Premise: A patient with fat malabsorption will have enhanced absorption of oxalate, which is excreted in the urine, causing a hyperoxaluria.
Molecular weight of sodium oxalate: 134 g (45.96 g sodium; 88.04 g oxalate)
1 mmol of oxalate = 88.04 mg; 1 mg = 0.01138 mmol = 11.38 µmol
Method:
(1) The patient is on an unrestricted diet, although the diet should not be high in calcium or oxalate.
(2) The patient receives 300 mg of sodium oxalate orally with 200 mL water daily at lunch and dinner for 2 days (total dose 1,200 mg sodium oxalate).
(3) A 24 hour urine specimen is collected on the second day.
Exclusions:
(1) failure to collect a complete 24 hour specimen (as indicated by a 24 hour urine creatinine excretion of < 679 mg (< 6 mmol, with 1 mmol creatinine = 113.12 mg)
(2) failure to take the total dose of sodium oxalate
(3) oliguria or anuria
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24 Hour Urine Oxalate Excretion
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Interpretation
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<= 0.44 mmol per 24 hour (<= 56 mg per 24 hour)
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normal
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> 0.44 mmol per 24 hour (> 56 mg per 24 hour)
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hyperoxaluria indicative of steatorrhea
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Performance:
• The predictive value of a normal urinary oxalate excretive for normal fecal fat excretion is 89%.
• The predictive value of an elevated urinary oxalate excretive for abnormal fecal fat excretion is 74%.
