The assessment of postoperative cognitive function

Postoperative cognitive function (POCD) has been subject to extensive resea rch. In the literature, large differences are apparent in methodology such as the test batteries, the interval between sessions, the endpoints to be a nalysed, statistical methods, and how neuropsychological deficits are defin ed. Traditionally, intelligence tests or tests developed for clinical neuro psychology have been used. The tests for detecting POCD should be based on well-described sensitivity and suitability in relation to surgical patients . In tests using scores, floor/ceiling effects may compromise the evaluatio n if the tests are either too easy or to difficult. Uncontrolled testing fa cilities and change of test personnel may affect the test performance. Prac tice effects are pronounced in neuropsychological tests but have generally been ignored. The use of a suitable normative population is essential to al low correction for practice effects and variability between sessions. Missi ng follow-up may severely compromise valid conclusions since subjects unabl e or unwilling to be examined are particularly prone to suffer from POCD. I n the statistical analysis of the test results, the evaluation should be ba sed on differences between pre- and postoperative performance. Parametric s tatistical tests are not relevant unless the appropriate Gaussian distribut ions are present, perhaps after transformation of data. The definition of c ognitive dysfunction should be restrictive and the criteria should be fulfi lled in only a small proportion of volunteers. In the literature, these req uirements often have not been fulfilled. This precludes a reasonable estima tion of the incidence of POCD and the conclusions of comparative studies sh ould be interpreted with great caution. In this review article, we present a number of recommendations for the design and execution of studies within this area. In addition, the critical reader may use these recommendations i n the evaluation of the literature.