A method of utilizing the chymotrypsin level of an individual as a measure of the success of secretin, other neuropeptides, and peptides or digestive enzyme administration to such individuals, and in particular, as a prognosticative of potential secretin, other neuropeptides, peptides, and digestive enzyme administration for persons having ADD, ADHD, Autism and other PDD related disorders. In one aspect, a method for determining the efficacy of secretin, other 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 secretin, other neuropeptides, peptides, or digestive enzyme administration. In another aspect, a therapeutic method for treating an individual diagnosed with a PDD pervasive developmental disorder comprises determining the efficacy of secretin, other neuropeptides, peptides, and digestive enzyme administration for the treatment of the individual based on a measure of the individual's chymotrypsin level, and administering secretin, other neuropeptides, peptides, or digestive enzymes to the individual based on the determination of the measure of the individual's chymotrypsin level.
