What test involves mixing dilutions of patient plasma with a known factor deficient substrate to determine the amount of correction in Prothrombin Time (PT) or Partial Thromboplastin Time (PTT)?

Factor Assay: The Test Described in the Question

The test described in the question is a Factor Assay (answer C).

A factor assay is a specific laboratory test that measures the amount of correction in PT or PTT when patient plasma is mixed with known factor deficient substrate at various dilutions and compared to a normal reference curve 1.

How Factor Assays Work

Factor assays are designed to quantitatively measure the activity of specific coagulation factors by:

1. Dilution Series: Creating various dilutions of patient plasma
2. Substrate Addition: Mixing these dilutions with plasma specifically deficient in the factor being tested
3. Correction Measurement: Measuring how much the patient plasma corrects the prolonged clotting time of the factor-deficient substrate
4. Reference Curve Comparison: Comparing results to a normal reference curve to determine the percentage of factor activity

This differs from other coagulation tests in the following ways:

Differentiation from Mixing Studies

Mixing studies (answer A) involve mixing patient plasma with normal pooled plasma (typically in a 1:1 ratio) to determine if a prolonged clotting time corrects, which helps distinguish between factor deficiencies and inhibitors 1. Unlike factor assays, mixing studies:

• Use normal pooled plasma rather than factor-deficient substrate
• Are qualitative rather than quantitative
• Don't use multiple dilutions to create a reference curve
• Don't specifically identify which factor is deficient

Differentiation from Tests for Circulating Anticoagulants

Tests for circulating anticoagulants (answer B) focus on detecting inhibitors that interfere with coagulation. These tests typically involve:

• Immediate and incubated mixing studies to detect time-dependent inhibition
• Calculation of indexes like the index of circulating anticoagulant (ICA) 2
• Looking for non-correction patterns rather than quantifying factor activity

Differentiation from Lupus Inhibitor Screens

Lupus anticoagulant (LA) screens (answer D) are specialized tests designed to detect antiphospholipid antibodies that interfere with phospholipid-dependent coagulation tests. According to the International Society on Thrombosis and Haemostasis guidelines 2, LA testing involves:

• Screening tests (dRVVT or sensitive aPTT)
• Mixing tests
• Confirmatory tests with increased phospholipid concentration
• Not the use of factor-deficient substrates or dilution series

Clinical Significance of Factor Assays

Factor assays are essential for:

• Precise diagnosis of specific factor deficiencies
• Determining the severity of factor deficiencies (severe <1%, moderate 1-5%, mild >5% to <40%) 1
• Monitoring replacement therapy
• Distinguishing between different types of hemophilia and other factor deficiencies

Common Pitfalls in Factor Assay Testing

• Pre-analytical variables (improper sample collection, processing, or storage) can affect results
• Lupus anticoagulants can interfere with factor assays, potentially causing falsely low results
• Heparin or direct oral anticoagulants in the sample can interfere with testing
• Factor assays should be performed regardless of mixing test results to confirm specific factor deficiencies 1

Factor assays remain the gold standard for quantifying specific coagulation factor activity and are essential for diagnosing and managing patients with suspected coagulation factor deficiencies.