In 1977, the World Health Organisation (WHO) published in its Technical Report Series Proposals for standardization in the control of oral anticoagulation. These proposals are based on extensive clinical and laboratory experience and were prepared under the auspices of the International Committee on Thrombosis and Haemostasis (ICTH) and the International Committee on Standardization in Haematology (ICSH)1). The WHO guidelines cover the conversion, for patients under long-term oral

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... n of any prothrombin time found in patients with any given thromboplastin into a scale that is common to all thromboplastins and all modifications of the prothrombin time (PT) test. The basic assumption here is that (Fig. 1 ) the clotting time ratios found with two different modifications of the prothrombin time test (called here method 1 and method 2) in patients on long-term oral anticoagulant therapy (ratio being the quotient patient time over normal time) display a rectilinear relationship. Extensive experience shows that this basic assumption holds for all modifications of the prothrombin time test investigated thus far. If method 1 referred to in the slide is a well-standardized method using an internationally accepted reference thromboplastin, this forms the common scale, i. e. the WHO pro-