Galactosylsphingosine does not interfere with the quantitation of plasma glucosylsphingosine levels in Gaucher patients. 
It has been shown that the plasma level of glucosylsphingosine (Lyso GL-1) is a useful biomarker for the diagnosis and monitoring of Gaucher disease.  Potentially interfering with the quantitation of Lyso GL-1 is its isobaric structural isomer, galactosylsphingosine (psychosine).  The contribution of psychosine is generally not accounted for in the determination of Lyso GL-1, due to the difficulty in separating these two isomers.  Few methods have been presented in the literature to distinguish the two isomers, and those available tend to be tedious and time-consuming.  Here, we developed a LC/MS/MS method able to chromatographically separate Lyso GL-1 and psychosine reproducibly and combine it with a simple, high-throughput sample preparation technique.  We also show that the separation of these two isomers in the plasma of Gaucher patients is not necessary for the quantitation of Lyso GL-1 levels, as the relative psychosine level is <3% of Lyso GL-1.