What is the purpose of the PTT-LA (Partial Thromboplastin Time - Lupus Anticoagulant) test?

Purpose of the PTT-LA Test

The PTT-LA (Partial Thromboplastin Time - Lupus Anticoagulant) test is a specialized aPTT assay with low phospholipid content and silica as an activator, specifically designed to detect lupus anticoagulant (LA) antibodies in patients with suspected antiphospholipid syndrome. 1

Key Characteristics of PTT-LA

• Specialized Design: PTT-LA is formulated with low phospholipid content and silica as an activator, making it highly sensitive for detecting lupus anticoagulant 1
• Second-line Test: It is recommended as the second test of choice (after dRVVT) for LA detection according to the International Society on Thrombosis and Haemostasis guidelines 1
• Enhanced Sensitivity: The low phospholipid content increases sensitivity to LA antibodies, which interfere with phospholipid-dependent coagulation reactions 2

Clinical Applications

PTT-LA is used in the following clinical scenarios:

• Antiphospholipid Syndrome Diagnosis: For detecting LA in patients with suspected antiphospholipid syndrome 3
• Unexplained Thrombosis: Particularly in young patients (<50 years) with unprovoked venous or arterial thrombosis 1
• Pregnancy Complications: For women with recurrent pregnancy loss or late pregnancy complications 1
• Autoimmune Diseases: For patients with systemic lupus erythematosus and other autoimmune conditions who have thrombotic events 3
• Unexplained Prolonged aPTT: As a follow-up test when routine coagulation screening shows prolonged aPTT without obvious cause 1

Testing Protocol

The proper LA testing protocol using PTT-LA involves:

1. Screening Phase: Initial testing with PTT-LA to detect potential LA presence
2. Mixing Studies: If PTT-LA is prolonged, mixing with normal pooled plasma to differentiate between factor deficiencies and inhibitors
3. Confirmatory Testing: Addition of excess phospholipids to neutralize LA effect and confirm its presence 1

Improved Diagnostic Approach

Research has shown that calculating a ratio between PTT-LA and a less sensitive aPTT reagent (aPTT-ratio) significantly improves diagnostic performance:

• Sensitivity increases from 82.1% for PTT-LA alone to 92.3% for the aPTT-ratio approach 2
• This ratio method provides better discrimination between LA and other causes of prolonged aPTT 2

Clinical Significance

Detection of beta2-glycoprotein I-dependent lupus anticoagulant using specialized tests like PTT-LA has strong clinical relevance:

• Patients with beta2-glycoprotein I-dependent LA have a dramatically increased risk of thrombosis (odds ratio 42.3) 4
• Proper identification helps guide anticoagulation therapy decisions in patients with thrombotic events 5

Important Considerations and Limitations

• Anticoagulant Interference: Heparin and other anticoagulants can interfere with test results, though some PTT-LA reagents contain neutralizers for heparin up to 0.8 U/mL 1
• Monitoring Challenges: Using PTT-LA to monitor anticoagulation therapy in patients with baseline elevated aPTT can be unreliable and potentially dangerous 5
• Confirmation Requirement: A single positive result is insufficient; confirmation requires repeat testing after >12 weeks 1
• Pre-analytical Variables: Proper sample collection with double centrifugation is essential to ensure minimal platelet contamination 1

Patient Selection

Testing should be limited to patients with significant probability of antiphospholipid syndrome, with appropriateness graded as:

• High priority: Young patients with unprovoked thrombosis, thrombosis at unusual sites, late pregnancy loss, or thrombosis in patients with autoimmune diseases
• Moderate priority: Unexplained prolonged aPTT, recurrent early pregnancy loss, provoked VTE in young patients
• Low priority: Thromboembolism in elderly patients 1

Indiscriminate testing in asymptomatic individuals is discouraged due to the risk of false-positive results 1.