A pregnant woman at 28 weeks has a random plasma glucose of 190 mg/dL and glucosuria on dipstick. Her renal threshold for glucose appears reduced. Why does glycosuria appear before the theoretical transport maximum for glucose is fully saturated?

• A The filtered glucose load exceeds tubular capacity uniformly across all nephrons
• B Pregnancy increases the number of SGLT2 transporters in the proximal tubule
• C Nephrons have different transport maxima, causing splay in the titration curve ✓
• D Glucose is secreted by the distal tubule during pregnancy

Explanation

The glucose titration curve shows splay rather than a sharp cutoff because individual nephrons differ in their transport maxima and because transporter affinity is finite. Some nephrons spill glucose at plasma levels well below the average Tm of about 375 mg/min, so glycosuria typically begins around a plasma glucose of 180 mg/dL. Pregnancy raises GFR, increasing the filtered load, which lowers the apparent renal threshold.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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