Prothrombin time

The prothrombin time or PT is an assessment of the extrinsic and common pathways. The PT is the specific and only lab test used to measure the effectiveness of coumarin-type anticoagulant drugs, such as warfarin sodium (Coumadin). The most common methods to report the PT are time (in seconds) and the International Normalized Ratio (INR).

As oral coumadin has a relatively long onset of action, it takes several days of therapy to achieve a therapeutic level. When a patient will be managed on oral coumadin, he or she may be given both heparin and coumadin, and then heparin is discontinued. If a PT is increased beyond a therapeutic level, the coumadin dose may be reduced, or the patient may be given parenteral Vitamin K. Drugs that increase or prolong the PT time include antibiotics, cimetidine (Tagamet), salicylates, and sulfonamides. Barbiturates, oral contraceptives, and Vitamin K in multivitamin preparations or in liquid nutritional supplements decrease the PT time.

When therapy with coumadin is begun, the dose is guided by monitoring the prothrombin time. Therapeutic levels are generally between 1 1/2 and 2 times normal, depending on the patient's need for anticoagulation. Expressed in terms of the INR, the range is between 2.0 and 3.0. A usual dose requirement is between 5 to 7.5 mg. daily. In some patients with coumadin resistance, the dosage may be much higher. If a patient with a prolonged PT must have surgery, it is important that the PT be brought within a normal range before surgery. This is often done with Vitamin K injections. Whole blood or fresh frozen plasma should be available for the surgical patient with an abnormal PT.

Read about issues regarding anticoagulant therapy testing in International Normalized Ratio (INR) (Emergency Nursing World ! Robert C. Knies, RN, MSN, CEN)

Instant Feedback:
The INR is increasingly becoming an important way to measure and report prothrombin time.


RnCeus Homepage | Course catalog | Discount prices | Login | Nursing jobs | Help