Overview
Thrombin Time (TT), also known as Thrombin Clotting Time, is a laboratory test that measures how long it takes for plasma to clot after adding excess thrombin. It evaluates the stage of the clotting process where soluble fibrinogen is converted into insoluble fibrin threads, which form the structural basis of a blood clot.
Unlike many other coagulation tests, TT does not require calcium or phospholipids. Instead, it is highly sensitive to thrombin inhibitors, fibrinogen abnormalities, fibrin degradation products (FDPs), hypoalbuminemia, and paraproteins. The reference range depends on the reagent used:
- Thrombin reagent: 12 – 14 seconds
- Batroxobin (snake venom derivative): 15 – 20 seconds
Normal TT values rule out quantitative fibrinogen disorders like afibrinogenemia and hypofibrinogenemia, but qualitative defects (dysfibrinogenemia) may still require further testing.
Symptoms
Patients are advised to undergo a TT test when they exhibit clinical signs of clotting or bleeding abnormalities. Symptoms include:
- Prolonged bleeding from minor cuts or injuries.
- Excessive or unexplained bruising.
- Recurrent miscarriages, linked to coagulation abnormalities.
- Thrombotic episodes, such as unexplained blood clots.
- Frequent nosebleeds or gum bleeding.
- Delayed clotting after trauma or surgery.
- Symptoms of liver disease, which may present with clotting disorders.
- Breathing difficulty or chest pain, in cases of deep vein thrombosis or pulmonary embolism.
These symptoms usually warrant coagulation studies, including TT, PT, and APTT, for accurate diagnosis.
Causes
A prolonged Thrombin Time may occur due to a range of hematological and systemic conditions:
- Liver failure, impairing fibrinogen synthesis.
- Dysfibrinogenemia, where fibrinogen structure is abnormal.
- Paraproteinemia in multiple myeloma, interfering with fibrin formation.
- Heparin administration or contamination, prolonging clotting time.
- Decreased fibrinogen levels (hypofibrinogenemia or afibrinogenemia).
- Elevated fibrin degradation products (FDPs), which inhibit fibrin polymerization.
- Use of thrombin inhibitors.
- Disseminated Intravascular Coagulation (DIC), where clotting factors are consumed rapidly.
- Congenital fibrinogen deficiency in newborns with bleeding disorders.
- Malignancy, contributing to clotting abnormalities.
Critical values (TT > 100 seconds) require urgent medical consultation, as they indicate severe coagulation dysfunction.
Risk Factors
Several factors increase the risk of abnormal TT values:
- Liver diseases, such as cirrhosis or acute liver failure, leading to impaired fibrinogen synthesis.
- Inherited fibrinogen disorders, including congenital afibrinogenemia or dysfibrinogenemia.
- Use of anticoagulant medications, particularly heparin and thrombin inhibitors.
- Malignancies, especially multiple myeloma, where paraproteins interfere with clot formation.
- Pregnancy complications, including recurrent miscarriage linked to clotting defects.
- Severe trauma or surgery, where excessive bleeding may occur.
- Disseminated Intravascular Coagulation (DIC), causing simultaneous clotting and bleeding.
- Sample handling errors, since improper collection or processing can alter TT results.
Prevention
While TT abnormalities reflect underlying conditions rather than a disease itself, preventive strategies help reduce the risk of clotting or bleeding disorders:
- Regular health check-ups: Routine coagulation tests for at-risk patients (liver disease, anticoagulant therapy).
- Monitoring anticoagulant therapy: Close INR and TT monitoring in patients on heparin or thrombin inhibitors.
- Liver health management: Avoiding alcohol abuse, treating hepatitis early, and maintaining liver wellness.
- Awareness during pregnancy: Women with recurrent miscarriages should be screened for coagulation abnormalities.
- Cancer monitoring: Patients with myeloma or other malignancies should undergo regular TT assessment.
- Accurate laboratory handling: Ensuring correct anticoagulant-to-blood ratio (1:9 with citrate/oxalate tubes) and timely processing to maintain result accuracy.
- Prompt intervention in DIC: Early recognition and treatment of disseminated intravascular coagulation can prevent life-threatening outcomes.
