Arriving at diagnoses from Serum Sodium and Urine Osmolality

A. Conditions with Hypernatremia: Both the following have low Sodium in urine.

Central Diabetes Insipidus [CDI]-loose a lot of water in urine due to central issue causing high serum sodium.

Nephrogenic Insipidus [NDI] – Central Diabetes Insipidus [CDI]-loose a lot of water in urine due to kidney issue causing high serum sodium.

B. Conditions with Hyponatremia:

SIADH-Euvolemic, loses Na in urine so high Na in urine is seen.

Psychogenic Polydypsia [PP]-drinking too much water diluting serum and urine with both showing low Na

Cerebral salt wasting disease [CSWD]-Salt is wasted in urine so high Na in urine is seen.

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You may be given the following values

  1. Urine Osmolality
  2. Urine Osmolality after DDAVP [Desmopressin]
  3. Serum Sodium
  4. Urine Sodium
  5. Urine osmolality after water deprivation

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Normal values:

Sr Na=135-145

Urine osmolality=50-1200

Urine Na=20-40

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Arriving at a diagnosis: Tip to remember: UO UOD SS

Step 1: Look at Urine Osmolality.

Urine Osmolality is Low in conditions-CDI, NDI, PP

Urine Osmolality is High in conditions-SIADH, CSWD

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Step 2: Look at Urine Osmolality after DDAVP [Desmopressin].

DDAVP mimics the actions of endogenous ADH, acts as a selective agonist of V2 receptors expressed in the renal collecting duct to increase water re-absorption and reduce urine production.

DDAVP is also given as treatment for Platelet dysfunction in ESRD, against Rapid Na correction to prevent Osmotic Demyelination Syndrome and in Mild vWD.

Urine osmolality goes up in CDI, PP,CSWD.

Urine osmolality remains the same in NDI. [Kidney is not responding]

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Step 3: Look at Serum Sodium

Serum Sodium is High in CDI, NDI

Serum Sodium is Low in PP, SIADH, CSWD.

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Urine Na is low in Hepatorenal syndrome, PP.

Urine Na is high in SIADH, CSWD

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Lets do some questions

Question 1: Urine Osmolality=70, Post DDAVP urine osmolality=650, Serum Sodium=145, Urine Sodium =12, Post water restriction urine osmolality=75

Step 1: Look at Urine Osmolality, 70 is low. It could be CDI, NDI or PP.

Step 2: Look at Urine Osmolality after DDAVP [Desmopressin], 650 is improved to normal which means kidney is working fine. So it is a central problem like CDI or PP.

Step 3: Look at Serum Sodium, 145 is high consistent with CDI.

So, the answer is CDI.

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Question 2: Urine Osmolality=64, Post DDAVP urine osmolality=650, Serum Sodium=130, Urine Sodium =14, Post water restriction urine osmolality=550

Step 1: Look at Urine Osmolality, 64 is low, could be CDI, NDI, or PP

Step 2: Look at Urine Osmolality after DDAVP [Desmopressin], 650 which is improved due to DDAVP working like ADH. This happens in CDI or PP.

Step 3: Look at Serum Sodium, 130 is low which happens in PP.

So, the answer is PP.

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Practice Question 3: Urine Osmolality=70, Post DDAVP urine osmolality=75, Serum Sodium=150, Urine Sodium =14, Post water restriction urine osmolality=80

What is the diagnosis here?

Answer is NDI.

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