Coagulation Studies Flashcards
What is the Prothrombin time used to evaluate and what are the normal values?
Used to evaluate the adequacy of the extrinsic system and common pathway in the clotting mechanism
Normal PT = 11.0 -12.5 sec.
Normal INR = 0.8 – 1.1
Full anticoagulation therapy > 1.5 -2 times the control value
What is the first reaction with active bleeding?
First reaction to bleeding is blood vessel constriction
What does the primary phase of clotting involve?
involves platelet aggregation to the blood vessel which activates the 2nd phase of clotting
Name the four pathways in secondary hemostasis?
First Reaction - Intrinsic Pathway
Second Reaction - Extrinsic Pathway
Third Reaction - Common Pathway
Fourth Reaction – Clot Formation
What is the process of the intrinsic pathway?
Factor XII and other proteins form a complex on the subendothelial collagen in the injured blood vessel.
Activated factor XI is formed and activates factor IX. In a complex formed by factors VIII, IX, and X, activated factor X is formed.
What is the process of the extrinsic pathway?
At the same time as the intrinsic pathway is occurring, the extrinsic system is activated. A complex is formed between tissue thromboplastin (factor III) and factor VII resulting in activated factor VII.
This can directly activate factor X or can activate factors IX and X together.
What is the process of the common pathway?
Factor X is activated by proteases formed by the two prior reactions and by activated factor IX.
This links the intrinsic and extrinsic pathways.
What is the process of clot formation?
Prothrombin is converted to thrombin by activated factor X in the presence of factor V, phospholipid, and calcium.
Thrombin converts fibrinogen to fibrin in clot stabilization but also stimulates platelet aggregation and activates factors V, VIII, and XIII.
Once fibrin is formed it polymerizes into a stable gel, and factor XIII crosslinks the fibrin polymers to form a stable clot.
Almost immediately three major activators of the fibrinolytic system act on plasminogen which was absorbed into the clot to form plasmin.
Plasmin degenerates the fibrin into fragments which are cleared by macrophages.
What does the PT/INR measure?
PT measures the clotting ability of factors I (fibrinogen), II (prothrombin), V, VII, and X – the extrinsic and common pathways.
When these clotting factors are deficient, the PT/INR is prolonged
What causes decreased level of Factors I, II, V, VII, X
Obstructive biliary diosease, hepatocellular disease, coumadin (interferes with production of clotting factors)
Obstructive biliary disease (bile duct obstruction due to tumor or gallstones, or intrahepatic cholestasis due to sepsis or drugs) – obstruction causes bile required for fat absorption to not enter the gut so fat malabsorption occurs. If vitamin K is given, the PT will normalize in obstructive biliary disease but not in hepatocellular disease as the liver cells are still incapable of synthesizing the clotting factors no matter how much vitamin K is present.
How is coumadin therapy monitored?
Therapy is monitored by following the patient’s PT/INR.
PT results are given in seconds along with a control value
The control varies from day to day because the reagents used vary. To have uniform results, the PT now includes the use of the International Normalized Ratio (INR), which is independent of the reagents or methods used.
Preffered Internation normalized ratio (INR) values
Indication Preferred INRDVT Prophylaxis 1.5-2.0 Orthopedic surgery 2.0-3.0 DVT 2.0-3.0 Atrial fibrillation 2.0-3.0 Pulmonary embolus 2.5-3.5 Prosthetic valve prophylaxis 3.0-4.0
PT/INR interfering factors?
Alcohol – prolongs PT, diminishes liver function. Lesser quantities of factors cause prolonged PT
A diet high in fat or leafy vegetables may shorten the PT. Vitamin K absorption is enhanced so factors are made at increased levels.
Diarrhea or malabsorption syndromes can prolong PT, as vitamin K is malabsorbed
Drugs may increase or decrease the PT and also may interact with coumadin
Causes of Prolonged PT/INR?
Liver disease (cirrhosis, hepatitis) – factor synthesis is decreased Hereditary factor deficiency Vitamin K deficiency Bile duct obstruction Coumadin ingestion Disseminated intravascular coagulation (DIC) – factors are consumed Massive blood transfusion – banked blood has anticoagulant, factors are diluted out of the banked blood Salicylate intoxication
What is the Partial Thromboplastin time (PTT) and what is the normal range?
Used to assess the intrinsic pathway and the common pathway of clot formation
Also used to monitor heparin therapy
Normal PTT = 60-70 seconds
Patients receiving anticoagulant therapy: 1.5-2.5 times the control value in seconds
