- Intrinsic, Extrinsic, & Common Coagulation Pathways
- Prothrombin Time ( PT, Factor II )
- (Activated) Partial Thromboplastin Time (PTT, aPTT)
- Thrombin Time (TT)
- Fibrin D-Dimer
PROTHROMBIN TIME (PT) — The prothrombin time (PT) (figure 1) is used to assess the extrinsic pathway of clotting, which consists of tissue factor and factor VII, and coagulation factors in the common pathway (factors II [prothrombin], V, X, and fibrinogen).
Reporting of Test Result from Lab:
PT expressed as International Normalized Ratio (INR) – In order to promote standardization of the PT for monitoring anticoagulation with warfarin and other vitamin K antagonists, the World Health Organization (WHO) developed an international reference thromboplastin, currently recombinant tissue factor, and recommended that the PT ratio be expressed as the INR . This allows values of the PT from various locations to be directly compared, as may happen when a patient taking warfarin has blood sampled at different laboratories
Normal Results: INR <1.0 is normal. Clinical signficance in bleeding generally does not appear until INR >1.5.
Causes of prolonged PT (INR) — In addition to the administration of warfarin, there are other causes of PT prolongation. These include:
● Vitamin K deficiency due, for example, to poor nutrition or prolonged use of broad spectrum antibiotics. When vitamin K deficiency is mild, only the PT may be prolonged due to a predominant effect on factor VII. However, in severe vitamin K deficiency, both the PT and aPTT may be prolonged.
● Liver disease, which decreases the synthesis of both vitamin K-dependent and -independent clotting factors. When liver disease is mild, only the PT may be prolonged due to a predominant effect on factor VII. However, in severe and/or chronic liver disease, both the PT and aPTT may be prolonged.
● Disseminated intravascular coagulation depletes coagulation factors, which may prolong the PT and aPTT.
● Deficiency or inhibition of factors VII, X, II (prothrombin), V, or fibrinogen.
● The infrequent antiphospholipid antibodies (aPLs) (lupus anticoagulant phenomenon) with antiprothrombin activity. In such patients, the acquired prothrombin deficiency may be severe enough to cause clinical bleeding.
● Polycythemia (ie, hematocrit >55 percent) can artificially prolong the PT if the amount of citrated anticoagulant solution in the collection tube is not appropriately decreased.
● While treatment with heparin does not normally prolong the PT (due to the addition of heparin-neutralizing materials to the PT reagent), the PT may be transiently elevated after bolus administration of heparin.
● The parenteral direct thrombin inhibitor argatroban causes prolongation of the PT.
● Oral direct thrombin and factor Xa inhibitors also cause prolongation of the PT.
The activated partial thromboplastin time (aPTT or PTT) is used to assess the integrity of the intrinsic coagulation pathway (prekallikrein, high molecular weight kininogen, factors XII, XI, IX, VIII) and final common pathway (factors II, V, X, and fibrinogen), and to monitor therapy with unfractionated heparin.
The test is performed by recalcifying citrated plasma in the presence of a thromboplastic material that does not have tissue factor activity (hence the term partial thromboplastin) and a negatively-charged substance (eg, celite, kaolin, silica), which results in contact factor activation, thereby initiating coagulation via the intrinsic clotting pathway.
Results: Normal Range for aPTT: 25-35 seconds
Causes of prolonged aPTT — Prolongation of the aPTT can occur with a deficiency of, or an inhibitor to, any of the clotting factors except for factor VII.
Patients with von Willebrand disease (VWD) can have a prolonged aPTT because factor VIII levels may be low, but a normal aPTT can also be seen in VWD.
In addition, certain lupus anticoagulants, which are antibodies directed against plasma proteins bound to anionic phospholipids, cause aPTT prolongation by interfering with the in vitro assembly of the prothrombinase complex. This in vitro event is paradoxically associated with an increased risk of venous and arterial thrombosis.
Medications that prolong the aPTT include heparin, direct thrombin inhibitors, and direct factor Xa inhibitors. Warfarin has a weak effect on most aPTT reagents, but will increase the sensitivity of the aPTT to heparin effect.
Monitoring heparin therapy — Heparin is an indirect thrombin inhibitor that complexes with antithrombin (AT), converting this circulating protein from a slow to a rapid inactivator of thrombin, factor Xa, and, to a lesser extent, factors XIIa, XIa, and IXa.
Historically, the goal of maintenance heparin therapy has been to maintain the aPTT in the range of 1.5 to 2.5 times the patient's aPTT baseline value. However aPTT results are not standardized across different reagent/instrument systems, which may result in significant variability in aPTT results for a given heparin concentration.
Monitoring parenteral direct thrombin inhibitor activity — Parenteral direct thrombin inhibitors (eg, argatroban, lepirudin [a recombinant form of the medicinal leech salivary anticoagulant hirudin]) have short half-lives and are administered intravenously; monitoring is required.
The aPTT test is used to monitor these agents, with target values that depend on the specific drug. For argatroban, the target is 1.5 to 3 times the control, but not to exceed 100 seconds. Testing is done two hours after initiation of therapy or any dose change. Lepirudin production was discontinued in 2012.
THROMBIN TIME (TT) — The TT measures the final step of the clotting pathway, the conversion of fibrinogen to fibrin.
The test is performed by recalcifying citrated plasma in the presence of dilute bovine or human thrombin and recording the time (in seconds) for a clot to form.
Thrombin Time Normal Values: for most labs, 14-22 seconds.
Causes of prolonged TT — The following conditions cause a prolongation of the TT:
● Anticoagulants including heparin, direct thrombin inhibitors (eg, bivalirudin or argatroban), or heparin-like compounds (eg, danaparoid)
● The presence of fibrin/fibrinogen degradation products
● Hypofibrinogenemia (<100 mg/dL), dysfibrinogenemia, or hyperfibrinogenemia (>400 mg/dL)
● High concentrations of serum proteins, as occurs in multiple myeloma or amyloidosis
● Bovine thrombin antibodies from prior exposure to bovine thrombin (only when tested using bovine thrombin reagent; TT will be normal if tested using human thrombin reagent). Patients who have been exposed to bovine thrombin during surgery may develop antibodies to bovine thrombin. In general, such antibodies are specific for bovine thrombin and are usually not associated with an increased bleeding risk unless they cross-react with human thrombin or the patient has coexisting inhibitors to other clotting factors.
Occasionally, patients exposed to topical bovine thrombin develop antibodies to bovine factor V present in the topical thrombin preparation, which crossreact with human factor V, causing prolongation of the prothrombin time (PT) and activated partial thromboplastin time (aPTT) and a significant bleeding risk
Reptilase time (RT) — Reptilase is an enzyme similar to thrombin that is found in the venom of Bothrops snakes. However, it differs from thrombin by generating fibrinopeptide A, but not fibrinopeptide B, from fibrinogen and by resisting inhibition by heparin via antithrombin (AT).
The RT is similar to the TT in measuring the conversion of fibrinogen to fibrin.
The RT is useful for detecting abnormalities in fibrinogen (in which case the TT is also prolonged) and in detecting the presence of heparin (heparin will cause prolongation of the TT but not RT). Thus, the RT is most useful for determining if heparin is the cause of a prolonged TT.
Similar to heparin, direct thrombin inhibitors prolong the TT but not the RT. Inadvertent presence of a direct thrombin inhibitor is less likely to be clinically relevant, but RT could be used to test for this possibility.
Plasmin cleaves fibrin at multiple sites and releases fibrin degradation products (FDPs). One of the major FDPs is D-dimer, which consists of two D domains from adjacent fibrin monomers that have been crosslinked by activated factor XIII.
Since D-dimer is generated from crosslinked fibrin, but not from fibrinogen, an elevated plasma concentration of D-dimer indicates recent or ongoing intravascular blood coagulation.
Normal D-Dimer Levels: < or = 250ng/mL D-Dimer Units (DDU)
Increased D-dimer values are abnormal but do not indicate a specific disease state. D-dimer values may be increased as a result of:
- -Clinical or subclinical disseminated intravascular coagulation/intravascular coagulation and fibrinolysis
- -Other conditions associated with increased activation of the procoagulant and fibrinolytic mechanisms such as recent surgery, active or recent bleeding, hematomas, trauma, or thromboembolism
- -Association with pregnancy, liver disease, inflammation, malignancy or hypercoagulable (procoagulant) states
The degree of D-dimer increase does not definitely correlate with the clinical severity of associated disease states. A list of the various conditions that lead to an increased D-dimer level is shown in the table
— Thromboelastography (TEG) tests both platelet function and coagulation by assaying several parameters of clot formation dynamically in whole blood. There are several devices available commercially to measure TEG at the bedside or point of care. While the specifics of the devices vary, they all measure the same general parameters. As examples:
●The TEG and related rapid TEG (r-TEG) devices monitor the interaction of platelets within the fibrin mesh of the clot during clot formation and lysis over time. The physical property of the clot is measured by use of a cylindrical cup that holds a whole blood sample at 37ºC and is oscillated to and fro with a rotation cycle of 10 seconds. As the clot forms, the torque of the rotating cup is transmitted to an immersed pin. The degree of pin rotation is converted to an electrical signal via a transducer and monitored via a chart recorder. The strength of the developing clot increases the magnitude of the output, whereas during clot lysis, the bonds between the cup and the pin are broken, and the signal decreases. The forces that are generated are used to measure the clotting time, kinetics of clot initiation, clot strength, and clot lysis over time.
●The ROTEM (Rotational Thromboelastometry) device is an adaptation of the TEG in which the cup remains stationary and the pin rotates directly in the sample. Results obtained are essentially identical to the TEG.
TEG has been utilized successfully as a point of care test within surgical departments (especially in trauma and orthotopic liver transplantation) and to predict for thromboembolic events in surgical patients. The usefulness of TEG in general hematologic practice remains uncertain. Further information regarding the use of TEG in surgical patients is presented separately.
Of note, the TEG instrument may be insensitive to samples from patients who have taken aspirin
A big Thank You to UpToDate for the bulk of this information, including tables.