Which specimen is appropriate for measuring inorganic phosphate given diurnal variation?

• A. 24-hour urine
• B. Serum
• C. Saliva
• D. Cerebrospinal fluid

Answer: (A)

Diurnal variation means the level changes with the time of day. For inorganic phosphate, serum levels fluctuate during the day, so a single measurement can be misleading. A 24-hour urine collection integrates phosphate excretion over an entire day, smoothing out those peaks and troughs and providing a more reliable picture of the body's phosphate handling and renal excretion. Urine phosphate reflects how the kidneys respond to dietary intake and regulatory hormones across the day, making it the preferred specimen when diurnal variation is a concern. Saliva and cerebrospinal fluid don’t reliably reflect systemic phosphate status, and a one-time serum sample remains subject to time-of-day effects. So, the appropriate specimen is a 24-hour urine collection.