Asecond oral lactose tolerance test is a useful discriminatory test for intestinal lactase activity. When the maximum blood glucose rise changes between tests by more than 1.0 mmol/litre intestinal lactase activity is almost certainly normal; if the maximm rise is initially low and changes by less than 0.5 mmol/litre on repeat testing it is highly probable that intestinal lactase activity is low. Subjective stress may play a role in false low oral lactose tolerance tests.