texrmate is based on the Debye-Hückel equation, which is, though complicated enough, only really applicable to solutions with ionic strength up to 100mmol/l, while normal plasma has an ionic strength of about 160mmol/l. The result of the calculation on this page has thus to be taken with a grain of salt. Nevertheless, it gives a good idea about the kind of influence grossly abnormal serum Na+ values can have on your measurements.