Abstract: The present disclosure describes a method of quantifying amounts of phosphopeptides using isotopically-enriched peptides as internal standards. A kit comprising at least one isotopically-enriched phosphorylated peptide can be used to quantify changes in amounts of phosphopeptides using parallel reaction monitoring mass spectrometry techniques. The invention can be used to indicate the pathologic mechanism, severity of the disease, and treatment response of a subject. The invention can also be used to identify subjects who require more aggressive therapeutic interventions or alternative treatments.