A diagnostic composition for the detection of phenyl ketones especially phenyl pyruvic acid in body fluids comprises a bibulous carrier which has been impregnated with a solution comprising (a) ferric sulphate, ferric ammonium sulphate or ferric chloride, (b) cyclohexylsulphamic, gluconic, lactic, maleic, malic, malonic, saccharic or succinic acid and (c) magnesium sulphate, aluminium sulphate or aluminium ammonium sulphate, said solution having a pH of 1,2-1,8. The impregnating solution may be prepared with water only as solvent but an alcohol content, e.g. of ethyl p alcohol up to 20%, n-and iso-propanol up to 10% or n-butanol up to 1%, aids in even distribution on the carrier. Examples describe the preparation of suitable solutions containing (1) ferric ammonium sulphate, magnesium sulphate and cyclohexylsulphamic acid (2) ferric ammonium sulphate, aluminium ammonium sulphate and gluconic acid and (3) ferric sulphate magnesium sulphate and d -glucono-lactone. Strips, e.g. of filter paper or wood, having an impervious ethyl cellulose barrier extending from one end to 1/2 inch from the other end are prepared by dipping said other end into the prepared solution, allowing to stand in drying racks at room temperature and then drying in an oven at 90 DEG -110 DEG C.