Testing of Coagulation
Numerous tests are used to assess the function of the coagulation system:
- Common: aPTT, PT (also used to determine INR), fibrinogen testing (often by the Clauss method), platelet count, platelet function testing (often by PFA-100).
- Other: TCT, bleeding time, mixing test (whether an abnormality corrects if the patient's plasma is mixed with normal plasma), coagulation factor assays, antiphosholipid antibodies, D-dimer, genetic tests (e.g. factor V Leiden, prothrombin mutation G20210A), dilute Russell's viper venom time (dRVVT), miscellaneous platelet function tests, thromboelastography (TEG or Sonoclot), euglobulin lysis time (ELT).
The contact activation (intrinsic) pathway is initiated by activation of the "contact factors" of plasma, and can be measured by the activated partial thromboplastin time (aPTT) test.
The tissue factor (extrinsic) pathway is initiated by release of tissue factor (a specific cellular lipoprotein), and can be measured by the prothrombin time (PT) test. PT results are often reported as ratio (INR value) to monitor dosing of oral anticoagulants such as warfarin.
The quantitative and qualitative screening of fibrinogen is measured by the thrombin clotting time (TCT). Measurement of the exact amount of fibrinogen present in the blood is generally done using the Clauss method for fibrinogen testing. Many analysers are capable of measuring a "derived fibrinogen" level from the graph of the Prothrombin time clot.
If a coagulation factor is part of the contact activation or tissue factor pathway, a deficiency of that factor will affect only one of the tests: Thus hemophilia A, a deficiency of factor VIII, which is part of the contact activation pathway, results in an abnormally prolonged aPTT test but a normal PT test. The exceptions are prothrombin, fibrinogen, and some variants of FX that can be detected only by either aPTT or PT. If an abnormal PT or aPTT is present, additional testing will occur to determine which (if any) factor is present as aberrant concentrations.
Deficiencies of fibrinogen (quantitative or qualitative) will affect all screening tests.
| Condition | Prothrombin time | Partial thromboplastin time | Bleeding time | Platelet count |
|---|---|---|---|---|
| Vitamin K deficiency or warfarin | Prolonged | Normal or mildly prolonged | Unaffected | Unaffected |
| Disseminated intravascular coagulation | Prolonged | Prolonged | Prolonged | Decreased |
| Von Willebrand disease | Unaffected | Prolonged | Prolonged | Decreased and/or Rejected |
| Hemophilia | Unaffected | Prolonged | Unaffected | Unaffected |
| Aspirin | Unaffected | Unaffected | Prolonged | Unaffected |
| Thrombocytopenia | Unaffected | Unaffected | Prolonged | Decreased |
| Liver failure, early | Prolonged | Unaffected | Unaffected | Unaffected |
| Liver failure, end-stage | Prolonged | Prolonged | Prolonged | Decreased |
| Uremia | Unaffected | Unaffected | Prolonged | Unaffected |
| Congenital afibrinogenemia | Prolonged | Prolonged | Prolonged | Unaffected |
| Factor V deficiency | Prolonged | Prolonged | Unaffected | Unaffected |
| Factor X deficiency as seen in amyloid purpura | Prolonged | Prolonged | Unaffected | Unaffected |
| Glanzmann's thrombasthenia | Unaffected | Unaffected | Prolonged | Unaffected |
| Bernard-Soulier syndrome | Unaffected | Unaffected | Prolonged | Decreased or unaffected |
| Factor XII deficiency | Unaffected | Prolonged | Unaffected | Unaffected |
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