How Much Quality Control is Enough?

Abstract

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...