The role of inhibitory antibodies on metabolic biomarker and sleep disordered breathing (SDB) amongst ERT treated MPS I patients. Bars represent means with p values presented. For correlation plots, linear regression lines of best fit were drawn and correlation coefficients were calculated with Pearson’s r, with p values representing significantly non-zero lines of best fit. HSCT treated Hurler patients: closed circles. Hurler patients treated with ERT: Half shaded triangles. ERT patients with clinically significant inhibitory antibodies were defined as a cellular uptake inhibition greater than 30% & titres greater than 4000 (Open triangles). ERT treated patients without such an antibody response represented by closed triangles. (A). Correlation plot with linear regression lines of best fit for cellular uptake inhibition and DS:CS ratio. Increasing activity of inhibitory antibodies correlates strongly with poorer substrate clearance (urinary DS:CS ratio performed within 6 months of antibody status) (r2 = 0.74, p = 0.002). 30% uptake inhibition is a suggested percentage with a measurable biochemical effect based on intersection of upper confidence intervals at baseline and lower CI. (B). Urinary DS:CS ratio within 6 months of antibody status was significantly improved in individuals without inhibitory antibodies (mean 0.61, S.D 0.14) compared to those with inhibitors (mean 1.8, S.D 0.76). As a reference, ERT attenuated patients without inhibitors have as successful metabolic outcomes as HSCT treated Hurler patients (mean 0.47). (C). SDB as measured by ODI4% within 6 months of antibody status was significantly lower in individuals without inhibitory antibodies (mean 2.03, S.D 1.68) compared to those with inhibitors (mean 16.85, S.D 7.23).