Kosten made her international debut at the European LC Championships 2000 in Helsinki, Finland where she ended fifth in the 4×100 meter freestyle relay, she did so alongside Thamar Henneken, Chantal Groot and Wilma van Hofwegen.

At the European Short Course Swimming Championships 2001 in Antwerp, Belgium she won the silver medal in the 4×50 meter freestyle, together with Suze Valen, Hinkelien Schreuder and Inge de Bruijn.

She ended fourth in 4×50 meter freestyle at the European Short Course Swimming Championships 2002 in Riesa, Germany alongside Marleen Veldhuis, Suze Valen and Chantal Groot.

At the 2003 World Aquatics Championships in Barcelona, Spain Kosten teamed up with Manon van Rooijen, Marleen Veldhuis and Chantal Groot for the 4×100 meter freestyle relay, in which they reached the fourth position.

At the 2004 Summer Olympics in Athens, Greece Kosten swam in the heats of the 4×100 meter freestyle relay alongside Inge Dekker, Marleen Veldhuis and Chantal Groot.