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deamino-8-D-arginina vasopresina (DDAVP) sobre el tiempo de sangria en pacientes uremicos. Rev Med Chil 113:763-768, 1985. 654. Alfonsi A, SO Schwartz, D Silver: What routine preoperative tests for bleeding tendencies? J A M A 236:2547, 1976. Letter and responses. 655. Anonymous: The bleeding-time and the haematocrit. Lancet 1:997-998,1984. 656. Austen DEG, IL Rhymes: Laboratory diagnosis of blood coagulation disorders. In: Biggs R, CR Rizza, Eds: Human Blood Coagulation, Haemostasis and Thrombosis, Blackwell Scientific Publications, Oxford. 1984, pp 170-171. 3rd edition. 657. Åberg M, U Hedner, S-E Bergentz: The antithrombotic effect of dextran. Scand J Haematol Suppl. 34:61-68, 1979. 658. Bachmann F, R McKenna, ER Cole, H Najafi: The hemostatic mechanism after open-heart surgery. I. Studies on plasma coagulation factors and fibrinolysis in 512 patients after extracorporeal circulation. J Thorac Cardiovasc Surg 70:76-85, 1975. 659. Bachmann F: Diagnostic approach to mild bleeding disorders. Dent, ZM Ruggeri, TS Zimmerman: The heterogeneity of type IIA von Willebrand's disease: studies with protease inhibitors. Blood 68:1207-1212, 1986. 663. Bergqvist D, S Arvidsson, CO Esquivel, B Lindblad, U Haglund: The effect of serotonin inhibition on initial microvessel hemostasis and platelet aggregation in vivo. Thromb Haemost 49:173-175, 1983. 664. Bergqvist D: Dextran and haemostasis. Acta Chir Scand 148:633-640, 1982. 665. Bloom AL, IR Peake, RA Furlong, BL Davies: High potency Factor VIII concentrate: more effective than cryoprecipitate in a patient with von Willebrand's disease and inhibitor. Thromb Res 16:847-852, 1979. 666. Bloom AL: The von Willebrand syndrome. Semin Hematol 17:215-227, 1980. 667. Bowie EJ, CA Owen: The significance of abnormal preoperative hemostatic tests. In: Spaet TH, Ed: Progress in Hemostasis and Thrombosis, vol. 5. Grune and Stratton, New York. 1980, pp 179-209. 668. Bowie EJ, CA Owen: Abnormalities of Factor VIII. In: Triplett DA, Ed: Laboratory Evaluation of Coagulation, American Society of Clinical Pathologists Press, Chicago. 1982, pp 115-142. 669. Bowie EJW, CA Owen: The bleeding time. In: Spaet TH, Ed: Progress in Hemostasis and Thrombosis, vol. 2. Grune and Stratton, New York. 1974, pp 249-271. 670. Bowie EJW, CA Owen: Standardization of the bleeding time. Scand J Haematol 37 (Suppl.):87-94, 1980.

To the Editor: attention this review brings to an important clinical problem is most welcome. However, the reader may be misled in certain respects. Unfortunately, Lind did not distinguish between our own meta-analytical We noted with interest the appearance of Lind's review concerning the bleeding time, appearing in a recent issue.' The added

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Quality assurance testing represents a substantial proportion of the clinical laboratory budget, but current guidelines are based on criteria that pertain to analytic error rather than to optimization of the cost-effectiveness of patient care. A general Bayesian mathematical model for the cost-effectiveness of assay quality control has been developed, and is demonstrated using previously published data. The cost-effectiveness of quality assurance as defined here depends upon the prevalence of disease, the shapes of the distributions of test results observed in the non-diseased and diseased populations, the decision limit selected for labeling results positive or negative, the costs and benefits associated with each of the possible therapeutic outcomes, the magnitude of random and systematic analytical errors, the statistical power of the quality control test in use, the costs associated with delays due to re-assay, and the proportion of total test cost attributable to quality control procedures. Given current clinical laboratory practice, much of this information will not be routinely available. The model combines these factors into a simple equation with three terms: one for the cost of the original and any required repeat laboratory analyses, one for the cost of delay entailed by the rejection of an assay batch, and one for the change in total costs consequent to rejection of erroneous assay results.In the mid 1970s, the proportion of medical care expense in the United States directly attributable to laboratory testing was 8%, disregarding any additional costs that may have resulted from inappropriate therapy administered on the basis of misleading results.'3 Laboratory testing thus represents a major component of total health care cost. Recent studies have compared the effects of clinical laboratory testing and other new diagnostic technologies on the rate of rise of health care costs.6~14,zoThe proportion of laboratory expense attributable to quality control, estimated to be between 30 and 40%, has remained constant in the face of rapidly increasing laboratory utilization over the past three decades. In 1982, the total annual cost of clinical laboratory quality control in the United States was estimated to be between 6 and 8 billion dollars.22 Guidelines for optimizing the cost-effectiveness of quality control would clearly be of significant economic importance.Despite the substantial economic impact of quality control, no attempt has yet been made to develop a formal analysis of the cost-effectiveness of laboratory quality control, probably at least partly because of the fragmented nature of medical economics. The laboratory is generally concerned with the costs of its immediate operations 12 and is isolated from the longterm costs consequent to the utilization of laboratory results. Legal and voluntary professional regulations require laboratories to comply with arbitrary quality control guidelines which are generally based upon consideration of analytic error rather than clinical impact.21Clinical laborato...

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