Fluid & Electrolyte Disorder

Test

Interpretation

Notes

Electrolytes (Na, K, Cl)

Calculate Stool Osmotic Gap (SOG) 290-[ 2([Na]+[K])]

· Secretory type diarrhea SOG

· Osmotic type diarrhea SOG >75-100

· If SOG negative, suggests presence of additional anion like phosphate

· If [Cl] as high or higher than [Na] suggestive of congenital chloride diarrhea

Order as ‘electrolytes, fluid’ select ‘stool’

Magnesium

· Obtain when unexplained osmotic type diarrhea.

· >75 mEq/L suggests Mg containing substance

Order as ‘magnesium, fluid’ select ‘other’ and in comments write ‘stool’

Osmolality

· Should be close to serum osmolality

· If substantially less than serum, suggests contamination with water or urine

Order as ‘osmolality, fluid’ select ‘stool’; send on ice

Test

Interpretation

Notes

Sodium

FENa = UNa + PCr x100

PNa + UCr

· Amount of excreted sodium is determined by RAAS and intake;

· Urine Na concentration will depend on water reabsorption; FENa+ corrects for water reabsorption

· If suspect intravascular volume contraction but clinical data unclear, FENa

· FENa inaccurate when patients are on IV fluids

Or diuretics

· If clinical euvolemia but urine Na+ is

Order as ‘Electrolytes, urine’ which gets you Na, K, and Cl; order ‘Creatinine, urine spot’; Also order Chem 7 on same day

Potassium

Transtubular K gradient (TTKG)

Uk + Posm

Pk + Uosm

· Concentration depends on water content of urine, which depends on volume status

· TTKG corrects for water reabsorption

· TTKG is indirect measure of K secretion in the distal nephron

· TTKG depends on dietary K and the activity of aldosterone

· >10 suggests hyperaldosterone state, as in hypovolemia

· TTKG not important when serum potassium is normal—used to characterize normal or abnormal kidney response in states of hyper and hypokalemia

Order as ‘Electrolytes, urine’ which gets you Na, K, and Cl; order ‘Creatinine, urine spot’; Also order Chem 7 on same day

Hypokalemia

Appropriate TTKG

Hyperkalemia

· TTKG should be high, indicating appropriate kidney response

· consider decreased GFR, acid/base disturbance, insulin deficiency as cause in this case

· If it is abnormally low say 2-5, the kidney is not excreting K appropriately. Sometimes this is due to insufficient urine sodium. Urine sodium needs to be over 40 (ideally) to allow distal secretion of K.

Chloride

· Differential diagnosis of metabolic alkalosis

· Urine chloride is low in patients with metabolic alkalosis when it is due to vomiting, NG suction (loss of HCl), or chronic diuretic use leading to chloride depletion, laxative abuse, CF, and congenital chloride diarrhea. Also in patients with certain salt-wasting disorders such as Bartter or Gitelman syndrome.

· Urine chloride is usually NOT low (>40) in patients with metabolic alkalosis and volume expansion due to certain forms of monogenic hypertension, including primary aldosteronism, Liddle’s syndrome, excess licorice intake, and apparent mineralocorticoid excess.

Specific gravity

· Ratio of the weight of a solution to weight of water

· Water = 1.000

· Range 1.001 – 1.035

· SG and Osm rise in parallel

· Large molecules increase SG disproportionately (like contrast and proteins)

· If s.g. >1.035, consider glucose, contrast exposure.

Osmolality

· Non-infant kidneys should be able to concentrate to 1200 and dilute to 50 mOsm/Kg

· In setting of hyponatremia, urine osmolality can distinguish between SIADH (osmolality high) and excessive water ingestion (osmolality low)

Order as ‘osmolality, urine’