Filing Date: 2003-10-10.--Priority Data: ES (2002-10-16) (P200-20-23)[ES]La metodologia objeto de la invention se basa en el empleo de 4-galactosil-xilosa (4-O-p-galactopiranosil-D-xilosa). Este compuesto funciona como un analogo estructural de la lactosa, el substrate fisiologico de la lactasa, de manera que tras su administración oral es hidrolizado por el enzima de la mucosa intestinal, pasando los productos de la reaction a la sangre y uno de ellos, la xilosa, aparece en la orina, donde puede ser valorado mediante una simple determinación colorimétrica. Alternativamente, en la presente invention se describe la valoración de xilosa en sangre puede determinarse igualmente la presencia de xilosa en sangre, para lo que bastan unas pocas gotas de la misma, dada la elevada sensibilidad del método de valoración de xilosa. Esta alternativa supone una ventaja substancial especialmente en el caso de lactantes y nines en general, puesto que al evitar la toma de orina de varias horas resulta menos molesto y proporciona mayor fiabilidad.[EN]The invention relates to the use in humans of a method which is based on the use of 4-galactosyl-xylose and which involves an alternative technique to determining xylose in urine. The method consists in measuring the concentration of xylose in the blood following the oral administration of 4-galactosyl-xylose to the patient, the use of said method being even more suitable in the case newborns. The invention also relates to the technology used for the in vivo evaluation of intestinal lactase activity, by way of a non-invasive test for the diagnosis of a deficiency of said enzyme, which consists in directly evaluating the overall activity of the enzyme in the individual and not in assessing the consequences of said deficiency. The inventive method does not require specialist equipment, does not cause any obvious discomfort to patients with a lactase deficiency and is extremely reliable, thereby eliminating the inconveniences of currently-used diagnostic tests.