Questions about diabetes insipidus

[bulldoc]

---

Members do not see this ad.

I don't remember exactly, but this might have been a question on NBME form 2, so if you don't want a spoiler, I'm giving you an out right !




















Is water deprivation the best test to distinguish central vs nephrogenic? I imagine ADH administration would work just as well...just curious as to your thoughts on this subject.

 

[BlackNDecker]

The best test is administration of ADH. It easiest to think about it logically...if you give ADH and they respond then it is definitely Central DI, thus ruling out Nephrogenic DI.

How would water deprivation diagnose Central or Nephrogenic DI??? All it would show is that the Urine Osm are less than the Plasma Osm FOR BOTH DISORDERS...this simply reinforces the suspicion of DI.

 

[bulldoc]

The best test is administration of ADH. It easiest to think about it logically...if you give ADH and they respond then it is definitely Central DI, thus ruling out Nephrogenic DI.

How would water deprivation diagnose Central or Nephrogenic DI??? All it would show is that the Urine Osm are less than the Plasma Osm FOR BOTH DISORDERS...this simply reinforces the suspicion of DI.

on page 286 of Goljan's RR of path, he has a pink box that states that "The water deprivation test distinguishes central DI from nephrogenic DI." Then he goes on to talk about ADH injections....

Do you have any references to back up your statement, or is it more of a logical hypothesis?

I had the same thought...

 

[BlackNDecker]

The best test is administration of ADH. It easiest to think about it logically...if you give ADH and they respond then it is definitely Central DI, thus ruling out Nephrogenic DI.

How would water deprivation diagnose Central or Nephrogenic DI??? All it would show is that the Urine Osm are less than the Plasma Osm FOR BOTH DISORDERS...this simply reinforces the suspicion of DI.

Wow...I'm a dumba$$.

Forgot that the H2O deprivation is a two-step test. OK, if you infuse ADH without first depriving the pt of water...if you get no response there is the possibility that they are volume overloaded and ANP is counteracting the ADH. I believe the rationale for the "water deprivation" is to prevent the ANP inhibition of anti-diuresis.

 

[bulldoc]

One more thought...

I think the best screening test is water deprivation, where you measure the serum ADH levels after deprivation. If high, nephrogenic. If low, central. ADH administration will theoretically work, but in reality only helps confirm the diagnosis...and only really works when the disease is VERY central or VERY nephrogenic (you know what i'm saying? lol)

What do you guys think?

 

[bulldoc]

Wow...I'm a dumba$$.

Forgot that the H2O deprivation is a two-step test. OK, if you infuse ADH without first depriving the pt of water...if you get no response there is the possibility that they are volume overloaded and ANP is counteracting the ADH. I believe the rationale for the Water deprivation test is to prevent ANP inhibition of anti-diuresis.

Thanks for the feedback. 👍

 

[BlackNDecker]

Thanks for the feedback. 👍

Thanks for making me go back and look this up...

I can't think of any other reason for the water deprivation.

 

[Idiopathic]

you dont ever measure adh levels.

water deprivation test - urine concentrates/serum Na+ stays normal, dx is PRIMARY POLYDIPSIA

if urine still does not concentrate (I think ~50% is what you look for) and serum sodium rises, then administer DDAVP. if urine concentrates/serum Na+ starts to return to normal, then dx is CENTRAL DI

otherwise, NEPHROGENIC DI

 

[bulldoc]

you dont ever measure adh levels.

water deprivation test - urine concentrates/serum Na+ stays normal, dx is PRIMARY POLYDIPSIA

if urine still does not concentrate (I think ~50% is what you look for) and serum sodium rises, then administer DDAVP. if urine concentrates/serum Na+ starts to return to normal, then dx is CENTRAL DI

otherwise, NEPHROGENIC DI

Idiopathic, where did you read that you never measure serum ADH? my google results (emedicine, etc) show that serum ADH can be part of the diagnosis.

Now I'm more confused than ever. So if a question pops up asking, "what is the best test to distinguish central from nephrogenic DI," what would you put? Exogenous ADH or water deprivation?

Must be a bad question...with all the focus on water deprivation in the diagnosis of DI, my gut instinct tells me that that is the answer.

 

[DOCTORSAIB]

I hope this doesn't add to the confusion. Here goes:

Dx of DI is made using the water deprivation test. This test INCLUDES giving vasopressin (ADH). Here's how it works:

1) Hold all water, check hourly Urine osm.
2) When Urine osm stable for 3 hours, check Serum osm, THEN give vasopressin.

3) Normal people: Urine osm after deprivation > Serum osm. Vasopressin administration causes INCREASED Urine osm of LESS THAN 10%. (Read that again, it makes sense).

4) Central DI: Urine osm AFTER deprivation no greater than Serum osm. However, vasopressin administration causes INCREASED Urine osm of GREATER THAN 10%. (That makes sense too since ADH was low/non-existent to begin with)

5) Nephrogenic DI: Urine osm after deprivation no greater than Serum osm AND vasopressin administration DOES NOT increase Urine osm. (yup, since there was plenty of ADH already floating around)

I hope this clears up any confusion.

This info came from B&W for Step 1. G'luck.

 

[Idiopathic]

Idiopathic, where did you read that you never measure serum ADH? my google results (emedicine, etc) show that serum ADH can be part of the diagnosis.

Now I'm more confused than ever. So if a question pops up asking, "what is the best test to distinguish central from nephrogenic DI," what would you put? Exogenous ADH or water deprivation?

Must be a bad question...with all the focus on water deprivation in the diagnosis of DI, my gut instinct tells me that that is the answer.

exogenous ADH administration is the best test to distinguish between the two types of DI. measuring ADH levels is actually pretty worthless, IMHO, especially for a cerebral test like USMLE.

water deprivation is the first test in suspected DI, becaues you need to rule out that the person is simply drinking too much H2O, especially if senility/drugs are a concern. after that dx has been ruled out, then DDAVP is given to distinguish between the two DI's. You could also measure serum ADH levels, but these fluctuate, and you would still have to do the water test first or your result would look like central DI (pees all the time, low ADH).

 

[Kashue]

on page 286 of Goljan's RR of path, he has a pink box that states that "The water deprivation test distinguishes central DI from nephrogenic DI." Then he goes on to talk about ADH injections....

Do you have any references to back up your statement, or is it more of a logical hypothesis?

I had the same thought...

Black's right, it's Vasopressin administration. I have Goljian RR - he says oyu need to first water restrict them, then do the ADH test.

He says water deprivation, and then ADH administration, you should get > 50 % in Uosm with central vs < 50% with Nephrogenic.