How is the calcium-loading test interpreted in the workup of hypercalciuria?

Updated: Apr 23, 2019
  • Author: Stephen W Leslie, MD, FACS; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Note that in this testing series, hypercalciuria is defined as the excretion of more than 200 mg of urinary calcium per 24 hours on the 400-mg calcium-restricted diet.

Absorptive hypercalciuria

Patients with absorptive hypercalciuria normalize their urinary calcium excretion while on a fasting diet but greatly increase their urinary calcium excretion after the calcium load. Therefore, their fasting calcium/creatinine ratio is 0.11 or less, but their post–calcium load samples are greater than 0.20, demonstrating an exaggerated calcium absorption and subsequent excretion.

Patients with absorptive hypercalciuria type I typically do not normalize their urinary calcium excretion to less than 200 mg per 24 hours on the 400-mg calcium restricted diet, whereas patients with type 2 hypercalciuria do demonstrate less than 200 mg of urinary calcium per day while on that same diet.

Renal leak and resorptive hypercalciuria

Patients with either renal leak or resorptive hypercalciuria are hypercalciuric regardless of oral calcium intake. Consequently, they show more than 200 mg of urinary calcium excretion per 24 hours on the calcium-restricted diet and demonstrate high calcium/creatinine ratios in both phases of the calcium-loading test.

The serum calcium level, however, can be used to differentiate between these 2 diagnoses. Patients with renal leak hypercalciuria have low serum calcium levels, whereas those with resorptive hypercalciuria, which occurs in patients with hyperparathyroidism, are hypercalcemic. Table 3, below, provides a guide to interpreting calcium-loading tests.

Table 3. Calcium-Loading Test Interpretation Guide (Open Table in a new window)

Criteria

Absorptive Type I

Vitamin D–Dependent (Classic Form)

Absorptive Type I

Vitamin D–Dependent (Variant Form)

Absorptive Type II

Dietary Calcium Responsive

Absorptive Type III

(Renal Phosphate Leak)

Renal Calcium Leak

Resorptive

Urinary calcium on regular diet*

High

High

High

High

High

High

Urinary calcium on low-calcium diet

High

High

NL

High

High

High

Urinary calcium fasting

NL

High

NL

High

High

High

Urinary calcium after 1-g calcium load§

High

High

NL

High

High

High

Serum PO4 (fasting)

NL

NL

NL

Low

NL or high

Low

Serum calcium (fasting)

NL

NL or high

NL

NL or high

NL or low

High

Serum PTH

NL or low

NL or low

NL

Low

High

High

Serum PTH after 1-g calcium load

NL or low

NL or low

NL

Low

High

High

Serum vitamin D-3 (calcitriol)

NL

High

NL

High

High

High

Fasting normocalciuria while on ketoconazole

No

Yes

No

Yes

No

No

Bone calcium density

NL

NL or low

NL

NL or low

Low

Low

NL = normal; PO4 = phosphate; PTH = parathyroid hormone.

* Regular diet is unrestricted calcium and sodium intake. Normal upper limit calciuria is < 4 mg/kg body weight per day.

Low-calcium diet is 400 mg calcium and 100 mEq of sodium per day. Normal upper limit calciuria is < 200 mg/day.

Fasting is a 12-hour fast. Normal upper limit is < 0.11 mg calcium/mg creatinine.

§ After 1-g calcium load, normal upper limit is < 0.20 mg calcium/mg creatinine.


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