Intended for healthcare professionals

Rapid response to:

Practice Practice Pointer

Diabetes insipidus

BMJ 2019; 364 doi: https://doi.org/10.1136/bmj.l321 (Published 28 February 2019) Cite this as: BMJ 2019;364:l321

Rapid Response:

Re: Diabetes insipidus

I had initially planned to write to point out the incompatible advice in this article: The text states that serum osmolality >295mOsmol/Kg is significant whilst the highlight box states that <295mOsmol/Kg is significant. The correct advice being that a high serum osmolality due to excessive water loss is important [i.e. >295mOsmol/L]. This point had already been made.

There is however another simple test that can differentiate polyuria due to psychogenic polydipsia from polyuria due to diabetes insipidus [an important distinction because the treatments are diametrically different - restrict water vs. do not restrict water]. Patients with diabetes insipidus tend to have high (above upper limit of reference range) serum sodium concentrations whilst those with psychogenic polydipsia have low sodium concentrations. In my experience, this simple test is just as effective as a water deprivation test at identifying those who have to urinate to get rid of their excess water load, from those who have to drink to keep up with their excessive urination.

Competing interests: No competing interests

06 March 2019
Tim Reynolds
Consultant Chemical Pathologist
Queen's Hospital
Belvedere Rd.