What is the diagnostic blood test for lactose intolerance (lactose malabsorption)?

From the Guidelines

The lactose hydrogen breath test is the most widely used and recommended method for diagnosing lactose intolerance, as it is relatively easy to perform and has similar sensitivity and specificity to the mucosal lactase assay. This test involves ingestion of 25–50 g lactose dissolved in 200–500 ml water after an overnight fast, and end expiratory breath samples are taken at 15–30 minute intervals for three hours 1. A rise of hydrogen concentration from basal levels by more than 20 parts per million is compatible with a positive diagnosis. Some key points to consider when using the lactose hydrogen breath test include:

• The test is not invasive and is relatively easy to perform, particularly if an electrochemical cell is used for analysis 1
• A negative result does not exclude the diagnosis, due to a false negative rate of up to 25%, and hence a trial of a lactose free diet should be considered if the diagnosis is still suspected 1
• The lactose tolerance test, which measures serum glucose or galactose in response to an oral lactose load, is another option, but it is probably inferior to the lactose hydrogen breath test 1
• Other methods, such as assay of mucosal lactase, may be available, but the lactose hydrogen breath test is the most widely used and recommended method 1

From the Research

Diagnostic Methods for Lactose Intolerance

• The hydrogen breath test (HBT) is a commonly used method for diagnosing lactose intolerance, as it is non-invasive, inexpensive, and highly sensitive and specific 2.
• Genetic testing, such as genotyping for the lactase gene, can also be used to diagnose lactose intolerance, particularly in populations with a high prevalence of lactase-persistent alleles 3.
• The lactose tolerance test and quick lactase test are other methods that have been used to diagnose lactose intolerance, although they are less commonly used than HBT and genetic testing 2.

Comparison of Diagnostic Methods

• A study comparing HBT and genetic testing found that genetic testing had a higher negative predictive value and specificity than HBT, although HBT had a higher positive predictive value and sensitivity 3.
• Another study found that genetic testing and HBT had a high degree of concordance, although there were some cases where the results did not agree 4.
• A study investigating the use of a modified algorithm that includes genotyping for the diagnosis of adult-type hypolactasia found that genotyping can be used as a first step to diagnose lactose intolerance, and can accurately identify patients who need further breath testing 5.

Limitations of Diagnostic Methods

• HBT may not be suitable for populations with a high prevalence of lactase-persistent alleles, as it may produce false positive results 3.
• Genetic testing may not provide information on actual patient symptoms, and may not be suitable for patients who are heterozygous for the lactase gene 6.
• The lactose breath test may require prolonged monitoring of hydrogen excretion, which can be inconvenient for patients 6.

Blood Tests for Lactose Intolerance

• There is no specific blood test that can diagnose lactose intolerance, although genetic testing can be used to identify individuals who are at risk of developing lactose intolerance 2, 3, 5, 6, 4.