Other tests which can aid in the diagnosis of GSID but which are not truly diagnostic for the disease are the sucrose breath test, and a genetic test which tests for the absence of certain genes which are thought to be responsible for GSID.Sucrose (also called saccharose) is a disaccharide and is a two-sugar chain composed of glucose and fructose which are bonded together. A more familiar name is table, beet, or cane sugar. It was believed that most cases of sucrose intolerance were due to an autosomal recessive, genetic, metabolic disease. Based on new data patients with heterozygous and compound heterozygous genotypes can have symptom presentation as well. GSID involves deficiency in the enzyme sucrase-isomaltase, which breaks the bond between the glucose and fructose molecules. When disaccharides are consumed, they must be broken down into monosaccharides by enzymes in the intestines before they can be absorbed. Monosaccharides, or single sugar units, are absorbed directly into the blood.A deficiency of sucrase may result in malabsorption of sugar, which can lead to potentially serious symptoms. Since sucrase-isomaltase is involved in the digestion of starches, some GSID patients may not be able to absorb starches as well. It is important for those with sucrose intolerance to minimize sucrose consumption as much as possible. Dietary supplements or medications may be taken as a substitute for the missing enzyme or to introduce healthy bacteria into the immune system. Diagnosis
Chronic gastrointestinal symptoms that are fairly common but difficult to diagnose may be caused by congenital sucrase-isomaltase deficiency (CSID).
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.