There are no data to substantiate that patients must have any particular test once/day. Nonetheless, the practice of getting laboratories twice/day on all critically ill patients is not only a waste of money, but it also subjects patients to risk through repeated, unnecessary phlebotomy. Accordingly, the available data suggest that patients only require laboratories as they are needed. In most ICUs, a CBC and electrolytes are performed once/day and more often only if indicated (e.g. a parameter that is changing and might become critical, requiring an intervention, before 24 hours). If successive tests, are stable, you can reasonably reduce the frequency of checks unless it becomes indicated to get them more frequently. A CBC and electrolytes may not even be required every day if they have been stable and there is no risk of them changing (e.g. related to bleeding, diuresis, renal failure etc.).

Chest radiographs are much the same. We obtain them every day on mechanically ventilated patients, in part to assure that "hardware" has not changed to a dangerous position (e.g. ETT tube too high or low, feeding tube in the esophagus). However, in patients with several sequential days without changes and no other indication, (e.g. changes in oxygenation, patient discomfort, altered respiratory mechanics) and especially in those with tracheostomy and surgical enterostomy who are stable, daily radiographs aren't always necessary.