The invention relates to a dyeing composition comprising a fluorescent disulphide dye and to a method of dyeing keratin materials, with a lightening effect, that employs said composition. A first sealing member sealing a gap between the sealing body and the inner peripheral wall surface is disposed on an inner peripheral surface of the annular sealing body and a second sealing member sealing a gap between the sealing body and the outer peripheral wall surface is formed on an outer peripheral surface of the annular sealing body. 1,5-anhydroglucitol in a sample containing glucose, and a reagent and a reagent kit useful in the method.