Tryptophan degradation is a key metabolic pathway controlling immune reactions and evidence suggests that during cancer progression generation of tryptophan metabolites may be fundamental for immune escape promoting the malignant phenotype of cancer cells in an autocrine fashion. The present invention relates to methods of measuring mass tag labelled tryptophan and metabolites thereof and methods using the labelled molecules for monitoring in a subject the effectiveness of a treatment and of disease recurrence after treatment, for stratifying patients and for diagnosing suppression of an immune response in a subject.