CCM 62: Urine Osmolality and Electrolytes

Question 1

What is the expected urine osmolality in patients with DI, excess water administration, polydipsia?

Answer 1

< 200 mOsm/kg

Question 2

How will synthetic colloids affect urine specific gravity?

Answer 2

will make it read falsely high - cannot use USG after giving HES

Question 3

How do you calculate the fractional excretion of sodium?

Answer 3

FeNa (%) = 100 x [ (urine Na x plasma creatinine) / (plasma Na x urine creatinine)]

start with urine and sodium

Question 4

What FeNa cutoff is suggestive of volume responsive versus intrinsic renal injury?

Answer 4

< 1% - volume responsive
> 1% intrinsic damage

Question 5

What is indicated by a urine Na concentration < 20 mmol/L?

Answer 5

kidneys are trying to conserve Na and volume –> aldosterone working against a decreased ECV

Question 6

What is indicated by a urine Na cc of > 40 mmol/L?

Answer 6

normal ECV

or if hypovolemia–> suggestive of hypoadrenocorticism

see Na Drobatz ER book for more differentials (Na chapter)

Question 7

When is urine Cl instead of urine Na cc recommended to assess for adequate ECV?

Answer 7

alkalosis –> would increase urine Na cc regardless of ECV as it is needed for concurrent bicarbonate secretion

Question 8

What urine measurements can be used to assess for renal versus extrarenal causes of potassium derangements?

Answer 8

urine K cc
* hypokalemia <15-20&raquo_space; nonrenal loss
* hypokalemia > 40&raquo_space; renal K wasting
* hyperkalemia > 40&raquo_space; nonrenal cause

Transtubular potassium gradient
* hyperkalemia > 6 nonrenal cause
* hyperkalemia < 6 renal cause (aldosterone deficiency!!)
* hypokalemia < 3 nonrenal cause
* hypokalemia > 7 renal K wasting

Question 9

How do you calculate the TTKG?

Answer 9

= (urine K x plasma osmolality) / (urine osmolality x plasma K)

Question 10

How do you calculate the urinary free water clearance?

Answer 10

urine volume x [1- (urine Na + urine K)/ serum Na]

Question 11

How can you differentiat chloride-responsive from chloride-resistant metabolic alkalosis?

Answer 11

urine chloride cc

chloride-responsive alkalosis - caused by chloride loss and will improve with Chloride administration
* urine Cl concentratin < 15-25 mmol/L

chloride-resistant or unresponsive alkalosis - caused by HCO3- gain and won’t improve with Chloride administration
* urine Cl concentration > 15-25

Question 12

What does a positive versus free water clearance indicate?

Answer 12

positive&raquo_space; water excretion e.g., absence of ADH e.g., during hypoosmolality

negative&raquo_space; water retention e.g., ADH activity