APTT Mixing Studies for Coagulation Factor Deficiencies and Inhibitors

Basic Principle and Methodology

APTT mixing studies involve mixing patient plasma with normal plasma and measuring the APTT of this mixture, which is a diagnostic test that distinguishes between coagulation factor deficiencies and the presence of inhibitors, according to the International Society on Thrombosis and Haemostasis 2, 3
The test is essentially a modified APTT mixing study using 50% patient plasma and 50% normal plasma, as recommended by the British Committee for Standards in Haematology 4
The mixing study includes three key components: an activator, phospholipids, and calcium, which are the same components used in standard APTT reagents, as stated by the American Society of Hematology 4

Mixing studies are performed both immediately and after incubation, which is crucial because certain inhibitors are time and temperature-dependent, according to the College of American Pathologists 2, 3
Prolongation of the APTT in the mixture after incubation compared to the immediate mix is typical of factor VIII autoantibodies, as reported by the European Hematology Association 2
The APTT will correct when mixed with normal plasma in cases of factor deficiency, as the normal plasma provides the missing factors, according to the American College of Medical Genetics and Genomics 1, 2

APTT mixing studies are used to differentiate between factor deficiencies and inhibitors when a patient presents with prolonged APTT, as recommended by the International Society on Thrombosis and Haemostasis 1, 2
The tests are used to diagnose specific conditions like Acquired Hemophilia A, which is characterized by factor VIII inhibitors, according to the World Federation of Hemophilia 2, 3
APTT mixing studies are also used to detect lupus anticoagulant, which is important in diagnosing antiphospholipid syndrome, as stated by the Antiphospholipid Syndrome Alliance 5, 6

Immediate correction of APTT with normal plasma does not completely exclude inhibitors, especially in patients with suggestive clinical presentations, according to the College of American Pathologists 2
Anticoagulant therapy can interfere with mixing study results, as reported by the European Hematology Association 5, 6
Lupus anticoagulant can cause artifactual lowering of factor levels due to inhibition of phospholipid in the assay, as stated by the International Society on Thrombosis and Haemostasis 2

Regardless of mixing study results, specific factor assays should be performed to facilitate accurate diagnosis, as recommended by the American Society of Hematology 2, 3
For suspected inhibitors, quantification using assays like the Bethesda assay may be necessary, according to the College of American Pathologists 2
In cases of suspected lupus anticoagulant, specific confirmatory tests should be performed, as stated by the Antiphospholipid Syndrome Alliance 1, 2