Maybe you want to use polarimeter instead or a refractometer. The polarimeter measure only compounds with optical activity while the refractometer measure also salts. Of curse, if all your solutions have the same salt content, then the difference will be due to the dextran content.

Polarimeters are standard in the sugar industry. You could also use colorimetric methods, like the classic Phenol-H2SO4 method. You could calibrate with your own dextran standard. Just be sure you have really dry standards to increase accuracy.

Good luck!

http://www.lifespantech.com/products/dextran-assays/S-7500%20Dextran%20ELISA%20Kit%20Rev5.pdf

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