THROMBIN TIME: RATIONALE, PROCEDURE, PATHOLOGIES - BIOLOGY - 2024

The thrombin time (TT) is a laboratory test that consists of quantifying the time it takes for the conversion of fibrinogen to fibrin. The expected normal or physiological value ranges from 13 - 17 seconds, although it may vary from one laboratory to another.

Thrombin time (TT) along with prothrombin time (PT) and partial thromboplastin time (PTT) make up the laboratory test called the coagulation screen. However, the most commonly used tests are PT and PTT, the use of TT being often avoided, being restricted to special cases.

Images taken from the video: thrombin time posted by Kike Álvarez on YouTube.

The study of the complete coagulation screen is recommended in patients with some type of hemorrhage or unexplained bleeding.

Coagulation tests (PT, PTT, and TT) can determine which clotting pathway is affected or which factor is probably deficient. Therefore, these tests help to elucidate the origin of the bleeding according to the test that is altered.

Blood coagulation is a complex process that consists of the intrinsic pathway, the extrinsic pathway and a common pathway where the two previous pathways converge. In the case of thrombin time, it evaluates the last stage of the coagulation cascade (common pathway).

Therefore, the thrombin time TT will be altered in the following cases:

-Presence of dysfunctional or decreased fibrinogen, -Exaggerated fibrinolytic activity, -Patients treated with anticoagulants or fibrinolytic drugs.

Rationale for the thrombin test

To carry out this test, it is necessary to obtain calcium-free or decalcified plasma. For this, trisodium citrate (C ₆ H ₅ O ₇ Na ₃) at 3.2% or 3.8% is used as an anticoagulant. The ratio used is one part anticoagulant with 9 parts blood (1: 9).

This anticoagulant preserves clotting factors and the way to prevent them from being consumed is by inhibiting the ionization of calcium present.

The test is based on pre-incubating the calcium-free plasma at a temperature of 37 ° C and later facing it with a portion of the reagent containing standardized thrombin in solution.

Thrombin will act on the patient's fibrinogen, and if it is in adequate concentration and its function is normal, the conversion of fibrinogen to fibrin will be activated.

The time it takes for fibrinogen to convert to fibrin under normal conditions should be in the following range: 13-17 seconds.

Process

Take the blood sample in blue cap plastic tubes (with trisodium citrate anticoagulant). Mix the sample and centrifuge to separate the plasma.

Reconstitute the thrombin vial provided by the commercial company. Follow the manufacturer's instructions.

Measure 0.2 ml of plasma and place it in a 12 x 75 tube and incubate in a water bath at 37 ° C for 2 minutes.

Add 0.2 ml of thrombin reagent to the tube and immediately start the stopwatch, stopping when the clot is visible.

The procedure is repeated and the 2 readings obtained in seconds are averaged.

A control plasma must also be mounted, it is also done in duplicate and the readings are averaged.

The thrombin time will be considered long when the difference from the patient's average minus the control average is greater than 2 seconds.

Pathologies

-Elongated thrombin time

Afibrinogenemia

Congenital afibrinogenemia is a rare rare disorder. It is characterized by the total absence of fibrinogen, which results in the appearance of significant bleeding.

In the case of acquired afibrinogenemia it can be caused by the suffering of certain bacterial infections (typhoid fever), in some types of cancer and in burns.

In this case, no clot is formed in the thrombin time test.

Hypofibrinogenemia

Acquired hypofibrinogenemia can occur during labor or in pregnant patients with a placenta previa and / or a stillborn fetus.

Fibrinogen is decreased in typhoid fever, an infection caused by Salmonella Typhi.

Dysfibrinogenemia

In this case, the fibrinogen concentration may be normal but it is dysfunctional. It is usually due to the excessive presence of sialic acid residues in the fibrinogen structure caused by a mutation, which interferes with coagulation. The PT and PTT are normal but the TT is elongated.

Hypodysfibrinogenemia

It is the combination of hypofibrinogenemia and dysfibrinogenemia. This means that the patient has a low concentration of fibrinogen and also what little it has is abnormal (non-functional).

Disseminated intravascular coagulation

It is characterized by the generation of thrombin in a pathological way. It manifests with bleeding, the appearance of petechiae, ecchymosis or the formation of thrombi (thrombosis).

It can originate from bacterial septicemia, liver cirrhosis, neoplasms, extracorporeal cardiac bypass, transfusion reactions, retention of a stillborn fetus, severe trauma, anaphylactic reactions, acute leukemias, extensive third degree burns, snake bites, among others.

Secondary fibrinolysis

The term fibrinolysis refers to the destruction of fibrin and fibrinogen by the action of plasmin. This occurs physiologically to prevent the clot from overdeveloping and causing thrombi.

But when this is caused by external factors then it is called secondary fibrinolysis and it can cause clotting problems.

In some conditions, such as myocardial infarction with an elevated ST fragment, specific or non-specific fibrinolytic drug treatment can be placed to help break up the clot.

Presence of antithrombins (heparin treatment)

Heparin inhibits the function of thrombin. Therefore, patients who have heparin therapy have lengthened PT and TT.

Liver disease

Different liver diseases can produce an abnormal synthesis of fibrinogen, as well as other coagulation factors. Among the most frequent liver diseases that affect coagulation are: liver cirrhosis, chronic hepatocellular disease, viral hepatitis.

-Shortened thrombin time

Hyperfibrinogenemia

It can be generated from any acute bacterial infection, except typhoid fever. Increased fibrinogen accelerates the erythrocyte sedimentation rate.

recommendations

-To perform the TT test it is not necessary for the patient to be fasting.

-Icteric, lipemic or hemolyzed samples interfere with the test when the reading is done automatically (photo-optical detection), but it does not affect if they are done manually.

-The blood / anticoagulant ratio must be respected. This step is essential to obtain a reliable result.

-The plasma should be separated quickly and the test processed with fresh plasma.

-The material used during the test must be perfectly clean and dry to avoid errors.

-The sample must be taken in plastic or silicone glass tubes.

-Each laboratory must establish its own reference values, since these are influenced by various factors, such as: taking and keeping the sample, technique, commercial kit, etc.

-If when performing a duplicate test there is a difference greater than 5% between the two readings of the same sample, the entire procedure must be repeated and the readings obtained must be ignored.

References

1. Ángel A, Ángel M. (1996). Clinical interpretation of the Laboratory. 5th Edition. Editorial Médica Panamericana, Bogotá Colombia.
2. Wiener Laboratories. (2000). Thrombin time. Available at: wiener-lab.com.ar
3. López S. Coagulation tests. Acta Pediatr Mex. 2016l; 37 (4): 241-245. Available at: scielo.org.
4. Téllez-Ávila Félix I, Chávez-Tapia Norberto C, Torre-Delgadillo Aldo. Coagulation disorders in the cirrhotic. Rev. invest. clin. 2007; 59 (2): 153-160. Available at:.scielo.org
5. Majluf A. Liver disease and hemostatic abnormalities. Gac Méd Méx, 2000; 132 (2): 29-30. Available at: anmm.org.mx
6. Junker G. Fibrinolytic treatment in acute myocardial infarction. Rev. Urug.Cardiol. 2013; 28 (3): 430-436. Available in: scielo.