The PT is a screening test for the extrinsic clotting system, i.e. factor VII. It will also detect deficiencies of factors, prothrombin, V, X, and fibrinogen. It is mainly used to monitor patients receiving warfarin
anticoagulation.
Principle of the test
Plasma or capillary blood is added to a thromboplastin and calcium chloride reagent at 37°C and the time taken for a clot
to form is measured. The clotting time in seconds is converted to the International Normalized Ratio (INR), usually by reference to a table provided by the manufacturer of the reagent OR by the formula:
INR= (PT PATIENT/PT CONTROL)^ISI
ISI is provided by reagent manufacturer
Reagents
Thromboplastin calcium chloride combined reagent.
Several different thromboplastin calcium combined reagents are available depending on the source of the thromboplastin and whether the test is to be performed using plasma or capillary whole blood. Some manufacturers supply a thromboplastin calcium reagent that can be used with both capillary blood and plasma.
Capillary blood PT testing:
This avoids the need to collect venous blood when monitoring patients being treated with warfarin. It should not be used
however when a patient is anaemic or polycythaemic. Plasma should then be used.
See also
Lablink 