Which electrolyte is described as requiring fasting before measurement?

• A. Calcium
• B. Phosphate
• C. Magnesium
• D. Sodium

Answer: (B)

Phosphate is the electrolyte whose blood level is most affected by recent meals, so fasting helps obtain a stable, baseline value. After you eat, phosphate from dietary sources is absorbed into the bloodstream, causing a temporary rise in serum phosphate. If you measure phosphates right after a meal, the result can be higher than the true fasting level, making it harder to assess whether someone truly has hypophosphatemia or hyperphosphatemia. By fasting, you minimize this postprandial fluctuation and get a value that better reflects the body's steady-state phosphate handling, which is important for evaluating disorders of phosphate metabolism and renal phosphate management.

Calcium, magnesium, and sodium are not routinely required to be fasting for measurement because their levels are either less influenced by a single meal or are interpreted with consideration of other factors (for calcium, albumin-corrected levels; for the others, their regulation is relatively stable in the short term).