Prothrombin Time (PT)

by: Joe Hing Kwok Chu

Reference Range      Normal range: 13.2~17 seconds; warfarin (e.g. Coumadin) therapy: 2 to 3 X normal

This is a test that measures the time required for a fibrin clot to form after the addition of a source of tissue extract (thromboplastin) and calcium ions to the patient's citrated plasma.

Three  (factors II, VII and X) of the five factors (factors V,VII,X, II and fibrinogen) are vitamin K-dependent and  are depressed by warfarin-like drugs. Thus the PT is the most commonly used test for warfarin (Coumadin) administration.

If the patient has received a subcutaneous heparin injection, it is necessary to wait 6 hours for the effects to disappear before performing a PT.

Laboratory tests can differ because of different reporting methods. The result of the tests can be reported in terms of seconds, ratios or percentage activity.⁽¹⁾  Sources of tissue thromboplastins used can be different: human brain, rabbit brain, bovine brain, and others and prepared by different methods. These may result in their varied sensitivities to the reduction of the vitamin K-dependent clotting factors⁽²⁾.  For example, rabbit brain thromboplastins, commonly used in North America, are generally less sensitive than the human brain thromboplastins more commonly used in European and U.K. laboratories. ⁽³⁾

In the early stages of Coumadin (warfarin) administration, the test will not reflect the effectiveness of warfarin accurately until the three vitamin K-dependent factors that have been formed earlier have been depleted. Thus heparin can be used during the early period of using warfarin (e.g.Coumadin) based on the results of PT tests. 

Bibliography:

Chen Xin Qian, Jin You Yu et al , New Edition of Pharmacology, 1996; 340  (in Chinese)

(1) Loeliger EA, Poller L, Samama M, Thomson JM, Van den Besselaar AMHP, Vermylen J, Verstraete M. Questions and answers on prothrombin time standardisation in oral anticoagulant control, Thromb Haemost 1985;54:515-517

(2) Hirsh J. Oral anticoagulant drugs. N Engl J Med 1991;324; 1865-1875

(3) Hirsh J. , Poller L, Deykin D, Levine M, Dalen JE,. Optimal therapeutic range for oral anticoagulants. Chest 1989; 95; 5S-11S