Diagnostic devices do more than just passively register facts. They intervene in the situations in which they are put to use. The question addressed here is what this general remark may imply in specific cases. To answer this question a specific case is being analysed: that of the blood sugar measurement device that people with diabetes may use to monitor their own blood sugar levels. This device not only allows the patients concerned to better approach normal blood sugar levels, but alters what counts as normal in the first place. Using the device may shift people's attention away from their physical sensations towards the numbers measured, but it may also help them to increase their own physical self-awareness. Self-monitoring finally (something that the devices have made possible) makes patients less dependent on professionals, but it requires them to engage in self-disciplining and binds them to the outcomes of their measurement activities: their own blood sugar levels.