Kidney Essentials Season 1 Episode 3 What’s in a Urine?
A podcast for Medical students, residents, and advanced practitioners at the University of Colorado and beyond
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Sophie- tell us about our mission
Mission statement (Sophie):
- Make nephrology more accessible, less intimidating
- Provide concise nephrology “pearls” in each episode to help listeners understand renal pathophysiology
- “Making nephrology sexy one episode at a time”
Legal disclaimer (Judy):
This podcast is for educational purposes only. The views and statements expressed on this podcast are solely those of the hosts. This podcast should not be used as medical advice or for treatment purposes
Getting to know your hosts-
This is the part of the podcast where for our audience to get to know us better we start off with a fun non-medical question for our hosts. This episodes question is:
What is your earliest childhood memory?
Podcast episode 3 we have several cases to elucidate how nephrologists use urine studies and chemistries to evaluate patients
First Case:
A 70 yo man is found down in his home by his daughter who came to check on him. He has a medical history of hypertension and early dementia. She had spoken to him earlier in the week and he had reported having the “flu”. His BP 145/89, P of 90 and the medicine team calls because he has not produced much urine since admission
Sophie, what urine study might be helpful in this scenario
Sophie: Una: used to assess effective circulating volume
Normal Una is 40-220meq/day
A low Una < 15meq/L suggest avid reabsorption of sodium by the kidney proximally in response to reduced effective circulating volume. Importantly, the fact that the kidney can get teh uNa this low tells us the cells are doing what they are supposed to and we have INTACT SODIUM REGULATION BY THE KIDNEYS
Sarah: and I just want to point out Sophies use of the word effective circulating volume, because you can have a lot of salt and water but not be effectively perfusing your kidneys (eg CHF and cirrhosis) but this is NOT due to inadequate salt intake
Judy, what things can mess up the Una and make it not reflective of effective circulating volume?
Judy: 1. Extreme urine volume
a) as we discussed in our first episode and mini bonus episode. If you are having a water diuresis and producing ~ 7 liters of UOP- even though you are not salt deficient and excreting 100meq of NA a day on a spot urine this will come back as 14meq/L which in the absence of knowing the patient is making 7 liters of urine would suggest ineffective renal perfusion
b) highly concentrated urine eg 25meq/Iliter with a total of 1 liter os UOP suggests a Na excretion which is quite low
2. Nonreabsorbable anions ie ketone, sodium bicarbonate (check pH)
Sarah: okay so to recap extreme urine volumes and anions- negatively charged molecules which can not be reabsorbed alongside Na and so lead to an obligatory increase in U na
so the team sends of a Una and it returns at 10 meq/L. What does this tell you?
Sophie: since the team already told us that this patient is making VERY LITTLE URINE, a low urine sodium like 10meq/L suggests that the kidney is NOT being perfused. Based on the clinical history, volume depletion from the “flu” is my greatest suspicion.
Sarah: wHAT DOES THE LOW UNA HELP DISTINGUISH FROM?
Sophie: While there are always exceptions to the rule, at this point, I feel fairly comfortable that this is not acute tubular necrosis (otherwise lovingly referred to as ATN).
Sarah: Judy, what other entities other than volume depletion could cause a low Una?
Judy: CHF, cirrhosis, extensive burns, early ATN, after contrast, early obstruction
Sarah: you & your team do an h& P and there is no evidence of cirrhosis, burns or CHF. The team calculated the FENA and it was <1%. Sophie, what is the FENA?
Sophie: Fena Una x P creatinine/Pna X U creatinine. Literally is the percent of salt filtered that is excreted in the urine. Also a measure of avid reabsorption and ineffective circulating volume
Sarah: Judy, given the Una of 10 meq/L and the FeNA of 1%, what is your assessment and how would you treat the patient
Judy: both suggest the patient will be volume responsive. excluded other causes of a low Na. cautious of rate
- discuss intermittent bolus of fluids over maintenance
Sarah: okay so this case highlights how the Una and Fena can be used to determine if an oliguric patient will be volume responsive. Just a reminder- disease states other then volume depletion that can give you a low Una and fena which should not be treated with volume include CHF and cirrhosis
Case 2
A 27 yo woman presents to the ER with near syncope. She reports fluid retention with swelling of the face, hands, trunk, and limbs. She was evaluated by an outside physician who found no evidence of cardiac, hepatic, or renal disease. She was prescribes a loop diuretic (furosemide 40mg po bid) for her symptoms
Sarah: Sophie- would a Una help us figure out if this patient is volume depleted and whether that is the cause of her near syncope
Sophie:
This patient was given furosemide, a loop diuretic, more aptly named lasix. Lasix works by blocking sodium, potassium and chloride reabsorption in the thick ascending loop of henle. In doing so, that means that those electrolytes will remain in her urine instead of being reabsorbed into her bloodstream. Therefore, it will cause sodium loss in her urine causing volume depletion while elevating sodium measurements in her urine and FeNa calculations. So, if her Una is LOW it would be dx of volume depletion, but if it is not low, she may still be volume depleted but uNa is high due to the diuretic.
Judy, Is there a test we can use to help determine if this patient is volume depleted in the setting of a loop diuretic?
Judy: Fractional excretion of urea
Fe Urea= UureaX P creatinine/ Purea X Ucreatinine
FEurea< 35% suggests decreased effective volume
Sarah: okay so a Fe urea can be helpful to determine if some one is volume depleted when a loop has been administered.
There are some situations when FE urea is NOT helpful and those include:
- Acetazolamide
- Osmotic diuresis
- Increased urea
- Elderly & sepsis- in these patient urea reabsorption is diminished in the proximal tubule and falsely high Fe urea
Case 3
A 55 yo male is admitted with weight loss and a pelvic abscess/possible mass. He reports having lost 50 lbs in the past year and several lbs in the past week. He has had fever, nausea and vomiting. His primary team placed an NGT which is on LIS. You are called for a rise in his creatinine from 1 to 1.4mg/dL. His UA is remarkable only for a UpH of 7. His chem 7 reveals a K of 3 and a hco3 of 30
How useful will a Una be in this setting?
Sophie- Una may not be helpful in this case. YOu can get a high Una in the setting of volume depletion if there is an nonreabsorbable anion such as Hco3. The U Ph and the chem7 suggest a metabolic alkalosis from his NGT. A high amount of filtered hco3 2/2 the metabolic alkalosis will lead to an increase In Una even if the patient is volume depleted
Judy- I s there a test which may be more helpful in this setting then U na
Judy: typically Una ~Ucl
because you have to excrete an anion with every cation and vice versa
but you may get a High U na and low Ucl in the setting of a metabolic alkalosis and volume depletion. Checking a Ucl here would be very helpful. Urine Cl of <20 or low Fecl would suggest volume depletion
Sophie- I would like to add that the ratio of Una to Cl in these patients can be suggestive. If the Una to Ucl ratio is > 1.6 this suggest volume depletion in the setting of a an extrarenal generation of metabolic alkalosis as in this case
Sarah: So to summarize
A Ucl or FeCl may be more helpful in determining volume depletion in the setting of a metabolic alkalosis from non renal causes such as this patient
learning objectives
1. Una and Fe na can help determine effective circulating volume
Una and fena are not indicative of body na in the setting of CHF and cirrhosis
Una may be misleading in the setting of extreme urine volumes
Fe urea can be used in place of Fena in the context of a patient who has been on a loop diuretic
U cl can be used in place of na when patient has a known resorbable anion which may lead to an elevated Una despite volume depletion
That ends this episode of Kidney Essentials. Thanks for tuning in!Sophie says goodbye
Judy says goodbye
Credits:
Seamus Klingsporn for editing
Josh strong for graphics
And of course the University of Colorado division of nephrology for giving us a job!
