She made her international championship début in the 800 metres at the 2010 African Championships in Athletics in Nairobi, but failed to make the final. In 2011, Sum set a personal best time of 1:59.66 in finishing second over 800 m at the Kenyan championships. This performance qualified Sum for the 800 m at the 2011 World Championships in Athletics in Daegu, South Korea, where she reached the semi finals.