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A Practical Guide to Laboratory Haemostasis

 

Dilute Thromboplastin Inhibition Assay



Introduction

The dilute thromboplastin inhibition test [DTI] is a test based upon the Prothrombin Time [PT] that is rarely performed today. Whilst the test is sensitive for a lupus anticoagulant [LA] it lacks specificity.

Principles & Method

The dilute thromboplastin test is based on the principle that diluting the thromboplastin reagent in a PT-based assay, increases its sensitivity to a lupus anticoagulant [LA]. In the test the thromboplastin [Tissue Factor + Phospholipid] is diluted 1:50 and 1:500. The diluted thromboplastin is then incubated 1:1 with normal or test plasma at 37°C after which calcium chloride is added and the time to clot formation recorded.
The ratio of the clotting time with the dilute thromboplastin [1:500] divided by the normal reagent[1:50] is compared to a normal control.

Interpretation

A PT ratio ≥1.3 suggests the presence of a LA whilst a ratio ≤1.1 is considered normal.
The test should should be interpreted with caution as it is sensitive to a number of variables including:
     - FVIII or FIX <10% [IU/dL]
     - Hypofibrinogenaemia
     - FVIII Inhibitors
     - Prolonged APTT

Reference Ranges

A PT ratio ≥1.3 suggests the presence of a LA whilst a ratio ≤1.1 is considered normal.

What Test Next

In individuals in whom a LA is identified, the test should be repeated in 12 weeks. It should also be remembered that not all tests including the DTI Time will identify all LAs and therefore, if the index of suspicion that a specific patient has a LA then other tests should be undertaken. Finally - the causes of a LA should be screened for e.g. ANA, drugs, viruses etc

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.