What's the next step after a mixing study shows no correction in a patient with a prolonged Activated Partial Thromboplastin Time (APTT)?

Next Step After Mixing Study Shows No Correction in a Patient with Prolonged APTT

The laboratory should perform a confirmatory assay that contains excess/high phospholipids as the next step to determine a diagnosis.

Clinical Scenario Analysis

This 42-year-old female presents with:

• Markedly prolonged APTT (125.9 seconds; reference range 23-36 seconds)
• Normal prothrombin time (11.6 seconds; reference range 11-13 seconds)
• Normal platelet count (256 x 103/μL; reference range 150-440 x 103/μL)
• No abnormal bleeding symptoms
• Mixing study showing no correction immediately and after incubation at 37 degrees

This pattern strongly suggests the presence of a lupus anticoagulant (LA).

Diagnostic Algorithm for Prolonged APTT

1. Initial screening tests: APTT, PT, platelet count (already completed)
2. Mixing studies: 1:1 mix of patient plasma with normal pooled plasma (already completed)
3. Confirmatory testing: Assay with excess/high phospholipids (next step)
4. Additional testing: Consider anticardiolipin and anti-β2 glycoprotein I antibodies

Rationale for Confirmatory Testing with High Phospholipid Concentration

The International Society on Thrombosis and Haemostasis (ISTH) guidelines recommend a confirmatory test with excess phospholipids as the next step after a mixing study fails to correct 1. This test is specifically designed to neutralize lupus anticoagulants, which are phospholipid-dependent antibodies.

The principle behind this test is that:

• Lupus anticoagulants bind to phospholipids in the test system
• Adding excess phospholipids provides additional binding sites for the antibodies
• This results in shortening (correction) of the clotting time in LA-positive samples

Why Other Options Are Not Appropriate

• D-dimer test: Not indicated as the primary concern is not thrombosis evaluation but determining the cause of the prolonged APTT
• Fibrinogen assay: Not indicated as the PT is normal, making hypofibrinogenemia unlikely
• Repeat platelet count: The platelet count is already normal and not the issue

Clinical Implications

The pattern of laboratory findings strongly suggests lupus anticoagulant, which is part of the antiphospholipid antibody family 2. Despite the name "anticoagulant," lupus anticoagulants are paradoxically associated with thrombotic risk rather than bleeding 3.

The ISTH guidelines recommend that LA test results should be considered in the context of a full antiphospholipid antibody profile, including anticardiolipin and anti-β2 glycoprotein I antibodies 1. The presence of medium-high titers of these antibodies of the same isotype (most often IgG) along with a positive LA identifies patients at high risk for thrombosis.

Common Pitfalls to Avoid

1. Misinterpreting LA as a factor deficiency: Unlike factor deficiencies, LA typically shows immediate non-correction in mixing studies that doesn't worsen with time 4

2. Medication interference: Certain medications like rivaroxaban can cause false-positive LA results 5. Always review patient medications when interpreting coagulation tests.

3. Inadequate phospholipid concentration: The confirmatory test must have sufficient phospholipid concentration to neutralize the LA. The ISTH recommends performing confirmatory tests in normal controls and using the mean of obtained clotting times to calculate the percentage of shortening 1.

4. Improper sample preparation: Careful preparation of platelet-poor plasma is essential to ensure maximum detection of LA 2.