Introduction & Principles
The Textarin/Ecarin ratio is a test for a lupus anticoagulant [LA] based on the differential dependence of these two snake venoms on phospholipid to activate coagulation. Textarin, a protein fraction isolated from the venom of the Australian Eastern brown snake (Pseudonaja textilis) directly activates prothrombin but requires factor V, calcium and phospholipid to do so; whereas, Ecarin, a venom from the saw-scaled viper (Echis carinatus) activates prothrombin to form meizothrombin in the absence of phospholipid. In the presence of a LA, the Textarin time is prolonged due to its phospholipid-dependence but the Ecarin time is not.
Individual textarin and ecarin clotting times are derived for a plasma sample and the ratio is then calculated.
|Platelet poor plasma||A source of coagulation factors, particularly thrombin and fibrinogen.|
|Textarin or Ecarin||Both venoms directly activate prothrombin. Ecarin does not require phospholipid to do so and converts prothrombin to meizothrombin which activates fibrinogen. Textarin requires both FV and phospholipid for its action on prothrombin.|
|Phospholipid||To provide a surface for thrombin generation. This should be diluted to a sufficient degree that it becomes the rate limiting step and any inhibition by APL prolongs coagulation.|
|Calcium||To initiate coagulation|
Textarin Time: PPP is incubated with reconstituted Textarin [containing phospholipid] at 37°C, 0.025M calcium chloride is added to initiate clotting and the time to clot formation recorded.
Ecarin Time: PPP is incubated at 37°C, ecarin previously warmed to at 37°C is added and the time to clot formation recorded.
The results are reported as a ratio of the Textarin/Ecarin times.
Deficiency of, or inhibitors to prothrombin will prolong both Textarin and Ecarin times and deficiency of, or inhibitors to FV will also prolong the Textarin time.
The two venom times are compared as a ratio. A high Textarin/Ecarin ratio (i.e. prolongation of the Textarin time relative to the Ecarin time) >1.3, suggests the presence of a lupus anticoagulant.
The Ecarin time has been suggested as an alternative confirmatory test for the Taipan venom time in patients on warfarin, instead of a platelet neutralisation/ phospholipid correction step similar to its use with as described here with Textarin.
Factor V deficiency and specific inhibitors to factor V will cause a prolongation of the Textarin time, but these appear to be the only factor deficiencies which cause a false-positive. Specific factor inhibitors or deficiencies (except prothrombin) do not affect the Ecarin time, because Ecarin acts directly on prothrombin independent of all other factors. However, heparinoids and heparin-like drugs activate heparin cofactor II (HCII) and function to inhibit thrombin, which can prolong the Ecarin clotting time (ECT).
In the presence of LA, the Textarin time is prolonged and the Ecarin time is unaffected. The mean Textarin:Ecarin ratio for a normal individual i.e. no LA is 0.8. A ratio of >1.3 is considered abnormal.
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 Textarin:Ecarin 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
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