In a statement sent to media, the Belgian WorldTour team said the 28-year-old rider notified the team of the positive test.
"Today, Lotto Soudal was notified by its rider Tosh Van der Sande of a positive doping test during the Six Days of Ghent where he took part in between 12 and 18 November 2018," the team said in today's media release.
Although the team's statement did not identify the substance involved, it did say the substance is "a permitted one and occurs in the nasal spray Sofrasolone, that is freely available and allowed to be used in competition if mentioned during a control."
The team said that after meeting with Van der Sande, the rider would be suspended "in expectation of the investigation executed by the UCI and to allow the rider to underpin his defense in the best possible way."
Lotto Soudal said they will not comment further until the UCI procedure has finished.
Van der Sande and six day partner Jasper de Buyst finished third at the Gent Six behind the Quick-Step duo of Elia Viviani and Iljo Keisse. The Baloise Insurance team of Robbe Ghys and Kenny de Ketele finished second.
Van der Sande has ridden for Lotto since 2012, the year after be beat Romain Bardet in the U23 Liège-Bastogne-Liège. Since then he's taken one win for the Belgian team, during stage 2 at the Tour de l'Ain in 2016.
Van der Sande has ridden the Vuelta a Espana four times with Lotto and the Giro d'Italia twice, with his best Grand Tour finish coming in the 2015 Vuelta, where he was 49th. He has ridden the elite Liège-Bastogne-Liège six times, finishing a career-best 55th in 2017.