a. Textarin:Ecarin Ratio
b. Taipan:Ecarin Ratio
Introduction & Principles
a. Textarin:Ecarin ratio. The Textarin:Ecarin ratio is a test for a Lupus Anticoagulant [LA] based on the differential dependence/requirement 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.
The Textarin clotting time is normal in the presence of Vitamin K Antagonists [VKA] such as Warfarin as the Prothrombin activator in the Textarin venom can activate the des-carboxyprothrombin present in the plasma of individuals on a VKA antagonist to Meizothrombin allowing clot formation.
Ecarin is also insensitive to VKA antagonists.
As both venoms are Prothrombin activators, they are completely unaffected by inhibitors of Factor Xa.
a. Taipan:Ecarin ratio: The Taipan:Ecarin ratio is very similar to the Textarin:Ecarin ratio. The Prothrombin activator present in the venom of the Coastal Taipan (Oxyuranus scutellatus) activates Prothrombin in the presence of Calcium and Phospholipid to the intermediate meizothrombin facilitating in vitro clot formation.
As with the Textarin clotting time, the Taipan clotting time is normal in the presence of Vitamin K Antagonists [VKA] such as Warfarin because again the Prothrombin activator in the Taipan venom can activate the des-carboxyprothrombin present in the plasma of individuals on a VKA antagonist.
As both venoms are Prothrombin activators, they are completely unaffected by inhibitors of Factor Xa.
Method
Individual Textarin [or Taipan] and Ecarin clotting times are derived for a plasma sample and the ratio is then calculated.
Component | Interpretation |
---|---|
Platelet poor plasma | A source of coagulation factors, particularly thrombin and fibrinogen. |
Textarin, Taipan or Ecarin | These venoms directly activate Prothrombin. Ecarin does not require Phospholipid to do so and converts Prothrombin to Meizothrombin which activates Fibrinogen. Textarin and Taipan both require both Factor V and Phospholipid for its action on Prothrombin. |
Phospholipid | To provide a surface for Thrombin generation. The PL should be diluted sufficiently that it becomes the rate limiting step and any inhibition by a LA prolongs the clotting time. |
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.
Taipan 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 clotting times or the Taipan:Ecarin clotting times.
Interpretation
A deficiency of, or inhibitors to Prothrombin will prolong both the Textarin/Taipan and Ecarin times and deficiency of, or inhibitors to Factor V will also prolong the Textarin/Taipan times.
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. Similarly a high Taipan:Ecarin ratio (i.e. prolongation of the Textarin time relative to the Ecarin time) 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 as described here with Textarin or Taipan.
Factor V deficiency and specific inhibitors to Factor V will cause a prolongation of the Textarin time and the Taipan time, but these appear to be the only factor deficiencies which cause a false-positive result. 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).
Reference Ranges
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 ratio or the Taipan:Ecarin 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.
The ISTH do not currently recommend LA screening assays based
on snake venoms such as Ecarin
and Textarin or Taipan because there are no standardized commercial
assays available based on this principle.