Evaluation of the coagulation system
Evaluation of the coagulation system may be useful for the preoperative assessment of coagulation status and measurement of anticoagulation therapy, as well as the diagnosis and management of intraoperative coagulopathy. The dual-cascade concept of coagulation, which involves the intrinsic and extrinsic pathways, is now understood to be an inadequate and incomplete representation of in vivo coagulation. The three-stage process of actual in vivo coagulation (activation, amplification, and propagation) involves complex interactions among the vascular endothelium, tissue factor, platelets, and soluble clotting factors. However, the classic dual-cascade view may still be beneficial in providing a reasonable model of in vitro coagulation tests, that is, the activated partial thromboplastin time (aPTT) and prothrombin time (PT) (Figure 147-1).
Preoperative assessment
The best method to screen patients preoperatively for bleeding disorders remains a thorough clinical history. Routine coagulation testing is not warranted without indications. Clinical indications for testing include congenital or acquired bleeding disorders, excessive bleeding during previous operations, liver disease, and the use of drugs or supplements with anticoagulant properties. Table 147-1 summarizes the most common preoperative coagulation studies used for assessing the coagulation status of patients.
Table 147-1
Studies Commonly Used Preoperatively for Assessing Patients’ Coagulation Status
| Test | Measured Aspect | Comments |
| PT | Extrinsic pathway and common pathway | The PT is prolonged if any of factors VII, X, V, II, and I are deficient, abnormal, or inhibited. The coagulant activity of these factors must be <30% of normal and the fibrinogen concentration must be <100 mg/dL for the PT to be prolonged. The PT may be used as a screening test for patients on oral anticoagulant therapy. The PT may be used to assess the synthetic function of the liver. |
| aPTT | Intrinsic pathway and common pathway | The aPTT is prolonged when any of factors XII, XI, IX, VIII, X, V, II, and I are deficient, abnormal, or inhibited. The coagulant activity of these factors must be <30% of normal and the fibrinogen concentration must be <100 mg/dL for the PT to be prolonged. The aPTT is prolonged by heparin therapy. The aPTT is prolonged in hemophiliacs and, usually, in people with von Willebrand disease. |
| Fibrinogen | Fibrinogen level; common pathway | Levels <100 mg/dL may be associated with the inability to form a clot and severe bleeding. |
| Platelet count | Quantitative platelet assessment |
