Laxatives

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 et al. Clin Biochem. 1993; 26: 179-181.

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