A method is provided for determining the level of an analyte in the blood of an individual based on determination of the level of the same analyte in non-blood sample (e.g. urine, saliva and hair) obtained from the individual. The non-blood sample contains red blood cells and the volume of the blood in the sample together with the amount of the analyte in the sample are the basis for calculating the level of the analyte in the individual's blood. Kits for carrying out the above method are also provided.