Disclosed is a method for correlating a biomarker in a non-blood bodily fluid with the same biomarker in the blood of an individual, including: measuring, in a first period in time, the biomarker in non-blood bodily fluid and measuring the same biomarker in the blood of the same individual to establish an R ratio of [NBBF1]/[BB1], where [NBBF1] is the biomarker concentration in non-blood bodily fluid in the first period in time, and [BB1] is the biomarker concentration in the blood in the first period in time; storing the ratio in a memory; measuring, in a second period in time, the biomarker in non-blood bodily fluid to determine [NBBF2], where [NBBF2] is the biomarker concentration in the non-blood bodily fluid in the second period in time; and correlating the measured [NBBF2] with the R ratio to generate a correlated [BB2] biomarker concentration in the blood of the individual in the second period in time. Also disclosed is a device, apparatus, and method for correlating the glucose concentration in a non-blood bodily fluid such as saliva with the glucose in the blood of an individual.