Which condition is most likely to cause a falsely high creatinine clearance result?

The situation where the patient empties his or her bladder at the start and then adds urine to the collection can lead to a falsely high creatinine clearance result due to the inclusion of urine that is not representative of the entire collection period. This method of collection essentially introduces urine that may have a different concentration of creatinine than that which would normally be excreted throughout the collection interval.

In a proper creatinine clearance test, the entire volume of urine collected over a specified time (usually 24 hours) must be representative of both the creatinine concentration and the total urine output during that time. By starting the collection with an empty bladder and then adding urine later, the initial output (which might contain different levels of creatinine) is excluded from the test results, thereby distorting the final measurement and leading to an artificially elevated clearance rate.

The integrity of creatinine clearance tests relies on accurately measuring both urine volume and creatinine concentration collected over the entire designated period. This ensures that the calculation reflects true kidney function and does not misrepresent it due to sampling errors or improper collection methods.