Reptilase Time (RT)

Introduction

The Reptilase time is a modification of the thrombin time in which the purified enzyme Reptilase is used to replace thrombin. Reptilase is isolated from the snake Bothrops atrox. Reptilase cleaves fibrinogen releasing fibrinopeptide A (FpA) generating fibrin. In contrast thrombin cleaves both fibrinopeptide A and fibrinopeptide B from fibrinogen to generate the fibrin clot.

Ancrod a similar enzyme from Agkistrodon rhodostoma can also be used to replace thrombin in the thrombin clotting time test.

Agkistrodon jpeg

Agkistrodon rhodostoma

Bothrops atrox

Principles

Reptilase is added to platelet poor plasma and the clotting time measured. The test requires no added phospholipid or calcium.

Method

1. Platelet poor plasma	See pre-analytical variables
2. Reptilase solution	Reptilase re-constituted according to the manufacture's instructions.

Reference Ranges

The reference range for the reptilase time is similar to the thrombin time and is generally in the range of 13-15s.

Interpretation of Results

The reptilase time is rarely performed in isolation and therefore, the results of this test should be considered together with other tests and in particular the thrombin time.

A prolonged reptilase time is seen in:

1. Inherited and Acquired Hypofibrinogenaemia	A low fibrinogen (usually <1.0g/L) will lead to a prolongation of both the thrombin time and the reptilase time.
2. Inherited Dysfibrinogenaemia	Most dysfibrinogenaemias will lead to a prolongation of both the thrombin time and the reptilase time. However, in some cases depending upon the site of the mutation - only one or other of the two tests may be abnormal.
3a. Following thrombolytic therapy due to the high levels of FDPs. 3b. DIC with elevated levels of FDPs	High levels of FDPs lead to defective fibrin polymerisation and therefore, prolongation of the both the thrombin and reptilase times. The thrombin and reptilase times may also be prolonged in DIC due to hypofibrinogenaemia.
5. Hypoalbuminaemia	A low serum albumin may be associated with a prolongation of both the thrombin time and reptilase time. This appears to be due to an in vitro phenomenon and correction of the low albumin either in vitro or in vivo corrects the abnormality. This problem was originally reported in patients with nephrotic syndrome but has also been reported in patients with HIV.
6. Liver disease	The thrombin time and reptilase time are often prolonged in patients with liver disease due to both hypofibrinogenaemia, elevated FDPs, and an acquired dysfibrinogenaemia due to abnormalities of fibrinogen sialic acid content.
7. Neonate	Neonatal fibrinogen differs from adult fibrinogen in its sialic acid content. As a consequence the reptilase and thrombin times maybe physiologically prolonged. A similar finding is seen in some patients with liver disease.
8. Myeloma	Some paraproteins can interfere with fibrin polymerisation and lead to a prolonged thrombin and reptilase time.

Comments

The Reptilase time is often used to exclude heparin contamination in a blood sample prior to embarking upon other more complex clotting tests tests. The finding of a prolonged thrombin time with a normal Reptilase time is strongly suggestive of heparin (unfractionated) contamination of the blood sample. Low Molecular Weight Heparins (LMWHs) do not prolong the reptilase time.
The Reptilase time is normal in patients receiving unfractionated heparin, Hirudin and Argatroban.
Inhibitors (antibodies) to bovine thrombin can develop in patients exposed to fibrin glue which historically contained bovine thrombin. In such patients the reptilase time is normal but the bovine thrombin time is prolonged. The thrombin time using human thrombin is normal.

What Test Next?

The following table summarises the abnormalities of the thrombin time and reptilase time. This can guide the the most appropriate investigation.

	Thrombin Time	Reptilase Time
Presence of unfractionated heparin	↑	Normal
Presence of LMWH	May show some prolongation	Normal
Presence of direct thrombin inhibitors	↑	Normal
Warfarin	Normal	Normal
Decreased/absent fibrinogen	↑	↑
Dysfibrinogenaemia	↑	↑
DIC	↑	↑
Liver disease	↑	↑
Heparin-like anticoagulants	↑	Normal
Paraproteinaemias	↑	↑
Thrombolytic therapy	↑	↑
Neonate	↑	↑
Amyloid	↑	↑
Hyperfibrinogenaemia	↑	↑
Hypoalbuminaemia	↑	↑

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