Unless it's been dropped, it's probably not mis-calibrated.
You can also put RODI water on it for a rough check.
Salinity of RODI should register VERY CLOSE to zero....if it's not VERY CLOSE then get some calibration fluid and dial it in correctly.....if it is VERY CLOSE then the refractometer is not your issue.
@Randy Holmes-Farley has a DIY recipe for calibration fluid if that's more your thing. I think all that's required is RODI, table salt, and an accurate gram scale. ($8 from ebay or amazon – useful in the kitchen too!)
@Brent Turner, assuming this doesn't turn out to be a tools issue, then just take out some salt water and replace it with RODI.
This still leaves the question of why salinity got that high – are you using saltwater to replace water lost to evaporation?