Lactose tolerance test

Specimen:

2 mL blood in lithium heparin tube prior to, and at 30 minute intervals.

Protocol:

The patient takes 1 g lactose per kg body weight up to a maximum of 50 g in a flavoured drink over 10 min.

Blood specimens are taken pre-test and half hourly for 2 h.

The specimens are analysed for glucose (spectrophotometry).

Lactose tolerance can also be assessed by measurement of breath hydrogen every half hour for 3 h after the lactose.

See Hydrogen breath.

Reference Interval:

Plasma glucose should increase by at least 1.0 mmol/L in normal individuals.

Application:

Investigation of lactose intolerance. This test is now rarely performed, as it frequently results in diarrhoea and abdominal pain in affected individuals.

It has been largely superseded by the assay of lactase in a small bowel biopsy.

See Disaccharidases - small bowel mucosa.

Interpretation:

A subnormal rise in plasma glucose indicates lactose intolerance.

Reference:

Scriver CR et al eds. The Metabolic and Molecular Bases of Inherited Disease. 7th ed. 1995. McGraw Hill.