  - No fasting required; maintain normal diet and hydration
  - Inform phlebotomist of all current medications
  - Report any recent infections or vaccinations (within past 2 weeks)
  - Remain seated or lying down for 5-10 minutes before blood draw if at risk for syncope
  - If repeat testing is needed at 12 weeks, schedule appointment and plan for blood collection

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