Abstract: The present invention concerns a method of detection and preferably of quantification of DNA, preferably comprising prokaryotic DNA, extracted from a stool sample of an individual, especially for the molecular determination of the composition of the intestinal flora in the stool. According to the invention, one controls the quality of the DNA extraction by verifying whether one detects a specific DNA of Methanobrevibacter smithii and one performs the quantification of said specific prokaryotic DNA for Archae Methanobrevibacter smithii, for the bacterial genus Lactobacillus, for the phylum Bacteroidetes and for the phylum Firmicutes, respectively, to provide a diagnosis and/or monitoring of the weight status of an individual. The present invention provides a method for carrying out the extraction of prokaryotic DNA in stools.