July 23, 2017
Calcium Absorption Test for Hypercalciuria Minimize

Calcium absorption test for hypercalciuria

Indication

This test is designed to differentiate between those subjects with renal, absorptive or idiopathic hypercalciuria. It should be considered in males with urine calcium >7.5 mmol/24h and females with urine calcium > 6.2 mmol/24h.

Contra-indications

none

Principle of test

Patients with post absorptive hypercalciuria (AHC) by definition have normal fasting calciuria (defined as molar calcium/creatinine ratio < 0.23). Patients with AHC have excessive calciuria after an oral calcium load. Patients with type I AHC cannot lower their calciuria by dietary calcium restriction unlike those with type II AHC.

Side effects

none

Preparation

No preparation required although patient may only drink water after 10 pm on the night before the test.

Requirements

  • Breakfast and calcium supplements to provide 1g elemental calcium.
  • 3 24h urine containers.
  • 1 plain tube for blood collection.

Procedure

Patient is allowed free water overnight after 10 pm.

0 hr

1: patient passes urine (sample 1: analysed for pH and microscopy)
2: blood sample taken without tourniquet for U&E, plasma bicarbonate, calcium, phosphate, urate and PTH

2 - 6 hr

1: free water allowed (at least 0.5L) and all urine collected (sample 2: analysed for calcium, phosphate & creatinine)
2: patient given breakfast with calcium supplement
3: free water allowed (at least 1L) and all urine collected for 4 hr (sample 3: analysed for calcium, phosphate & creatinine)

6 hr

end of procedure

Interpretation

 

Absorptive hypercalciuria type I

Absorptive hypercalciuria type II

Absorptive hypercalciuria type III

Renal hypercalciuria

plasma calcium

N

N

N

N

plasma phosphate

N

N

-

N

PTH

N

N

N

N

24 h urine calcium (1g diet)

high by definition !

fasting Ca/creatinine ratio

< 0.23

< 0.23

> 0.36

> 0.36

urine Ca/creatinine ratio (1g load)

> 0.77

> 0.77

> 0.77

> 0.77

24h urine calcium (restricted 400mg diet)

> 5 mmol

N ie < 5 mmol

> 5 mmol

> 5 mmol

Pathophysiology

hyperabsorption of calcium

diet-responsive hypercalciuria

phosphate leak leads to decreased renal resorption of calcium

decreased renal resorption of calcium is primary defect

Sensitivity and Specificity

  • Renal hypercalciuria is defined by fasting (ie sample 2) calcium/creatinine ratio > 0.36
  • Post absorptive hypercalciuria if calcium/creatinine ratio > 0.77
  • Normal fasting calcium/creatinine ratio < 0.23

References

  • Coe FL, Parks JH, Asplin JR. The pathogenesis and treatment of kidney stones. New Eng J Med 1992;327:1141-1152.
  • Pak CYC, Kaplan R, Bone H, Townsend J, Waters O. A simple test for the diagnosis of absorptive, resorptive and renal hypercalciurias. New Eng J Med 1975;292:497-500.
  

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