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Abstract Two stochastic microdisks were designed and validated for the DNA mismatch repair assessment in gastric and colon cancers, through determining simultaneously the concentrations of MLH1, MSH2, MSH6, PMS2, and of KRAS in whole blood, urine, saliva, and tumoral tissues. The active surface of the stochastic microdisks was composed of a thin layer of graphene paste decorated with nitrogen, boron, and sulfur and modified with inutec (SP) and with frutafit (FT), respectively. High sensitivities were recorded when the SP was used as modifier. All limits of determination were of fg mL−1 magnitude order. The paired student‐t‐test done at 99.00 % confidence level revealed that there are no significative differences between the results obtained using the two stochastic microdisks. Validation of the screening tests of whole blood, urine, saliva, and tumoral tissues was also done using the recovery tests when % recovery was higher than 87.00 %, with %RSD values lower than 1.00 %.