Background: Chitotriosidase (CT) is a surrogate plasma marker for Gaucher disease. The enzyme is released by storage cells and is on average thousand fold elevated in serum of Gaucher patients. Plasma CT level is measured with the substrate 4-methylumbelliferyl (4MU)-chitotriose or 4MU-chitobiose. Given the limitations associated with the use of these substrates, a novel substrate, 4MU-deoxychitobiose, has recently been conceived.Methods: Chitotriosidase activity was measured with all three substrates in serum samples from 91 type 1 Gaucher patients. Glucocerebrosidase and chitotriosidase genotypes were determined as well as disease parameters.Results: Chitotriosidase activity when measured with 4MU-deoxychitobiose gave higher values and was proportional to enzyme concentration over a much larger range as compared to the other two substrates. Patients that were carrier for the common CT mutation showed on average half the activity of those with wild type CT genotype. Plasma CT levels correlated best with combined liver and spleen volume: r=0.53 (p < 0.001).Conclusions: The use of 4MU-deoxychitobiose as substrate renders a substantial improved CT activity assay and may further facilitate accurate laboratory monitoring of Gaucher patients. (c) 2007 Published by Elsevier B.V.