LAXATIVES - urine, faeces

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Specimen: Random urine and faeces. If diarrhoea is a symptom, a diarrhoeal specimen should be collected.

Method: Urine: chromotography.

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

Application: Unexplained hypokalaemia, esp 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.

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