A 52-year-old woman with recurrent kidney stones and hypercalciuria (urine calcium >300 mg/day) is evaluated for stone-forming risk. To assess the functional capacity of the proximal convoluted tubule (PCT) for calcium reabsorption and identify whether the hypercalciuria is due to increased filtered load or impaired reabsorption, which investigation is most appropriate?

Explanation

## Investigation of Choice for Assessing Tubular Calcium Reabsorption ### Why Fractional Excretion of Calcium (FECa) is Correct **Key Point:** FECa quantifies the proportion of filtered calcium that is excreted in urine, allowing distinction between: - **Absorptive hypercalciuria** (normal FECa despite high urine calcium → increased intestinal absorption) - **Renal hypercalciuria** (elevated FECa → impaired tubular reabsorption in PCT) - **Resorptive hypercalciuria** (secondary to hyperparathyroidism) **High-Yield:** The FECa calculation is: $$FECa = \frac{[U_{Ca} \times S_{Cr}]}{[S_{Ca} \times U_{Cr}]} \times 100$$ Interpretation: - **FECa < 0.01 (< 1%)** = Normal tubular reabsorption; hypercalciuria is absorptive - **FECa > 0.02 (> 2%)** = Impaired tubular reabsorption; renal hypercalciuria ### Diagnostic Algorithm for Hypercalciuria ```mermaid flowchart TD A[Hypercalciuria 300+ mg/day]:::outcome --> B{Serum calcium & PTH}:::decision B -->|Normal Ca, normal PTH| C[Calculate FECa]:::action C -->|FECa < 1%| D[Absorptive hypercalciuria]:::outcome C -->|FECa > 2%| E[Renal hypercalciuria]:::outcome B -->|High Ca, high PTH| F[Resorptive hypercalciuria]:::outcome ``` **Clinical Pearl:** - **Absorptive hypercalciuria** (most common, ~60% of stone-formers): Treat with dietary calcium restriction and thiazide diuretics - **Renal hypercalciuria** (impaired PCT reabsorption): Treat with thiazides to reduce filtered load - **Resorptive hypercalciuria** (hyperparathyroidism): Treat underlying PTH excess **Tip:** FECa directly assesses the **functional capacity** of the proximal tubule to reabsorb calcium — it is the investigation that answers "Is the problem in the filtrate or in the tubule?"