Definitions and methods of cost assessment: an intensivist's guide. ESICM section on health research and outcome working group on cost effectiveness.

OBJECTIVE: To define the different types of costs incurred in the care of critically ill patients and to describe some of the most commonly used methods for measuring and allocating these costs.
DESIGN: Literature review. Definitions for opportunity, direct and indirect, fixed, variable, marginal, and total costs are described and interpreted in the context of the critical care setting. Two main methods of costing are described: the 'top-down' and 'bottom-up' methods together with a number of cost proxies, such as the use of weighted hospital days, diagnosis-related groups, severity and activity scores, and effective costs per survivor.
CONCLUSIONS: The assessment and allocation of costs to critically ill patients is complex and as a result of the different definitions and methods used, meaningful comparisons between studies are plagued with difficulty. When undertaking a study looking to measure costs, it is important to state: (a) the aim of the cost assessment study; (b) the perspective (point of view); (c) the type of costs that need to be measured; and (d) the time span of assessment. By being explicit about the rationale of the study and the methods used, it is hoped that the results of economic evaluations will be better understood, and hence implemented within the critical care setting.