I received this question on 9/14/24:
Hello George, Is there any condition where reptilase time is prolonged while thrombin time is normal?
My answer is, in a word, no. The thrombin time (TT, or TCT) typically uses 50–100 mg topical human or bovine thrombin, mixed 1:1 with plasma. A normal plasma/thrombin mix clots within ~20 seconds. Thrombin digests fibrinopeptides A and B, the peptides then bind to form a fibrin polymer. The TT is prolonged in the case of hypo- or afibrinogenemia or dysfibrinogenemia. The TT is also prolonged by heparin or anti-IIa anticoagulants such as dabigatran or argatroban.
The reptilase time (RT) employs the venom from the Bothrops atrox serpent in the place of thrombin. Reptilase digests only fibrinopeptide A and is therefore unaffected by the anticoagulants. Therefore, The RT can identify hypo-, afibrinogenemia, and dysfibrinogenemia. The RT is largely replaced by the Clauss fibrinogen assay, available from all facilities.
Dr. Emmanuel Favaloro provided his 9/16/23 response, “Yes, it would be unusual, but hard to know without actually seeing the test results. The TT may be just within range and the RT just outside range, in which case it would be just a range variation effect. This may also depend on the reagents used and various concentrations; e.g., maybe high thrombin concentration and too low a reptilase concentration in a patient with borderline normal/abnormal fibrinogen? But, yes, in general, we expect concordance in TT and RT, although normal RT with abnormal TT may point to heparin or anti-IIa inhibitor.”
It happened to us in the past. Once a plasma sample from a patient receiving Fondaparinux (an indirect FXa inhibitor) showed a normal TT and a mild RT prolongation (~ +5 sec). We did not further investigate this one-off situation since the other two plasma samples on Fondaparinux (lower dose though), did not show RT prolongation. It would be interesting to rule out the possibility of Fondaparinux interference with RT assay by a well-designed study.
Many thanks, George, I appreciate your prompt and very explicatory response. Doubt is solved. I read somewhere (I don’t remember the source) that RT could be prolonged when TT was normal.