Medical Information: Prothrombin Time/International Normalized Ratio

Prothrombin time and international normalised ratio are the parameters used to follow up the success of oral anticoagulation therapy. Here you will find a short explanation of the different terms used in the field of anticoagulation monitoring (1, 2).

Prothrombin Time (PT)

Either venous or capillary blood can be used to monitor anticoagulation therapy. To produce a reading, thromboplastin is added to the blood to activate coagulation. This causes a blood clot to form. The time it takes for this clot to form is measured in seconds and is known as the prothrombin time (PT).

In some European countries, blood coagulability was typically expressed in a unit known as the Quick value. In this case the measured prothrombin time is expressed in relation to the coagulation time of a healthy person. The value obtained is the "percentage of the standard value". In a person not receiving oral anticoagulation the "normal" Quick value is between 80 and 120%. A Quick value of only 30%, for instance, indicates that the blood coagulation time is longer than normal. The longer the patient's coagulation time, the lower the Quick value.

International Sensitivity Index (ISI)

Quick values measured with different thromboplastins cannot be directly compared with one another. To render coagulation times as comparable as possible, in 1983 the World Health Organization (WHO) approved a standard thromboplastin. Every manufacturer of thromboplastin must calibrate it against the WHO standard (2 references exist: one for human recombinant based thromboplastins, one for rabbit brain based ones). The value obtained is known as the International Sensitivity Index (ISI). This enables the various sensitivities of the thromboplastins to be ascertained.

International Normalised Ratio (INR)

As the Quick/PT value is very country- and laboratory- dependent, nowadays the value mostly measured is the INR value. It is the globally recommended unit for measuring thromboplastin time. INR makes coagulation measurements extensively comparable despite the numbers of different thromboplastins used.

INR = (Patient's PT / Normal mean PT) ^ISI

For example: The PT of a patient receiving oral anticoagulant is 64 seconds (= 18% Quick). The prothrombin time of a normal plasma is 22 seconds (= 100% Quick). The ISI of the thromboplastin used is 0.93. Substituting this value in the formula above gives the following INR:

(64) / (22) 0.93 = 2.7 INR

This signifies a coagulation time that is 2.7 times longer than the standard. The longer the patient's coagulation time, the higher the INR.

Initially, the tests are frequently needed, as - depending on the patient - the adjustment requires some time. Travelling to the healthcare professional or laboratory means time off for active people, less independence and a disruption of daily routine. Later, when the patient is properly adjusted and stabilized, PT/INR monitoring is commonly done at intervals of three to four weeks.

Statements based on references

1. Lutze G. Useful facts about coagulation. Mannheim: Roche Diagnostics GmbH, 2000. (data on file)
   
2. Wagner C, Dati F. Thromboplastinzeit (german article). In: Thomas L, ed. Labor und Diagnose: TH-Books Verlagsgesellschaft, 2000; 613-616.
   

Please do not hesitate to contact your local Roche representative if you need further information or references.