A timed specimen is commonly used to measure the concentration of which substance?

The use of a timed specimen is particularly relevant for measuring substances that can fluctuate significantly over time, and one of the primary substances for which this is crucial is creatinine. Creatinine measurement is often done over a 24-hour collection period to accurately assess kidney function, providing a comprehensive picture of how much creatinine is being produced over time. This is important because creatinine levels can vary due to factors such as hydration status, muscle mass, and diet, making a single measurement potentially misleading.

Collecting a timed specimen ensures that any diurnal variations or changes throughout the day are accounted for, leading to a more reliable and standard measurement of creatinine concentration. This is especially important in clinical scenarios where accurate kidney function assessment is critical for diagnosis and treatment planning. Other substances listed, such as cholesterol, hemoglobin, and platelets, typically do not require timed collections for meaningful results.