Laboratory Tests
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 FBC to see whether or not the patient is anaemic 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 |
|---|---|---|---|
| FBC (Full Blood Count)
|
Counts and evaluates size and shape of platelets, red and white blood cells (WBCs), types of WBCs; measures haemoglobin and haematocrit | 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 |
|
|
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
|
| 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. |
| International Normalised Ratio (INR) | Time to clot: a standardised version of the PT test, the results comparable between laboratories worldwide. | Used specifically to monitor warfarin or other coumarin anticoagulants.
|
The higher the ratio the less the patient is likely to clot but more to bleed.
|
| 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 platelets’ ability to be activated and stick to each other (agglutinate)
|
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 PFA100
|
An automated method to measure platelet function (this is the most widely used; there are also other analyzers)
|
Sometimes used as a pre-surgical 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 test; evaluates the extrinsic and common pathways of coagulation cascade
|
Investigate bleeding or thrombotic episode Pre-surgical screen for risk of excessive bleeding |
Prolonged PT may suggest need for further tests. May be elevated in inherited or acquired conditions.
|
| Ristocetin Cofactor (VWF:RCo)
|
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
VWF: Collagen binding test is often requested as an additional measure of VWF function |
| 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 (VWF: Ag)
|
Quantitative measure of VWF
|
Done together with VWF activity test (e.g. as measured by Ristocetin Cofactor); evaluate bleeding episodes | If low, may indicate von Willebrand disease, increased risk of bleeding
|
