Relationship between concentration and anticoagulant effect of heparin in plasma of hospitalized patients: Magnitude and predictability of interindividual differences *

The anticoagulant effect of heparin as reflected by the slope (S) of the relationship between heparin concentration and natural log of activated partial thromboplastin time (APTT) was determined in citrated plasma of 31 hospitalized, 21‐ to 80‐yr‐old patients (including many typical candidates for heparin therapy). Also determined were levels of factors II, V, VII to XII, albumin, individual globulins, calcium, antithrombin III, fibrinogen, α‐1‐acid glycoprotein, α‐1‐antitrypsin, and α‐2‐macroglobulin and prothrombin time and hematocrit. Baseline APTT was 24.1 to 60.3 sec and S was 1.80 to 4.27 ml/U.S correlated with baseline APTT, hematocrit, total protein, functional antithrombin III, prothrombin time, β‐globulin, and factors II, VII, X, XI, and XII. A multiple linear regression equation with baseline APTT, total protein concentration, and factor XI as independent variables was “best” for predicting the S of these patients (r = 0.807, P < 0.0001). A multiple linear regression equation with baseline APTT and hematocrit as independent variables, obtained in a previous study on healthy subjects, overpredicted the patients' S values. An equation with baseline APTT and γ‐globulin as independent variables yielded the best correlation between predicted and actual S values for the combined groups of patients and normal subjects (r = 0.715, P < 0.0001). Our observations indicate that it may be possible to predict the heparin concentration‐anticoagulant effect (APTT) relationship for individual patients before institution of heparin therapy. Clinical Pharmacology and Therapeutics (1982) 32 , 503–516; doi: 10.1038/clpt.1982.195