5+ Year Member
Jan 27, 2013
First Aid 2014 p325 states that giving water restriction test on central DI will increase urine osmolarity by >50%. I'm assuming this is because even in central DI, there is still some ADH action going on. However, in Pathoma's lectures he states that giving that same test in central DI would fail to increase urine osmolarity. Which one is correct? Or is this one of the cases where both are "kinda" correct? Kind of like Pathoma is theoretically correct (if absolute no ADH production) but in actuality FA is clinically correct (due to usually at least a little ADH production)?
Jun 30, 2015
Zurich, Switzerland

I think FA means by "water restriction test" also the second part of the test, where you give desmopressin.
After desmopressin, you will see a rise in urine osmolarity in central DI.

FirstAid said:
No water intake for 2–3 hr followed by hourly measurements of urine volume and osmolarity and plasma Na+ concentration
and osmolarity. DDAVP (ADH analog) is administered if normal values are not clearly reached.
You can find the ">50%"-statment also in the wikipedia article, here a screenshot:

Here is also a nice overview:

However, there is a true point to what you mention in the question. With "partial central diabetes insipidus", one can observe a rise in ADH induced by water restriction. This is perhaps due to receptor upregulation in full deprivation.

Does this make sense? Hopefully it helps.
Last edited:
Jun 30, 2015
Zurich, Switzerland
Just as a side note and to go one step further.
What helped me to understand all this conditions, was plotting them on a XY-diagram. Maybe this helps you too.
I made the following simple diagram:

Try to draw the following conditions:

  1. Line: Healthy patient, who is becoming thirsty and doesn't have access to water.
  2. Area: Where would you find patients with central or nephrogenic DI.
  3. Point: Patient with central/nephrogenic DI without access to free water. (See 6)
  4. Point: Patient with central/nephrogenic DI with access to free water. (See 7)
  5. Point: Primary polydipsia patient. (See 8)

    Water restriction/desmopressin test:
  6. Line: Where would patient "3" move with water restriction and then desmopressin.
  7. Line: Where would patient "4" move with water restriction and then desmopressin.
  8. Line: Where would patient "5" move with water restriction and then desmopressin.

  9. Point: Where would you find a patient with SIADH?
Maybe this helps!
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Aug 10, 2015
Haha I love this thread.
Normal guy posts thread asking question --> Maybe 3 sentences explaining something about 3 hours later
Hot girl posts thread asking question --> An entire page of information with follow up diagrams and further information within the hour.

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5+ Year Member
Apr 28, 2013

Water restriction test should increase the urine osmolarity by 25%, not 50%, in a normal patient, according to UWorld