Thread by @aishaikh

💥How to use Urine Electrolytes for assesment & diagnosis of Kidney Disorders?

A great article in @CJASN
👉🏽 cjasn.asnjournals.org/content/14/2/3…

⚡️Summary of the article is in this🧵
#MedEd
#FOAMed
#NephPearls

💥Urine Na is used for:

⚡️Assessment of effective circulatory volume
⚡️Differentiation of Pre-renal Azotemia vs. ATN

💥Spot Urine Na

⚡️Urine Na < 15 mEq/L suggests ⬇️ effective circulatory volume BUT the caveat is:

‼️Spot Urine Na is dependent on the amount of ‘water’ in the urine

⚡️Hence spot Urine Na can be ⬇️ in the setting of water diuresis & ❌ always mean ⬇️ volume

💥This is why we use Fractional Excretion of Sodium: FeNa

FeNa = (UNa x PCr/ PNa x UCr) x 100

⚡️FeNa provides a measure of Urine Na handling independent of urine concentration (meaning independent of the ‘water’ in the urine)

💥FeNa is used to differentiate b/w Pre-Renal Azotemia & ATN

⚡️FeNa < 1% suggests Pre-renal Azotemia & hence volume-responsive whereas a ⬆️ FeNa suggests ATN and hence not volume-responsive

‼️But FeNa is NOT always reliable in making this distinction

💥Limitations of FeNa:

⚡️There are conditions that cause ATN but also cause intense renal vasoconstriction resulting in ⬇️ filtered load of Na, resulting in
⬇️ FeNa despite presence of ATN👇🏽

⚡️Sepsis
⚡️IV contrast
⚡️NSAIDs
⚡️Rhandomyolysis

💥FeNa can also be misleadingly ⬇️ in the following conditions despite presence of ATN:

⚡️CHF
⚡️Cirrhosis
⚡️Extensive burns

‼️Intense neurohumoral activation =
Low FeNa

💥FeNa can be misleadingly ⬆️ despite ⬇️ effective circulatory volume in the setting of diuretic use (due to natriureis):

‼️In this situation, Fractional Excretion of Urea (FeUrea) can be used

💥FeUrea
= (Ur Urea x Pcr/ P Urea x Ucr)x 100
= < 35% suggests ⬇️ volume

⬇️ volume -> ⬆️ prox. tubular water & urea reabsorption ⬇️ FeUrea

‼️So w/ distal diuretic use, loop or thiazides, but NOT w/ proximal diuretic use, FeUrea can be of help

💥FeNa can be misleadingly ⬆️ despite ⬇️ volume when:

⚡️⬇️ volume is accompanied by ⬆️ in non-reabsorbable anions in urine, such as HCO3-, as HCO3- is excreted in urine paired w/ Na+

‼️Use Urine Cl and NOT FeNa to assess volume status in met. alkalosis

💥This is why in metabolic akalosis, Urine Cl is used as a marker to determine whether the metabolic alkalosis would be responsive to Cl-containing IVF or not:

⚡️⬇️ Urine Cl: responsive to IVF
⚡️⬆️ Urine Cl: not responsive to IVF

💥Urine Chloride (Cl) excretion mirrors Urine Na excretion & hence both move in the same direction in response to changes in the effective circulatory volume

‼️Except when volume changes are accompanied by Acid-Base disorders

💥If Urine Na to Urine Cl ratio is >1.6 in the setting of ⬇️ volume then:

⚡️An accompanying anion is causing obligatory Na loss in the urine despite an appropriately ⬇️ Urine Cl in response to neurohumoral activation due to ⬇️ effective circulatory volume

💥If UNa to Ur. Cl ratio is >1.6 in the setting of ⬇️ circulatory volume then:

‼️Check Urine pH: this will help identify the Non-reabsorbable Anion causing the obligatory Na loss

⚡️Ur. pH 7-8 = Bicarbonaturia
⚡️Ur. pH < 6 = Ketoanions or Drugs
👇🏽👇🏽👇🏽

💥If the Urine Na to Urine Cl ratio is
< 0.7 in the setting of ⬇️ effective circulatory volume then that suggests:

⚡️An accompanying cation in the urine causing obligatory Cl loss

⚡️Eg. ⬆️ ammonium excretion in urine as it is excreted as ammonium chloride

💥Urine Cl is helpful in work-up of:
⚡️Metabolic Alkalosis &
⚡️Metabolic Acidosis
👇🏽👇🏽👇🏽

💥Urine Potassium (K)

⚡️In Hypokalemia,
Step 1: ✅ determine if K loss is renal or extra-renal

⚡️Check Spot Urine K
< 5-15 mEq/L = extra-renal K loss
> 40 mEq/L = renal loss
‼️But Spot Urine K can be misleading as it can vary w/ urine concentration

💥To overcome this limitation of Spot Urine K one can use the following:

‼️Urine K to Urine Creatine ratio
⚡️Ratio of < 13 mEq/g
OR
⚡️Ratio of < 2.5 mEq/mmol
Both indicate extra-renal K loss

💥Once it is determined that hypokalemia is due to renal or extra-renal loss based on the:

Urine K to Urine Cr ratio then the next steps are:

Step 2: ✅Check BP & assess effective circulatory volume👇🏽

Step 3: ✅Plasma HCO3👇🏽

Step 4: ✅Urine Chloride👇🏽

💥Urine Anion Gap & Urine Osmolar Gap in Metabolic Acidosis

⚡️In normal anion gap matabolic acidosis, determine if the source of the acidosis is renal or extra-renal

⚡️Calculate Urine Anion Gap (UAG):
(Urine Na + Urine K) - Urine Cl

💥Positive UAG = source of met. acidosis is renal
💥Negative UAG = source of met. acidosis is extra-renal

‼️Why?

Because ⬇️ urine ammonium excretion by the kidney in the setting of metabolic acidosis means that the kidney is NOT getting rid of the acid

💥Why is Urine Anion Gap positive in the setting of ⬇️ urine ammonium excretion?

Because ammonium is excreted as ammonium chloride, now recall the UAG equation:

(Urine Na + Urine K) - Urine Cl

So ⬇️ ammonium excretion in urine = ⬇️ Urine Cl = Positive UAG

💥Urine Anion Gap can also be misleading - Why?

⚡️Recall that in the UAG equation we are only accounting for Urine Na, K & Cl
‼️So presence of ANY unmeasured ions (besides Na, K, Cl) can be misleading:
⚡️Ketoacids
⚡️Na Hippurate
⚡️D-lactate

💥When UAG is not reliable to assess urine ammonium excretion then use:
Ur. Osmolal Gap

Ur. Osm. Gap >100 mOsm/kg=⬆️ ammonium excretion= Met. acidosis cause is extra-renal

Limitations: Ur. Osm. Gap can be ⬆️ due to non-ammonium solutes (mannitol, alcohols)

💥This🧵is a review of how to use the following urine chemistries to assess & diagnose kidney disorders

⚡️Urine Sodium
⚡️Fractional excretion of Na
⚡️Fractional excretion of Urea
⚡️Urine Chloride
⚡️Urine Potassium
⚡️Urine Anion Gap
⚡️Urine Osmolar Gap
End/