Last reviewed: 21 Sep 2021
Last updated: 23 Nov 2018



History and exam

Key diagnostic factors

  • presence of risk factors
  • symptoms after ingestion of dairy products
  • diarrhoea
  • distension
  • lethargy
  • mouth ulceration
  • headache
  • poor short-term memory
  • poor concentration
  • muscle/joint pain
  • eczema
  • failure to thrive

Other diagnostic factors

  • abdominal pain/discomfort
  • borborygmi
  • flatulence
  • skin rashes
  • palpitations
  • history of asthma
  • history of peptic ulcer disease
  • history of gastro-oesophageal reflux disorder
  • nausea
  • vomiting
  • constipation
  • pallor
  • steatorrhoea
  • weight loss
  • short stature
  • progressive disability
  • flushing
  • sensory loss
  • oedema
  • hair loss

Risk factors

  • black, Native American, Asian, or Hispanic ethnicity
  • adolescence and early adulthood
  • family history of lactase deficiency
  • enteritis/gastroenteritis
  • comorbid predisposing disease
  • history of irritable bowel syndrome diagnosis

Diagnostic investigations

Investigations to consider

  • lactose hydrogen breath test
  • stool culture
  • faecal pH
  • faecal reducing substance/sugar
  • lactose tolerance test
  • small bowel biopsy

Emerging tests

  • genotyping
  • 13C-labelled lactose breath test

Treatment algorithm



Dr Anton Emmanuel, MD, FRCP

Academic Neuro-Gastroenterologist

University College London

Consultant Gastroenterologist

University College Hospital and the National Hospital for Neurology and Neurosurgery (Queen Square)




AE declares that he has no competing interests.


Dr Anton Emmanuel would like to gratefully acknowledge Dr Mohammad Azam and Dr Richard J. Farrell, previous contributors to this monograph. MA and RJF declare that they have no competing interests.

Peer reviewers

Anthony Lembo, MD

Assistant Professor of Medicine

Harvard Medical School

Beth Israel Deaconess Medical Center




AL declares that he has no competing interests.

Michele Di Stefano, MD

Coordinator of the Gastrointestinal Motility Laboratory

University of Pavia

1st Division of Medicine

IRCCS S. Matteo Hospital Foundation




MDS is the author of references cited in this topic.

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