Pathology (1979), 11, pp. 1-3
EDITORIAL
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
THE CONTROL OF ORAL ANTICOAGULANT THERAPY W. R. PITNEY School of Medicine, University of New South Wales
The oral anticoagulants, warfarin and phenindione, produce hypocoagulability by interfering with the action of vitamin K in the hepatic synthesis of coagulation Factors 11, VII, IX and X. Vitamin K facilitates gamma carboxylation of glutamic acid in the precursor molecules of these four factors’ and y carboxyglutamic acid is the binding site for Ca ions which are necessary for the conversion of the inactive proteins to serine proteases during physiological blood clotting. Oral anticoagulant therapy is associated with the presence in the blood of biologically inactive proteins with antigenic determinants related to Factors 11, VII, IX and X; they inhibit prothrombin conversion in test systems2 and may increase the anticoagulant effect of the drug in vivo (the PIVKA effect). It is essential to monitor the degree of hypocoagulability induced by the oral anticoagulants in order to prevent haemorrhage from overdosage and to ensure that satisfactory anticoagulation has been achieved. Most laboratories in Australasia use a modification of the Quick one-stage prothrombin time for this purpose3, a test which is deceptively simple to perform but often difficult to interpret. Problems with the one-stage test include the reporting procedure, the standardization of the test and a definition of the therapeutic range. The British Committee for Standards in Haematology4 recommended that results should be reported in the form of a ratio obtained by dividing the patient’s prothrombin time in seconds by the prothrombin time of a normal plasma pool. Thus a ratio of 2.0 would be reported if the prothrombin time of the patient’s plasma was 24 seconds and that of the control was 12 seconds. Other methods of reporting, which are not recommended, include calculating percentage ‘prothrombin’ from a saline or adsorbed plasma dilution curve prepared from normal plasma or using the prothrombin index, which is the reciprocal of the ratio reported as a percentage. These two other methods are likely to cause confusion as both report a percentage. In the example cited above, the ‘prothrombin’ concentration from a saline dilution curve would be about 25% while the prothrombin index would be 50%. The ratio method of reporting has not been adopted by all laboratories in Australasia despite acceptance as a satisfactory method by the International Committee for Standardization in Haematology . The prothrombin time is influenced by the type of thromboplastin used in the test. If the test is performed in duplicate on the same plasma but using two different sources of
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
2
PITNEY
Pathology (1979), 11, January
thromboplastin, the clotting times and the ratios will differ. Thromboplastins in general use are either ‘home-made’ from human brain or are commercially-produced from rabbit brain, some with added calcium. In general, rabbit brain thromboplastin is not as sensitive to Factor VII deficiency and to the PIVKA effect as is human brain. These ‘fast’ rabbit thromboplastins give shorter prothrombin times and lower ratios than do human brain thromboplastins for patients treated with oral anticoagulants. A further problem is the batch-to-batch variation in sensitivity which may occur with either human or rabbit thromboplastin. For some time now, a phenol-saline suspension of human brain has been available as a reference thromboplastin from the Division of Haematology, The Prince of Wales Hospital, Randwick, N.S.W. 2031. This material is produced in large volumes without significant variation in sensitivity between batches. If the supply was large enough, the ideal procedure would be for every laboratory in Australasia to use the reference thromboplastin as a working reagent. Since this is not practical, the material is designated the Australasian Reference Thromboplastin and it is intended to be used as a reference against which laboratories may calibrate their own human or rabbit-brain material. Calibration is performed by the method of comparative ratio testing and depends on the principle that a straight-line relationship is found to exist between the ratios obtained when a number of plasma samples from patients on oral anticoagulants are tested with two thromboplastins. Irrespective of the ratio obtained with the laboratory’s working reagent, it is desirable to report the result as a corrected ratio which makes allowance for the difference in sensitivity between the working thromboplastin and the reference reagent. The procedure has been described in standard laboratory manuals’ and details are available from The Prince of Wales Hospital. The Australasian Reference Thromboplastin, being a batchproduced liquid suspension. falls short of the criteria required for a thromboplastin standard. The material does lose potency on storage and each batch is tested against the previous batch, implying the possibility of a drift in potency. There is continuing international collaboration to produce freeze-dried human or rabbit thromboplastins with stable potency even on prolonged storage. Such freeze-dried material could serve as a primary standard against which the reference thromboplastin could be tested. A therapeutic range only has value if the type of thromboplastin is specified. Using the British Reference Thromboplastin, stated to be identical in potency to the Australasian reagent, a range of 2.0 to 4.0 has been suggested? Such a range indicates that at ratios below 2.0 oral anticoagulation is unlikely to be effective and above 4.0, the risk of bleeding is considerable. It must be stressed, however, that the degree of anticoagulation to aim for within this range depends upon clinical considerations. Patients at high risk from thromboembolic disease should be maintained at ratios which are higher than those for patients with relative contraindications to anticoagulation. In this issue of Pathology (pp. 39-44), Lam-Po-Tang and Starr report their experience with a modification of the one-stage prothrombin time test using capillary blood. There was good correlation between the capillary method and the standard test and the advantages of the former include avoidance of venepuncture, less equipment and rapidity in reporting the result. Similar problems of standardization, reporting and therapeutic range apply to both methods. Some well-known laboratories in Australasia use Thrombotest7 for control of oral anticoagulant therapy. This is a commercial reagent which is lyophilized and dispensed under vacuum in sealed containers. Batches of reagent are subjected to rigid quality control and there is little batch-to-batch variability. Thrombotest is simple to perform using either
3 capillary blood or whole, anticoagulated venous blood, and the capillary test is almost as rapid as the micromethod of Lam-Po-Tang and Starr. Clotting activity is recorded as a percentage from a graph supplied with each batch of material. The preferred therapeutic range is 8-12% with limits of 5-15%, the degree of anticoagulation in any particular patient again being influenced by clinical considerations. Disadvantages of Thrombotest are that it is an imported material, the accuracy of the calibration graph supplied with each batch by the manufacturer cannot be verified and the result is recorded as percentage coagulant activity, a term which is unable to be defined adequately. It must be remembered, too, that Thrombotest is used solely for the control of anticoagulant therapy and it is not suitable for the investigation of patients with disorders of haemostasis. Irrespective of the particular method for control of anticoagulant therapy employed in any laboratory, it is essential that reagents be dependable, standard techniques are used exactly as described and the result is given in a form which avoids any possibility of confusion to either clinician or patient.
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
EDITORIAL
References 1. STENFLO, J. & SUTTIE,J. W. (1977): Annu. Rev. Biochern. 46, 157. H. C., VELTKAMP, J. J. & LOELIGER, E. A. 2. HEMKER, (1968): Thrornb.Diathes.Haernorrh. (Stuttg.).19, 346. 3. LAM-PO-TANG, P. R. L. C. & POLLER,L. (1975): Thromb. Diathes. Haernorrh. (Stuttg.). 34, 419. 4. POLLER,L. (1970): The British comparative thromboplastin. The use of the national thromboplastin reagent for uniformity of laboratory control of oral anticoagulants and expression of results. Asso. Clin. Pathol. Broadsheet No. 71.
5 . DACIE,J. V. & LEWIS,S . M. (1975): Practical Haernatology, 5th ed. Churchill Livingstone, London. p. 406. 6. BLACKBURN, E. K. & SHINTON,N. K. (1978): Lancet. 1, 615. 7. OWREN,P. A. (1959): Lancet. 2, 754.
EDITORIAL
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
THE CONTROL OF ORAL ANTICOAGULANT THERAPY W. R. PITNEY School of Medicine, University of New South Wales
The oral anticoagulants, warfarin and phenindione, produce hypocoagulability by interfering with the action of vitamin K in the hepatic synthesis of coagulation Factors 11, VII, IX and X. Vitamin K facilitates gamma carboxylation of glutamic acid in the precursor molecules of these four factors’ and y carboxyglutamic acid is the binding site for Ca ions which are necessary for the conversion of the inactive proteins to serine proteases during physiological blood clotting. Oral anticoagulant therapy is associated with the presence in the blood of biologically inactive proteins with antigenic determinants related to Factors 11, VII, IX and X; they inhibit prothrombin conversion in test systems2 and may increase the anticoagulant effect of the drug in vivo (the PIVKA effect). It is essential to monitor the degree of hypocoagulability induced by the oral anticoagulants in order to prevent haemorrhage from overdosage and to ensure that satisfactory anticoagulation has been achieved. Most laboratories in Australasia use a modification of the Quick one-stage prothrombin time for this purpose3, a test which is deceptively simple to perform but often difficult to interpret. Problems with the one-stage test include the reporting procedure, the standardization of the test and a definition of the therapeutic range. The British Committee for Standards in Haematology4 recommended that results should be reported in the form of a ratio obtained by dividing the patient’s prothrombin time in seconds by the prothrombin time of a normal plasma pool. Thus a ratio of 2.0 would be reported if the prothrombin time of the patient’s plasma was 24 seconds and that of the control was 12 seconds. Other methods of reporting, which are not recommended, include calculating percentage ‘prothrombin’ from a saline or adsorbed plasma dilution curve prepared from normal plasma or using the prothrombin index, which is the reciprocal of the ratio reported as a percentage. These two other methods are likely to cause confusion as both report a percentage. In the example cited above, the ‘prothrombin’ concentration from a saline dilution curve would be about 25% while the prothrombin index would be 50%. The ratio method of reporting has not been adopted by all laboratories in Australasia despite acceptance as a satisfactory method by the International Committee for Standardization in Haematology . The prothrombin time is influenced by the type of thromboplastin used in the test. If the test is performed in duplicate on the same plasma but using two different sources of
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
2
PITNEY
Pathology (1979), 11, January
thromboplastin, the clotting times and the ratios will differ. Thromboplastins in general use are either ‘home-made’ from human brain or are commercially-produced from rabbit brain, some with added calcium. In general, rabbit brain thromboplastin is not as sensitive to Factor VII deficiency and to the PIVKA effect as is human brain. These ‘fast’ rabbit thromboplastins give shorter prothrombin times and lower ratios than do human brain thromboplastins for patients treated with oral anticoagulants. A further problem is the batch-to-batch variation in sensitivity which may occur with either human or rabbit thromboplastin. For some time now, a phenol-saline suspension of human brain has been available as a reference thromboplastin from the Division of Haematology, The Prince of Wales Hospital, Randwick, N.S.W. 2031. This material is produced in large volumes without significant variation in sensitivity between batches. If the supply was large enough, the ideal procedure would be for every laboratory in Australasia to use the reference thromboplastin as a working reagent. Since this is not practical, the material is designated the Australasian Reference Thromboplastin and it is intended to be used as a reference against which laboratories may calibrate their own human or rabbit-brain material. Calibration is performed by the method of comparative ratio testing and depends on the principle that a straight-line relationship is found to exist between the ratios obtained when a number of plasma samples from patients on oral anticoagulants are tested with two thromboplastins. Irrespective of the ratio obtained with the laboratory’s working reagent, it is desirable to report the result as a corrected ratio which makes allowance for the difference in sensitivity between the working thromboplastin and the reference reagent. The procedure has been described in standard laboratory manuals’ and details are available from The Prince of Wales Hospital. The Australasian Reference Thromboplastin, being a batchproduced liquid suspension. falls short of the criteria required for a thromboplastin standard. The material does lose potency on storage and each batch is tested against the previous batch, implying the possibility of a drift in potency. There is continuing international collaboration to produce freeze-dried human or rabbit thromboplastins with stable potency even on prolonged storage. Such freeze-dried material could serve as a primary standard against which the reference thromboplastin could be tested. A therapeutic range only has value if the type of thromboplastin is specified. Using the British Reference Thromboplastin, stated to be identical in potency to the Australasian reagent, a range of 2.0 to 4.0 has been suggested? Such a range indicates that at ratios below 2.0 oral anticoagulation is unlikely to be effective and above 4.0, the risk of bleeding is considerable. It must be stressed, however, that the degree of anticoagulation to aim for within this range depends upon clinical considerations. Patients at high risk from thromboembolic disease should be maintained at ratios which are higher than those for patients with relative contraindications to anticoagulation. In this issue of Pathology (pp. 39-44), Lam-Po-Tang and Starr report their experience with a modification of the one-stage prothrombin time test using capillary blood. There was good correlation between the capillary method and the standard test and the advantages of the former include avoidance of venepuncture, less equipment and rapidity in reporting the result. Similar problems of standardization, reporting and therapeutic range apply to both methods. Some well-known laboratories in Australasia use Thrombotest7 for control of oral anticoagulant therapy. This is a commercial reagent which is lyophilized and dispensed under vacuum in sealed containers. Batches of reagent are subjected to rigid quality control and there is little batch-to-batch variability. Thrombotest is simple to perform using either
3 capillary blood or whole, anticoagulated venous blood, and the capillary test is almost as rapid as the micromethod of Lam-Po-Tang and Starr. Clotting activity is recorded as a percentage from a graph supplied with each batch of material. The preferred therapeutic range is 8-12% with limits of 5-15%, the degree of anticoagulation in any particular patient again being influenced by clinical considerations. Disadvantages of Thrombotest are that it is an imported material, the accuracy of the calibration graph supplied with each batch by the manufacturer cannot be verified and the result is recorded as percentage coagulant activity, a term which is unable to be defined adequately. It must be remembered, too, that Thrombotest is used solely for the control of anticoagulant therapy and it is not suitable for the investigation of patients with disorders of haemostasis. Irrespective of the particular method for control of anticoagulant therapy employed in any laboratory, it is essential that reagents be dependable, standard techniques are used exactly as described and the result is given in a form which avoids any possibility of confusion to either clinician or patient.
Pathology Downloaded from informahealthcare.com by University of Otago on 01/10/15 For personal use only.
EDITORIAL
References 1. STENFLO, J. & SUTTIE,J. W. (1977): Annu. Rev. Biochern. 46, 157. H. C., VELTKAMP, J. J. & LOELIGER, E. A. 2. HEMKER, (1968): Thrornb.Diathes.Haernorrh. (Stuttg.).19, 346. 3. LAM-PO-TANG, P. R. L. C. & POLLER,L. (1975): Thromb. Diathes. Haernorrh. (Stuttg.). 34, 419. 4. POLLER,L. (1970): The British comparative thromboplastin. The use of the national thromboplastin reagent for uniformity of laboratory control of oral anticoagulants and expression of results. Asso. Clin. Pathol. Broadsheet No. 71.
5 . DACIE,J. V. & LEWIS,S . M. (1975): Practical Haernatology, 5th ed. Churchill Livingstone, London. p. 406. 6. BLACKBURN, E. K. & SHINTON,N. K. (1978): Lancet. 1, 615. 7. OWREN,P. A. (1959): Lancet. 2, 754.
