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  #1  
Old 03-07-2013
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Default pt w/ severe water deprivation vs. pt w/ SIADH

Which of the following would best distinguish an otherwise healthy person with severe water deprivation from a person w/ the syndrome of inappropriate antidiuretic hormone (SIADH)?

A) Free-water clearance (CH2O)
B) Urine osmolarity
C) Plasma osmolarity
D) Circulating levels of antidiuretic hormone (ADH)
E) Corticopapillary osmotic gradient

Answer & explanation, please. Thank you.
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Quote:
Originally Posted by luschka View Post
Which of the following would best distinguish an otherwise healthy person with severe water deprivation from a person w/ the syndrome of inappropriate antidiuretic hormone (SIADH)?

A) Free-water clearance (CH2O)
B) Urine osmolarity
C) Plasma osmolarity
D) Circulating levels of antidiuretic hormone (ADH)
E) Corticopapillary osmotic gradient

Answer & explanation, please. Thank you.
C plasma osmolarity
decreased in SIADH vs increased in water deprived healthy person
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But, I thought that in a water deprivation test...if you force a normal person not to drink, it should concentrate the urine....thus, having a urine osmolarity (high solutes, low water) > plasma osmolarity (low solutes, high water)?

Can you please explain how is it that the person with water deprivation will have a higher plasma osmolarity when compare to their urine osmolarity?

Thanks!
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Originally Posted by luschka View Post
But, I thought that in a water deprivation test...if you force a normal person not to drink, it should concentrate the urine....thus, having a urine osmolarity (high solutes, low water) > plasma osmolarity (low solutes, high water)?

Can you please explain how is it that the person with water deprivation will have a higher plasma osmolarity when compare to their urine osmolarity?

Thanks!
actually plasma osmolarity < urine osmolarity
but when u compare to SIADH patient who is producing large quantities of ADH he is going to have higher plasma concentration
cause SIADH patient is constantly diluting his plasma
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Water deprivation increased plasma osmolarity.

SIADH decreases plasma osmolarity.

Sometimes, the simplest logic is the right answer.

Plasma and Urine osmolarity differences come in play when you are comparing Primary Polydipsia and Diabetes Insipidus. Note that both disorders decrease urine osmolarity (increased free water clearance) but plasma osmolarity will be high in central DI (decreased ADH --> increased free water clearance, state of "dehydration" induced), whereas low in Primary Polydipsia (excessive water intake dilutes plasma solutes and also causes diuresis).
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Quote:
Originally Posted by ameerakhan View Post
Water deprivation increased plasma osmolarity.

SIADH decreases plasma osmolarity.

Sometimes, the simplest logic is the right answer.

Plasma and Urine osmolarity differences come in play when you are comparing Primary Polydipsia and Diabetes Insipidus. Note that both disorders decrease urine osmolarity (increased free water clearance) but plasma osmolarity will be high in central DI (decreased ADH --> increased free water clearance, state of "dehydration" induced), whereas low in Primary Polydipsia (excessive water intake dilutes plasma solutes and also causes diuresis).
Thank you for your answer.

However, I'm still unclear about the water deprivation test. I thought that in a water deprivation test...if you force a normal person not to drink, it should concentrate the urine....thus, having a urine osmolarity (high solutes, low water) > plasma osmolarity (low solutes, high water)?

Is there a different in urine/plasma osmolarity when conducting a water deprivation test for a health person vs. a DI patient?

Maybe this answer to the above question will clear up my misunderstanding for this subject.

Thanks again!
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Quote:
Originally Posted by luschka View Post
Thank you for your answer.

However, I'm still unclear about the water deprivation test. I thought that in a water deprivation test...if you force a normal person not to drink, it should concentrate the urine....thus, having a urine osmolarity (high solutes, low water) > plasma osmolarity (low solutes, high water)?

Is there a different in urine/plasma osmolarity when conducting a water deprivation test for a health person vs. a DI patient?

Maybe this answer to the above question will clear up my misunderstanding for this subject.

Thanks again!
Sure, here you go...

A water deprivation test is what you'd do when you have a patient having either Diabetes Insipidus or Primary Polydipsia and you want to find out what the diagnosis is.

Both conditions present with polyuria (urine osmolarity low) and low serum osmolarity. However, with Polydipsia, the urine osmolarity will increase on the water deprivation test (lower free water clearance as ADH kicks in) and become greater than the plasma osmolarity.

On the other hand, in case of Diabetes Insipidus, water deprivation will result in barely any ability to concentrate the the urine...and urine osmolarity will not show much improvement (beyond the little increase that's due to restricting some water - but the primary defect remains the same... ADH cannot conserve water).

You are right when you say a normal person on water deprivation will have a higher urine osmolarity than plasma osmolarity. In this test, the person with Primary Polydipsia really is a "normal person" in the sense that he's basically just drinking more water. There's nothing wrong with his urine concentrating ability... which is what becomes unmasked when you just hold back his water.

I hope that makes some sense?

Essentially, this test differentiates a "normal" person (he maybe a psychiatric condition like anxiety or OCD though, hence all that drinking) from someone with Diabetes Insipidus who, either doesn't have ADH (Central) or whose kidneys are resistant to ADH's effect (Nephrogenic).

Based on the water deprivation test, you can also further test ADH levels. Failure to concentrate the urine associated with low ADH -> Central DI
Failure to concentrate the urine associated with a high ADH -> Nephrogenic DI
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Quote:
Originally Posted by ameerakhan View Post
Water deprivation increased plasma osmolarity.

SIADH decreases plasma osmolarity.

Sometimes, the simplest logic is the right answer.

Plasma and Urine osmolarity differences come in play when you are comparing Primary Polydipsia and Diabetes Insipidus. Note that both disorders decrease urine osmolarity (increased free water clearance) but plasma osmolarity will be high in central DI (decreased ADH --> increased free water clearance, state of "dehydration" induced), whereas low in Primary Polydipsia (excessive water intake dilutes plasma solutes and also causes diuresis).
bear with me if you can.
we are comparing 2 people here right.
1)person being deprived of water 2) person with increased ADH.
so in 1) his urine osmolarity will increase and his plasma osmolarity will also increase ? - doubt.
for person 2) his urine osmolarity will increase and his plasma osmolarity will decrease.
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oh okay,is it because it is SEVERE water depriviation that 1) person's plasma osmolarity will also increase.?
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Originally Posted by samren17 View Post
oh okay,is it because it is SEVERE water depriviation that 1) person's plasma osmolarity will also increase.?
Yes... that's what I meant. Usually, the values given (as in Kaplan's endo section, there's a table highlighting these differences) show increase in plasma osmolarity on water deprivation. Remember that a person drinking excess water will have slightly lower plasma osmolarity that what is taken as normal, so when he stops drinking that water, the figure will "increase" towards normal while urine also, eventually, becomes concentrated (but more than plasma of course, since water now has to be conserved).
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  #11  
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awesome.this question better come in my exam now.
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