Coagulation studies: laboratory values aPTT, PT INR, Clotting Time, Platelet Count

 Coagulation studies

Activated partial thromboplastin time(aPTT)

• The aPTT evaluates how well the coagulation sequence is functioning by measuring the amount of time it takes in seconds for calcified citrated plasma to clot after partial thromboplastin is added to it. 
• The test screens for deficiencies and inhibitors of all factors, except VII and XIII. 
• Usually the aPTT is used to monitor heparin therapy and screen for coagulation disorders. 

Value: 20 to 36 seconds, depending on type of activator used. 

Nursing considerations

• If the client is receiving intermittent heparin, draw the blood sample 1 hour before the next scheduled dose. 
• Do not draw samples from an arm in to which heparin is infusing.
• Transport specimen to the laboratory immediately. 
• The aPTT should be between  1.5 and 2.5 times normal when the client is receiving heparin therapy, if the value is prolonged initiate bleeding precautions. 

Prothrombin time (PT) and international normalized ratio (INR)

• Prothrombin is a vitamin K - dependent glycoprotein produced by the liver that is necessary for fibrin clot formation. 
• Each laboratory establishes a normal or control value based on the method used to perform the PT test. 
• The PT measures amount of time it takes in seconds for clot formation and is used to monitor response to warfarin sodium therapy or to screen for dysfunction of the extrinsic clotting system resulting from liver disease, vitamin K deficiency, or disseminated intravascular coagulation. 
• A PT value within 2 seconds ( plus or minus) of the control is considered normal. 
• The INR standardized the PT ratio and is calculated in the laboratory setting by raising the observed PT ratio to the power of the international sensitivity index specific to the thromboplastin reagent used. 
• The INR measures the effects of oral anticoagulants. 

Values 

• PT: 9.6 to 11.8 seconds ( male adult): 9.5 to 11.3 seconds ( female adult ).
• INR: 2.0 to 3.0 for standard warfarin therapy. 
• INR: 3.0 to 4.5 for high dose warfarin therapy. 

Nursing considerations 

• A baseline PT should be drawn before anticoagulation therapy is started, note the time of collection on the laboratory form
• Provide direct pressure to the venipuncture site for 3 to 5 minutes if a coagulation defect is present. 
• Diets high in green leafy vegetables can increase the absorption of vitamin k, which shortens the PT. 
• Orally administered anticoagulation therapy usually maintains the PT at 1.5 to 2 times the laboratory control value.
• A PT longer than 30 second's place the client at risk for bleeding. 

Clotting Time

• The time required for the interaction of all factors involved in the clotting process. 

Value: 8 to 15 minutes 

Nursing considerations 

• The client should not receive heparin therapy for 3 hours before specimen collection because the heparin therapy will affect the result. 
• The test result is falsely prolonged by any anticoagulant therapy, test tube agitation, or exposure of the specimen to high-temperature changes. 

Platelet Count

• Platelet function in hemostatic plug formation, clot retraction, and coagulation factor activation. 
• Platelet are produced by the bone marrow to function in hemostasis. 

Value : 150, 000 to 400, 000 cells/mm3

Nursing considerations 

• Monitor the venipuncture site for bleeding in clients with known thrombocytopenia. 
• High altitudes, chronic cold weather, and exercise increase platelet counts. 
• Bleeding precautions should be instituted in clients with a low platelet count.