Coding specifies the work performed when providing patient care. Critical care services mostly use code 99291, and other codes specify additional time and procedures. Current Procedural Terminology defines critically ill as "a high probability of imminent or life-threatening deterioration in the patient's condition," a condition necessary for use of the critical care code. A patient may be critically ill for neurologic reasons even when stable from a cardiorespiratory status. Rules govern who can use these codes, whether they can be used by more than one physician, the locations where the code may be used, and what services are included and excluded. Physicians need to document the medical necessity of visits and nature of critical illness or high-risk medical decision making because auditors may not understand the nature of serious neurologic illness.