Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1127 page)

   Cases with an increased fibrinolytic tendency (bleeding, rapid dissolution of hemostatic clots)

Increased Values

   May result in a tendency to arterial or venous thrombosis

   Acquired: during acute thrombotic episodes, pregnancy, sepsis

   Congenital: rare congenital elevations have been described

   Limitations

   This test is a biologic assay that is difficult to perform reproducibly.

   The inhibitor has diurnal variations, with highest levels during morning hours (blood should be drawn fasting between 8
AM
and 12
PM
).

PLATELET AGGREGATION
^†

   Definition

   Platelets participate in primary hemostasis by forming aggregates at the site of injury. In vivo the platelets are stimulated by chemical substances called agonists or by interaction with damaged endothelial surfaces in the presence of von Willebrand factor and collagen. These properties are used in vitro to study the change in optical density as the platelets aggregate under the effect of added agonists (ADP, collagen, epinephrine, arachidonic acid, thrombin). Ristocetin is used to assess binding to von Willebrand factor, as reflected in platelets’ agglutination. The aggregometers are photo optical instruments that require platelet-rich plasma. The more advanced equipment can use whole blood and can also assess ATP release by chemiluminescence methodology, thereby better determining platelet functionality.

   
Normal range:
decrease in optical density of ≥65% (represented by graphs waves generated by the aggregometer). The results are also interpreted in relation to the role of each agonist in platelet physiology. The normal response to various agonists of ATP release in chemiluminescence assays is measured in nanomoles and reported as normal or abnormal.

   Use

   Platelet aggregation studies are indicated in patients with a bleeding diathesis, especially mucocutaneous bleeding (but without acquired thrombocytopenia), when a platelet defect or von Willebrand disease is suspected. By varying the amount of the ristocetin reagent, subtype 2B or platelet type of von Willebrand disease can be diagnosed preliminarily.