urine&electrolytes&and& osmolality& · •...

URINE ELECTROLYTES AND OSMOLALITY

Assoc. Prof. Damjan Kovač, MD, PhD University Medical Centre Ljubljana Department of Nephrology

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Urine electrolytes

•  Na+

•  K+

•  Cl-‐

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•  NH4+

•  Mg2+

•  Ca2+

•  PO4-‐

When are urine electrolytes indicated?

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In the following clinical circumstances: •  Acute kidney injury •  Disorders of intravascular volume •  Hyponatremia •  Hypernatremia •  Polyuria •  Acid – base disorders •  AbnormaliYes of serum potassium concentraYon

Remember! 1.  electrolyte and osmolality analysis should be done before insYtuYon of therapy 2. urine can be saved and analysed at a later Yme

Osmolality

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Number of moles of solute per kg of solvent (water).

Urine osmolality can vary in healthy person from 80 -‐1200 mOsm/kg H2O

Most important osmols in the urine are caYons Na+, K+, NH4+ with their

corresponding anions and urea. It can be calculated:

Uosm = 2 x (Na+ + K+ + NH4+) + (urea) + (glucose)

It is measured by an osmometer.

Osmolality [mOsm/kg H2O] = depression of freezing point [mK] ÷ 1.858

1.  Freezing point osmometers 2.  Vapor pressure osmometers 3.  Membrane osmometers

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Nonosmotic influences:

•  Drugs

•  ê efective arterial volume

•  Nausea

•  Postoperative pain

•  Pregnancy

High blood osmoticity

AnYdiureYc hormone (ADH)

ADH acYon

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ADH

H2O

H2O

H2O

urine renal interstitium

H2O

H2O

H2O

H2O H2O

H2O

Protein kinaze A

ATP

cAMP

d i s t a l t u b u l e

V2 receptor

OSMOLALITY vs. SPECIFIC GRAVITY

•  Osm: determines only number of parYcles •  SG: determines number of paYcles and their weight

•  Usually they change in paralel •  SG of 1.020-‐1.030 à osmolality 800-‐1200 mOsm/kg H2O

•  SG of 1.005 à osmolaity <100 mOsm/kg H2O •  DisproporYonate increase of SG if urine contains

–  glucose or proteins –  radiocontrast material

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Sodium excreYon

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•  Filtered in the glomerulus (25.000 mmol/day)

•  1-‐3 % of this filtreed load is excreted in the urine (FRACTIONAL EXCRETION)

•  Na+ determianYon in the urine is very suitable for differenYal diagnosis of AKI

•  Any acute kidney injury causes Na+ loss (é FE)

•  In hypovolemia and hypoperfusion Na+ is rebasorbed to a grater extent (ê FE)

•  DeterminaYon of 24-‐h Na+ excreYon is useful tool for determinaYon of salt consumpYon (17 mmol Na+ = 1 g salt)

Impact of hypovolemia on urine Na+

Hypovolemia

é Renin

Na+ tubular reabsorbYon

é Angiotensin II

é Aldosterone

é ADH

Concentrated urine with low Na+

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Diagnosing the cause of AKI by urine Na+

Prerenal azotemia? or Acute tubular necrosis? Urine Na+ < 20 mmol/L ê prerenal azotemia (osmolality > 500 mOsm/kg) Urine Na+ > 40 mmol/L ê acute tubular necrosis (osmolality < 350 mOsm/kg)

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Diagnosing the cause of AKI by urine Na+

STATES WHEN URINE Na+ CAN BE MISLEADING •  Metaboloc alkalosis with volume depleYon (vomiYng):

Urine Na+ is not maximally reduced à obligate Na+ excreYon with HCO3

-‐

Urine Cl-‐ is more accurate in such condiYons (Cl-‐ < 20 mmol/L). •  Prerenal azotemia in preexisYng CKD •  DiureYc use

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HCO3-‐

Na+

FracYonal excreYon of Na+

The most accurate test to disYnguish prerenal AKI from ATN in oliguric AKI (24h urine < 400 ml) DefiniYon: % of filtered Na+ excreted in the urine FE 1 % à 99 % of filtered Na+ reabsorbed, 1 % excreted in urine

urine Na+ X serum creaYnine FENa% = x 100

urine creaYnine X serum Na+

< 1%: prerenal AKI >2 %: ATN

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Piqalls of FENa It should allways be taken with cau8on. It is validated only in persons with oliguria. FENa in healthy person is allways <1%. Low in several renal condiYons not associated with volume depleYon:

•  GN •  contrast AKI •  pigment nephropathy •  vascular diseases •  vasculiYs •  liver disease

Some hypovolemic paYents will not have low FENa. Value of excreted sodium is related to GFR: as GFR decreases-‐ FENa increases. Elderly pts and pts with CKD are not able to generate FENa <1%.

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Hyponatremia

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•  The most aboundant electrolyte disturbance, present in 15% to 30% of hospitalized paYents

•  Hyponatremia represents an excess of water rela8ve to solute in the body.

•  Hyponatremia usually develops as a result of the acYon of ADH in the kidney to diminish free water excreYon.

•  Most common s8muli are nonosmo8c: drugs, pain, nausea, decreased effecYve arterial volume, strenuous exercise ...

•  Rarely develops as a consequence of •  consuming very low quanYYes of solute or •  polydipsia

Hyponatremia and anYdiureYc hormone

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280  285 290 Serum osmolality (mOsm/kg H20)

Serum [Na+] 135 mmol/L è max. suppressed ADH

ê

urine osmolality < 100 mOsm/kg

Nonosmotic ADH production

•  Drugs

•  ê effective art. volume

•  Nausea

•  Postoperative pain

•  Pregnancy

Physiological respons in

hyponatremia:

U-‐Osm < P-‐Osm

137 Na+ 145

Urine Na+ and osmolality in hyponatremia Urine sodium (mmol/L)

Urine osmolality (mOsm/kg)

True volume depleYon < 20 >300

EfecYve circulaYng volume depleYon

< 20 >300

SIADH > 40 if consuming regular diet >100-‐150, not maximaly dilute

Volume depleYon and SIADH Variable, may be <20 unYl volume depleYon corrected

>300

Water intoxicaYon Variable, may be low due to dilute urine

<100

Low solute intake (tea and toast) May be <20 Low (less than serum osmolality)

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Adapted from: Muser WP, Korzelius CA. Hosp Med Clin 2012; 1: e338-‐e352.

Urine chemistries in hypernatremia

Reason Urine osmolality (mOsm/kg) Urine sodium (mmol/L)

Water depleYon >600-‐800 Variable

Water and volume depleYon >600-‐800 <20

OsmoYc diuresis (hyperglicemia, manitol)

>300 Variable

Diabetes insipidus (central or nephrogenic)

<300 (<100 in complete nephrogenic DI)

Variable (depends on intake)

Salt intoxicaYon >600-‐800 >20 (very high)

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•  Deficit of free water relaYve to solute.

•  Inadequate free water intake because of impaired access, impaired thirst (in elderly paYents).

•  Most adult paYents with hypernatremia also have concurrent volume depleYon.


Urine chemistry in polyuria


Urine sodium (mmol/L)

Water diuresis < 300 Low (total daily excreYon matches intake)

Solute diuresis Usually > 300 Variable (high if sodium diuresis)

Water diuresis: polydipsia diabetes insipidus Solute (osmoYc) diuresis: i.v. NaCl

hyperalimentaYon hyperglycemia high protein intake recovery from AKI

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Diagnosing polyuria

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Water restricYon test: 1. At 8.00 a.m.: a) Empty bladder, record the volume, and send urine osmolality b) Take a serum osmolality c) Record the paYent's weight. 2. Then for the next 8 hours check: Urine osmolality and weight every hour

Serum osmolality every 2 hours Stop the test if the paYent's weight decreases by more than 3% of body weight (or by 4kg) or the serum osmolality rises >300 mOsm/kg/H2O. 3. If the urine osmolality at 4 p.m. remains <600mOsm/kg proceed with the desmopressin test. a) Desmopressin (DDAVP) 20 μg intranasally or 2 µg intramuscularly b) Can eat and drink freely c) Hourly urine volumes and osmolality unYl 8.30 p.m.

Polydypsia Central DI Nephrogenic DI

Serum osmolality (mOsm/kg)

<295 >300 >300


>600 (400-‐600 if chronic)

<300 ORen <200

<300

Post DDAVP urine osmolality (mOsm/kg)

No or mild response

>400-‐600 No response

Acid-‐base disorders

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Kidney role in maintaining acid-‐base balance: -‐ resorbYon or excreYon HCO3

-‐

-‐ secreYon of H+

-‐ producYon of NH3 Most acid-‐base disorders can be diagnosed and treated without measurement of urine electrolytes. ExcepYons: 1. METABOLIC ALKALOSIS Urine Cl-‐ more useful than urine Na+ to asses volume status. HCO3

-‐ excreted in urine and followed by Na+. Urine Cl-‐ < 20 mmol/L suggests volume depleYon. 2. NON-‐ANION-‐GAP METABOLIC ACIDOSIS Urine electrolyte measurement in non-‐anion-‐gap metabolic acidosis to disYnguish between bicarbonate loss and impaired renal acid excreYon.

HCO3-‐

Na+

Metabolic alkalosis

?

Urine anion-‐gap

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In steady state UAG is posiYve: with a mean 41 ± 9 mmol/L. NH4

+= 0.8 (Cl-‐-‐Na++K+) x Vu + 80 In severe diarrhea with metabolic acidosis urine NH3 will increase and be secreted with Cl-‐ à negaYve urine anion gap In type 4 and type 1 RTA NH3 excreYon is defecYve à posiYve urine anion gap

NH4+ NH4+

K+ K+

Na+ Na+

Cl-‐

Cl-‐

Other anions

Other anions

NegaYve anion gap

PosiYve anion gap

UAG = (urine Na+ + urine K+) -‐ (urine Cl-‐)

Diarrhea

RTA 4

NH3 + H+ à NH4+

éNH4+ à é Cl-‐

Potassium disorders

•  Daily K+ dietary load is 80-‐120 mmol, excreted mainly in the urine.

•  The amount of excreted K+ arises from the tubular excreYon in the distal and collecYng tubule – effect of aldosterone.

•  Daily K+ excreYon: 10 mmol -‐ 400 mmol.

•  The most important indicaYon for determinaYon of urine K+ is differenYal diagnosis of hypokalemia.

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Hypokalemia, UK+ < 10 mmol/L Hypokalemia, UK+ > 30 mmol/L

Reduced potassium intake Renal potassium loss

Extrarenal potassium loss

TRANSTUBULAR POTASSIUM GRADIENT

urine K+/serum K+ TTKG = urine osmolality/serum osmolality

High value: kidney is excreYng K+ Low values: conservaYon of K+ Urine must be isotonic or hypertonic to serum (300 mOsm/kg) Urine Na+ concentraYon must be >25 mmol/L

Potasium concentraYon in a urine spot is difficult to interpret without daily urine volume and urine concentraYon. •  to assess kidneys tendency to reabsorb or excrete potassium •  it is a surogate measure of aldosteron effect

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K+ K+

CollecYng duct

Distal capillary

TTG in hypokalemia and hyperkalemia

Hypokalemia

Hyperkalemia

Transtubular potassium gradient

< 2

> 2

< 7

> 10

Cause gastrointesYnal loss

hyper-‐ aldosteronism

hypo-‐aldosteronism

Normal aldosterone effect

Example Diarrhea Conn syndrome ACEI, spironolactone

Potassium shiz, overload

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Magnesium excreYon

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•  Mg++ concentraYon in serum does not provide accurate informaYon about the actual Mg++ content in the body.

•  Renal Mg++ excreYon is suprisingly accurate at regulaYng the total Mg++ content.

•  In Mg++ deficiency excreYon in the urine is reduced, therefore Mg++ excreYon can be used for diagnosing of Mg++ deficiency, even though Mg++ concentraYon is normal.

Hypomagnesemia, UMg++ < 0.5 mmol/L

Hypomagnesemia, UMg++ > 1.5 mmol/L

Extrarenal magnesium loss

Renal magnesium loss

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Conclusion

Urinary electrolytes and osmolality are useful tool in many clinical situaYons, and they have to be used more ozen, to show their full value.

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