Reducing Substances feces test?

Did your doctors order this malabsorption test? We just got a positive result but it’s hard to tell what it “means” exactly. My impression is it’s a fairly generic result that just means excess sugar is in the stool… which could mean a variety of things. My baby is 4 months and has fallen to below the 1st percentile in weight (from around 30 at birth). Anyway, just looking for your experiences!