[Roaming through methodology. XXV. Outcome measures, surrogate outcomes, and intermediate measures].

Clinical medicine is aimed at decreasing the current or future burden of disease. Disease outcome is best expressed as burden of disease or change thereof, and as such the only true 'outcome measure' in clinical research. However, outcome research is expensive in costs and time expenditure, and sometimes impossible. Therefore study results are often expressed in intermediate measures. These measures tell us something about the disease process and the pathophysiological consequences of the disease and should have a relation with outcome. If this relation is strong, the measure is called 'surrogate outcome'. Intermediate measures and surrogate outcomes have advantages and disadvantages. The reader of clinical trial results first has to decide whether the answer to the study question is relevant in his personal situation. If so, the applicability of a measure can be simply appraised by answering 3 questions: 'Is the measure truthful (relevant, unbiased)?'; 'Does it discriminate between situations that are of interest?'; 'Is the measure feasible in my setting?'