A method to determine the degradation percentage of a compound of formula by contact with part of a container for medical usage, comprising the following successive steps: i) preparing an aqueous solution comprising a compound of formula below in which: R 1, R 2, R 4 and R 5 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6) alkoxycarbonyl, aryl or a protecting group; n and m each represent independently of one another an integer from 0 to 10; R3 represents -(CH 2) 2 -COOH, -CH 2 -COOH, -(CH 2) 4 -COOH, -CH 2 -OH, -C -CH 3, -CH 2 -CO-NH 2, CH 2 -SH, -(CH 2) 4 -NH 2, -(CH 2) 2 -CO-NH 2, -CH 2 -CH 2 -S-CH 3, -(CH 2) 3 -NH-C -NH 2; or R 3 represents a group of formula below with R 6, R 7, R 8 and R 9 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6)alkoxycarbonyl, aryl or a protecting group; r and p each represent independently of one another an integer from 0 to 10; ii) adding to the solution prepared at step i) part of a container for medical usage during a period of time of 1 hour to 2 months, at a temperature in a range from 5 to 80°C; iii) analyzing by liquid chromatography the solution obtained at step ii); and iv) determining the degradation percentage. i) preparing an aqueous solution comprising a compound of formula below in which: R 1, R 2, R 4 and R 5 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6) alkoxycarbonyl, aryl or a protecting group; n and m each represent independently of one another an integer from 0 to 10; R3 represents -(CH 2) 2 -COOH, -CH 2 -COOH, -(CH 2) 4 -COOH, -CH 2 -OH, -C -CH 3, -CH 2 -CO-NH 2, CH 2 -SH, -(CH 2) 4 -NH 2, -(CH 2) 2 -CO-NH 2, -CH 2 -CH 2 -S-CH 3, -(CH 2) 3 -NH-C -NH 2; or R 3 represents a group of formula below with R 6, R 7, R 8 and R 9 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6)alkoxycarbonyl, aryl or a protecting group; r and p each represent independently of one another an integer from 0 to 10; R 1, R 2, R 4 and R 5 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6) alkoxycarbonyl, aryl or a protecting group; n and m each represent independently of one another an integer from 0 to 10; R3 represents -(CH 2) 2 -COOH, -CH 2 -COOH, -(CH 2) 4 -COOH, -CH 2 -OH, -C -CH 3, -CH 2 -CO-NH 2, CH 2 -SH, -(CH 2) 4 -NH 2, -(CH 2) 2 -CO-NH 2, -CH 2 -CH 2 -S-CH 3, -(CH 2) 3 -NH-C -NH 2; or R 3 represents a group of formula below with R 6, R 7, R 8 and R 9 each represent independently of one another H or a group (C1-C6)alkyl, (C2-C6)alkenyl or (C2-C6)alkynyl; X represents H or a group (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C1-C6)alkoxycarbonyl, aryl or a protecting group; r and p each represent independently of one another an integer from 0 to 10; ii) adding to the solution prepared at step i) part of a container for medical usage during a period of time of 1 hour to 2 months, at a temperature in a range from 5 to 80°C; iii) analyzing by liquid chromatography the solution obtained at step ii); and iv) determining the degradation percentage.