Online Companion: Fundamentals of Nursing Standards and Practice 2E

Frequently Asked Questions

Fasting for twelve hours before a test is a common preparation for some blood tests and some diagnostic procedures. What is the purpose for instructing the client to fast? How does the nurse know what tests require pre-procedure fasting?

A client is instructed to refrain from eating or drinking for as long as twelve hours before a test for one of several reasons. The blood component being tested may be affected by the recent ingestion of food, for example, the blood level of glucose after a twelve-hour fast is of diagnostic significance; eating food before the blood is drawn for this test would raise the glucose level and make the results inaccurate. A diagnostic procedure may rely for accuracy upon the client's stomach, and upper gastrointestinal tract being empty. The lining of the stomach cannot be visualized accurately with barium contrast if it filled with food. Another reason for requiring a fast is that the procedure may require the use of general anesthesia; the client is at risk for aspiration of recently ingested food or fluid during this type of anesthesia. The laboratory or diagnostic center that services the health care agency prepares a protocol manual that gives information on the preparation needed for each test. The nurse consults that manual for client teaching points.