If the cumulative amount of 13CO2 exhaled is below the normal level that occurs when sucrose is digested by sucrase, the person may have CSID and the person should undergo further examination by their doctor.The findings from a 13C-breath test are believed to be more definitive for CSID than the hydrogen/methane breath test although neither test is validated to provide, by themselves, a diagnosis for CSID. In both breath tests, the consumption of sugar may cause severe gastrointestinal symptoms in those who have CSID. For this reason, these breath tests should be conducted under the supervision of a healthcare professional. A relatively new, noninvasive test that shows promise is a genetic test for the gene SI, which codes for the enzyme sucrase-isomaltase. This test requires a swab of tissue from the inside of the cheek. So far, 37 aberrant variations of the SI gene have been found in patients who have been diagnosed with CSID.A positive genetic test for one or more of the 37 SI gene variations known to cause CSID can help confirm a diagnosis of CSID. However, a negative genetic test cannot rule out a diagnosis of CSID. There are more than 2,000 different variations of the SI gene, and many of these variations have not yet been investigated for their ability to cause CSID.
If the level of sucrase activity is below the level considered necessary for normal sucrase function, the patient will be diagnosed with CSID.Noninvasive diagnostic methods include two breath tests, which can be useful screening tools but are not specific enough for a confirmed diagnosis of CSID.The first of these is the hydrogen/methane breath test, which measures the amount of hydrogen and methane gases a person exhales after consuming sugary water. The exhaled breath is collected in sealed test tubes at 30-minute intervals over a three-hour period after drinking the sugary water. If a person has CSID and there are little or no working sucrase-isomaltase enzymes in the intestines, greater than normal levels of hydrogen and/or methane are generated and exhaled in the breath. However, there may be other reasons why the person is exhaling excess hydrogen and/or methane gas, such as an overgrowth of bacteria in the small intestine, called small intestinal bacterial overgrowth (SIBO).The second breath test is called the carbon-13 (13C) breath test. Carbon-13 is a stable isotope of carbon that occurs naturally in sucrose, making it possible to track a person’s ability to digest and absorb sucrose by measuring the amount of 13CO2 exhaled after drinking a sugar-water solution. In this breath test, the exhaled breath is collected in sealed test tubes at 30-minute intervals over a 90-minute period after drinking the sugar-water solution.