A method of utilizing the chymotrypsin level of an individual as a measure of the success of neuropeptides, and peptides or digestive enzyme administration to such individuals, and in particular, as a prognosticative of potential neuropeptides, peptides, and digestive enzyme administration for persons having ADD, ADHD, Autism and other PDD related disorders. A method for determining the efficacy of neuropeptides, peptides, or digestive enzymes for the treatment of an individual diagnosed with a pervasive developmental disorder (PDD) comprises obtaining a sample of feces from an individual, determining a quantitative level of chymotrypsin present in the sample, and correlating the quantitative level of chymotrypsin determined to be present in the sample with the PDD to determine the efficacy of treating the individual with neuropeptides, peptides, or digestive enzyme administration.