One of the most cited references regarding cellulase (saccharifying cellulase, endo-, exo- and betaglucosidase) assays (Ghose, 1987 & Wood and Bhat, 1988) recommend that OD readings of enzyme blanks be deducted from those of assay mixtures in cellulase assays to get the absolute OD of the reaction. In my experience so far, the OD of my enzyme blanks in most cases exceed that of the assay reactions thereby giving me negative values for my assay reaction. Furthermore, some of the samples gave negative OD values against both the substrate blank and enzyme blank. This implies that the equivalent glucose values would be zero or negative while this is not possible in reality (as it is not possible to have negative concentrations of glucose).

I want to know if anyone had a similar experience. My enzyme samples are cell-free supernatants of a microbe-hydrolyzed pretreated cellulosic substrate. I am thinking of using the direct OD readings of the enzyme assays without subtracting the blanks. Would this be correct?