Kolthoff was born on 27 January 1908 in Batavia, Dutch East Indies. She was the only child of father Abraham Kolthoff (1863-1940) who was director Officer of Health 1st class of the K.N.I.L. and worked in in Batavia. Her mother was Johanna Cornelia Brandon (1870-1947). They moved back to the Netherlands where she married to mechanical engineer Lucas Ruinen (1901-1993) on 20 August 1937 in The Hague. They had four children together.