What defines yarn as a DK weight?
Dk weight yarns are slightly heavier than sport weight yarns - they are called DK because they were created for doubleknit knitting - or using two strands at once, especially in intarsia type knitting. DK weight yarns are wonderful for accessories, like shawls, wraps - and can be combined color-wise to make up a worsted weight yarn.
DK Weight yarns are really mostly interchangeable with sport weight - sport is slightly thinner, in some cases, but so much depends upon the type of yarn (many DK weights bloom and become more of a light worsted - how's that for confusing!). But, pretty much count on a dk and a sport weight to be swappable.