So what I’m hearing is that, from a statistical standpoint, if something that we can measure (e.g., serum creatinine) contributes to prognosis, then we should try to use as much of the information we have about that parameter as possible when we’re studying prognosis. Since the definition of AKI includes a criterion that effectively “dichotomizes” this parameter (i.e., increase >1.5x baseline or not), the AKI definition is not ideal for the purpose of prognostication. For example, two patients, one of whose creatinine increases from 100 to 300 and the other whose creatinine increases from 100 to 600 over a certain number of days may have very different prognoses. Therefore, lumping these two patients together under the “AKI” heading in a study examining AKI prognosis will obscure this important prognostic difference. Furthermore, since two patients who experience a similar increase in creatinine over a similar short time frame may have very different prognoses depending on the underlying cause of the change, the “rate of change” criterion also might not be optimal for prognostication.
Coming back to the AKI definition itself, I guess “validity” here would refer to whether the three criteria above accurately identify patients who have sustained more than a certain amount of organ “damage” over a relatively short period of time. Even though I can see why using the AKI definition above could be problematic in a study examining prognosis, having some general consensus among physicians around what constitutes an abrupt and significant increase in creatinine can be helpful clinically with regard to differential diagnosis.