Something to know: All hydrophilic coatings contain multiple ingredients, some of which are not completely bound within. Even crosslinked coatings that purport to be chemically resistant still contain unreacted products from whatever reactions are used in the crosslinking. The article at Qmed makes us aware that these sorts of leftovers can either leach out or be extracted out, and there is a difference.
The author notes that leaching occurs under "normal" conditions of use, i.e. what will come out of your coating when you place it in the body at 37C for some length of time? Extraction is what happens during exaggerated conditions, i.e. what will come out of the coating at 50C in an acidic water bath, or an oil bath?
Extractables can give clues to what the leachables might be. They can also tell you how stable your material is, chemically. The presence of an extractable is not necessarily a show-stopper. Unwanted leachables might be harder to explain, however. Either way, both of these things will make up part of the larger picture of biocompatibility for your material.