Prothrombin time and INR
- Citrated plasma is mixed with tissue factor and phospholipid
- Calcium is added, reversing the citrate effect \(\to\) start the clock!
- Tissue factor combines with FVII to make extrinsic tenase \(\to\) activates FX (with FV, calcium and phospholipid) \(\to\) activates thrombin \(\to\) clot formation - stop the clock!
- Normal = 11-13 seconds
- Used for: monitoring liver synthetic function, coagulopathy from vitamin K deficiency, titrating vitamin K reductase inhibitors
Activated partial thromboplastin time
- Phospholipids and kaolin are added to citrated plasma
- Calcium is added, reversing the citrate \(\to\) start the clock!
- The negatively charged kaolin surface activates FXII \(\to\) FXI \(\to\) FIX \(\xrightarrow{\text{FVIII as cofactor}}\) FX (with FV, calcium and phospholipid) \(\to\) thrombin \(\to\) stop the clock!
- Normal = 25-35 seconds
- Used for: diagnosing intrinsic pathway factor deficiencies or inhibitors, titrating low-dose heparin (will never clot in the presence of high-dose heparin like in cardiopulmonary bypass!)
Activated clotting time
- Kaolin is added to whole blood \(\to\) start the clock!
- Kaolin activates FXII, which will then eventually activate the whole clotting cascade, including activating plaletets
- Wait until clotted \(\to\) stop the clock!
- Normal = 70-130 seconds
- Used for: titrating high-dose heparin
Anti-Xa
- Pre-activated Xa is added to patient plasma
- Some of the Xa is then inactivated by anti-Xa (either an ATIII+heparin complex, ATIII+LMWH complex, or a direct Xa inhibitor like apixaban)
- A substrate of Xa which changes color when cleaved is added \(\to\) the color change is measured
- A lookup table of optical absorbance constructed from normal plasma with known quantities of the drug (e.g. apixaban, heparin) is used to estimate the concentration of the drug in the plasma
Thromboelastography (TEG or ROTEM)
- Citrated whole blood is mixed with an activator, then with calcium.
- A stick is rotated through the blood (or the cup is rotated and the stick held still) and the force required plotted against time
- This allows quantification of the time to clot initation (depends on factors), maximum clot firmness (depends on fibrinogen and platelets), and degree of thrombolysis.
- The activator varies, which changes what the assay will detect. Using the ROTEM terminology...
- In EXTEM, the activator is tissue factor (analagous to PT)
- In INTEM, the activator is phospholipid (analagous to APTT)
- In FIBTEM, the activator is tissue factor again, but mixed with a platelet inhibitor, so the maximum clot firmness is determined by fibrinogen alone
- In APTEM, the activator is tissue factor again, but mixed with a antifibronolytic like aprotinin, which prevents fibrinolysis
- In HEPTEM, the activator is phospholipid, but mixed with a heparinase, which will remove the effect of heparin