When performing laboratory tests to evaluate bleeding disorders, the coagulation cascade can be thought of as having two branches: the extrinsic pathway and the intrinsic pathway. Each of these pathways utilizes different coagulation factors, proteins that are carried in an inactive form in the blood. These factors are sequentially activated down one pathway or the other and come together to complete the clotting process in the common pathway.

Using this approach, bleeding disorder testing is a step-by-step investigative procedure. If someone presents with a bleeding episode, a doctor may order a Prothrombin Time (PT), which evaluates the extrinsic and common pathways, Partial Thromboplastin Time (PTT), which evaluates the intrinsic and common pathways, and a CBC to see whether or not the patient is anemic and to evaluate the number of platelets present. If the PT is prolonged, further testing may be done to identify problems with factors involved in the extrinsic or common pathway. If the PTT is prolonged, then the doctor may follow-up with other testing to look for specific factor deficiencies in the intrinsic or common pathway and to see whether or not there may be factor inhibitors.

Some of the tests that may be ordered include:

Test	Measures	Ordered When/To	Abnormal Results May Indicate
CBC (Complete Blood Count)	Counts and evaluates size and shape of platelets, red and white blood cells (WBCs), types of WBCs; measures hemoglobin and hematocrit	Ordered as a routine screen, to check for any abnormalities	Decreased platelet numbers increase bleeding tendency
Coagulation Factors, Activity	Individual tests to measure the function of specific coagulation factors	Evaluate bleeding episodes	Decreased activity of one or more factors may increase risk of bleeding
Coagulation Factors, Antigen	Measures the quantity of individual factors	When factor activity is consistently low	Decreased production or increased use of one or more factors, increased risk of bleeding
D-dimer	Measures a specific type of cross-linked fibrin degradation	Evaluate blood clot formation during bleeding and clotting episodes	If elevated, indicates recent clotting activity may be due to acute or chronic condition, such as a thromboembolism or disseminated intravascular coagulation (DIC)
Factor Inhibitors	Individual tests for coagulation factor antibodies	Evaluate excessive bleeding and prolonged PTT	If present, may cause specific factor deficiencies and excessive bleeding
Fibrin degradation Products (FDP)	Reflection of clotting activity and breakdown	Evaluate bleeding and clotting	If increased, indicates recent blood clot formation and breakdown
Fibrinogen	Reflection of clotting ability and activity	Evaluate bleeding and clotting	If low, may indicate decreased production or increased use; may be elevated with infection and inflammation. It is an acute phase reactant.
Partial Thromboplastin Time (PTT)	Time to clot; evaluates the intrinsic and common pathways of coagulation cascade	- Investigate bleeding - Pre-surgical screen for risk of excessive bleeding - Monitor heparin anticoagulant therapy	Prolonged PTT suggests need for further tests. May indicate: - Coagulation factor deficiency - Specific inhibitor (such as Factor VIII antibody) - Nonspecific inhibitor (such as Lupus anticoagulant) - Patient on heparin and/or blood sample contaminated with heparin
Platelet aggregation (Platelet function test)	Evaluate platelet’s ability to adhere and form clumps	Evaluate bleeding, especially when platelet count normal	If abnormal, increases risk of excessive bleeding; may indicate presence of one of several disorders including von Willebrand’s disease
Platelet Function Analyzer	An automated method to measure platelet function (this is the most widely used; there are also other analyzers)	Sometimes used as a presurgical screen or to evaluate recurrent bleeding	Abnormal result may indicate acquired platelet disorder or von Willebrand’s disease. Indicates greater risk of excessive bleeding. This test has largely replaced the Bleeding time test.
Prothrombin Time (PT)	Time to clot tes; evaluates the extrinsic and common pathways of coagulation cascade	Investigate bleeding or thrombotic episode Presurgical screen for risk of excessive bleeding Monitor warfarin (coumadin) anticoagulant therapy	Most common use is monitoring warfarin anticoagulant therapy. Prolonged PT may suggest need for further tests. May be elevated in inherited  or acquired conditions.
Ristocetin Cofactor	Indirect measure of von Willebrand factor (vWF) activity/function	Evaluate bleeding episodes	vWF activity and decreased ability for platelets to adhere to injuries; may be due to von Willebrand’s disease, increased risk of bleeding
Thrombin Time (TT)	Time to clot; thrombin activates fibrinogen to fibrin stands; TT detects presence of inhibitors to this process	Help evaluate bleeding episode; sometimes when PTT prolonged; when heparin contamination suspected	If elevated, heparin may be contaminating blood sample; also elevated with FDP, with very low levels of fibrinogen, and with abnormal fibrinogen
von Willebrand Factor (vWF) Antigen	Quantitative measure of vWF	Done when activity (measured as Risocetin Cofactor) is low; evaluate bleeding episodes	If low, may indicate platelet-related acquired condition or von Willebrand disease, increased risk of bleeding