Thrombin Time [CO004600]

Related Information

• Activated Partial Thromboplastin Time
• D-Dimers and Fibrin Degradation Products
• Factor Inhibitors
• Fibrinogen
• Heparin Antifactor Xa Assay
• Heparin Neutralization
• Hypercoagulation Panel
• Reptilase® Time

Applies to Fibrinopeptide A; Fibrinopeptide B

Abstract Measures clotting time of the last step in the coagulation cascade, which is the conversion of fibrinogen into fibrin by thrombin. Useful for diagnosis of dysfibrinogenemia. Very sensitive to low amounts of heparin, hirudin, or argatroban anticoagulants.

Specimen Plasma

Container One blue top (sodium citrate) tube

Collection Routine venipuncture. If multiple tests are being drawn, draw blue top tubes after any red top tubes but before any lavender top (EDTA), green top (heparin), or gray top (oxalate/fluoride) tubes. Immediately invert tube gently at least 4 times to mix. Tubes must be appropriately filled. Deliver tubes immediately to the laboratory, on ice.

Storage Instructions Separate plasma from cells as soon as possible. Plasma may be stored on ice for up to 4 hours; otherwise, store frozen.

Causes for Rejection Specimen received more than 4 hours after collection, tube not filled, clotted specimens

Turnaround Time Less than 1 day

Reference Interval Approximately 10-13 seconds or 16-24 seconds, depending on thrombin concentration and ionic strength of the reaction conditions

Use Performed together with Reptilase® time to diagnose dysfibrinogenemia in patients undergoing evaluation for hypercoagulability and/or a bleeding tendency. Often performed only if an initial panel of tests excludes more common disorders, because dysfibrinogenemia is uncommon. The thrombin time is an older method for detecting heparin contamination in specimens; direct heparin neutralizing methods are now available for this purpose. The thrombin time has occasionally been used to monitor heparin therapy in patients for whom the PTT could not be used,¹ but now antifactor Xa assays are available for such situations. The thrombin time is often too sensitive to monitor heparin anticoagulation, and the assay is not standardized for this purpose.

Methodology Thrombin is added to patient plasma and the clotting time is measured in seconds. Thrombin cleaves fibrinogen, releasing fibrinopeptide A and fibrinopeptide B from fibrinogen and converting fibrinogen into fibrin clot. Assays use bovine or human thrombin.

Some laboratories use the thrombin time to detect unexpected heparin contamination in specimens. Even small amounts of heparin prolong the thrombin time, because heparin inhibits thrombin. If the thrombin time is prolonged, patient plasma can be mixed with an equal volume of normal plasma. The thrombin time of the mixture remains prolonged if the prolongation is due to heparin, fibrin degradation products (FDP), hirudin, argatroban, or other thrombin inhibitors. If the thrombin time of the mixture is normal, then the etiology of the prolongation is decreased fibrinogen or dysfibrinogenemia. The Reptilase® time is not prolonged by heparin. Many laboratories now use direct heparin neutralizing methods to detect heparin contamination rather than the thrombin time (see Heparin Neutralization).

Additional Information The thrombin time is prolonged when fibrinogen is decreased or dysfunctional, or when a thrombin inhibitor is present. Dysfibrinogenemia is an uncommon hereditary or acquired condition characterized by dysfunctional fibrinogen. Many different mutations are known to cause hereditary dysfibrinogenemia. Dysfibrinogenemia mutations can cause bleeding, thrombosis, or both, or they may be clinically asymptomatic. If bleeding is present it is usually mild, but severe bleeding has been reported. Dysfibrinogenemia has an estimated prevalence of 0.8% in patients with venous thrombosis.² Arterial thrombosis is less frequent than venous thrombosis in these patients. Most patients with hereditary dysfibrinogenemia are heterozygous. Rare homozygous cases have been reported. The thrombin time and Reptilase® time, which measure the clotting time during the conversion of fibrinogen into fibrin, are often prolonged in dysfibrinogenemia because fibrinogen is dysfunctional. Assays that measure fibrinogen function show lower levels than assays that measure fibrinogen quantity (immunological or "antigen" assays), because fibrinogen function is impaired but fibrinogen quantity is not. The PT and PTT may be prolonged in dysfibrinogenemia.^2,3,4 Causes of acquired dysfibrinogenemia include liver disease, hepatoma,⁴ or or acute phase reactions with generation of high levels of fibrinogen.⁵ The bleeding and thrombosis risk with acquired dysfibrinogenemia is uncertain. See Fibrinogen.

The thrombin time can be prolonged in disseminated intravascular coagulation (DIC) or thrombolytic therapy due to high levels of FDP and decreased fibrinogen. However, the thrombin time is not a necessary test for DIC diagnosis because fibrinogen and FDP can be measured directly. Prolongation of the thrombin time and Reptilase® time has been commonly observed with amyloidosis due to inhibition of fibrinogen conversion to fibrin.⁶ Patients exposed to bovine thrombin may develop thrombin inhibitors that prolong bovine-based thrombin times, and if the antibody cross-reacts against human thrombin, human-based thrombin times can also be prolonged (see Factor Inhibitors). The Reptilase® time is normal with these inhibitors. Rarely, heparin-like anticoagulants have been reported in patients with malignancies or other disorders, with prolonged thrombin times and normal Reptilase® times.

Footnotes

1. Ray MJ, Perrin EJ, Smith IR, et al, "A Proposed Model to Monitor Heparin Therapy Using the Concentrated Thrombin Time Which Allows Standardization of Reagents and Improved Estimation of Heparin Concentrations,"Blood Coagul Fibrinolysis, 1996, 7(5):515-21.

2. Haverkate F and Samama M, "Familial Dysfibrinogenemia and Thrombophilia. Report on a Study of the SSC Subcommittee on Fibrinogen,"Thromb Haemost, 1995, 73(1):151-61.

3. Cote HC, Lord ST, and Pratt KP, "gamma -Chain Dysfibrinogenemias: Molecular Structure-Function Relationships of Naturally Occurring Mutations in the gamma Chain of Human Fibrinogen,"Blood, 1998, 92(7):2195-212.

4. Galanakis DK, "Fibrinogen Anomalies and Disease. A Clinical Update,"Hematol Oncol Clin North Am, 1992; 6(5):1171-87.

5. Galanakis D, personal communication, 1999.

6. Gastineau DA, Gertz MA, Daniels TM, et al, "Inhibitor of the Thrombin Time in Systemic Amyloidosis: A Common Coagulation Abnormality,"Blood, 1991, 77(12):2637-40.

References

Olson JD, Arkin CF, Brandt JT, et al, "College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy. Laboratory Monitoring of Unfractionated Heparin Therapy,"Arch Pathol Lab Med, 1998, 122:782-98.