Laxatives - urine, faeces

Last updated: Wednesday, 07, April, 2004

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Item Process
Specimen

Random urine and faeces.

If diarrhoea is a symptom a diarrhoeal specimen should be collected.

Method

Urine: chromatography.

Faeces
magnesium: spectrophotometry, atomic absorption spectrophotometry
osmolality: freezing point depression.

Application

Unexplained hypokalaemia especially in a young anorexic patient.

Diarrhoea with no obvious cause. Surreptitious use of laxatives is common in these situations. By definition the patient denies taking laxatives.

Interpretation

Faecal magnesium >45 mmol/L, or the presence of phenolphthalein, anthraquinones or bisacodyl in urine indicates use of laxatives.

High faecal osmolality may indicate the use of non-absorbable sugars eg, lactulose.

Reference

Perkins SL and Livesey JF. Clin Biochem 1993; 26: 179-181.

Fine KD et al. N Engl J Med 1991; 324: 1012-1017.