Subsequently, Borst worked as a resident physician at the hospital Onze Lieve Vrouwe Gasthuis in Amsterdam where she specialized in pediatric medicine and immunohaematology.
Kemudian, Borst bekerja sebagai dokter residen di rumah sakit Onze Lieve Vrouwe Gasthuis di Amsterdam di mana ia mengkhususkan diri dalam pengobatan pediatrik dan imunohaematologi.