Metal colloid solution

A colloid solution of metal particles or metal compound particles being formed from one metal used in an integrity test for a virus removal membrane comprising:

• (1) from 0.0001 to 0.1 wt % of metal particles or metal compound particles being formed from one metal having an average particle diameter of 15-40 nm,
• (2) a water-soluble high molecular weight dispersant containing a pyrrolidone group,
• (3) water, and
• (4) an anionic surfactant and/or a chelating agent, wherein
• all of the metal particles or metal compound particles of the colloid solution are formed from said one metal have an average particle size diameter of 15-40 nm, and
• the colloid solution has the following properties (a) and (b):
• (a) a maximum absorption wavelength after 180 days storage at room temperature and at a constant pH in a range from pH4 to pH11 which differs from the maximum absorption wavelength prior to storage by −2.0 nm to +2.0 nm, and
• (b) a maximum absorption wavelength after one year storage at 50° C. and at pH5 which differs from the maximum absorption wavelength prior to storage by −2.0 nm to +2.0 nm.