Sorted By Test Name - Mayo Medical Laboratories

RNAUR
84522
+ potassium) unless there are exogenous factors inducing a change in this ratio, such as the presence of
other osmotic agents (magnesium sulfate, saccharides), or drugs inducing secretions, such as
phenolphthalein or bisacodyl. If the relationship between osmolality, potassium, and sodium (see equation
above) indicates an osmotic gap, diarrhea may be osmotically induced by exogenous agents such as
magnesium sulfate or saccharides. If there is no osmotic gap but sodium concentration is 2 to 3 times
normal, secretory diarrhea may be the cause, and agents such as phenolphthalein, bisacodyl, or cholera
toxin should be suspected.
Useful For: Determining the cause of chronic diarrhea
Interpretation: Typically, stool sodium is similar to serum since the gastrointestinal (GI) tract does
not secrete water. A useful formula is 2x (stool sodium + stool potassium) = stool osmolality + or - 30
mOsm. Increased fecal sodium content or daily excretion rate with normal fecal potassium and no osmotic
gap indicates secretory diarrhea. Normal fecal sodium and potassium in the presence of an osmotic gap
(>30 mOsm/kg) suggests osmotic diarrhea. Normal or low fecal sodium in association with high fecal
potassium suggests deterioration of the epithelial membrane or a bleeding lesion. High sodium and
potassium in the absence of an osmotic gap indicate active electrolyte transport in the GI tract that might
be induced by agents such as cholera toxin or hypersecretion of vasointestinal peptide. If sodium
concentration or 24-hour sodium excretion rate is 2 to 3 times normal and osmotic gap >30 mOsm/kg,
secretory diarrhea may be the cause. Agents such as phenolphthalein, bisacodyl, or cholera toxin should
be suspected. For very low stool osmolality, consider factitial diarrhea.
Reference Values:
0-15 years: not established
> or =16 years: 0-159 mEq/kg
Clinical References: 1. Phillips S, Donaldson L, Geisler K, et al: Stool composition in factitial
diarrhea: a 6-year experience with stool analysis. Ann Intern Med 1995;123:97-100 2. Ho J, Moyer T,
Phillips S: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092
Sodium, Random, Urine
Clinical Information: Sodium (Na+) is the primary extracellular cation. Na+ is responsible for
almost one half the osmolality of the plasma and, therefore, plays a central role in maintaining the normal
distribution of water and the osmotic pressure in the extracellular fluid compartment. The amount of Na+
in the body is a reflection of the balance between Na+ intake and output. The normal daily diet contains 8
to 15 grams of sodium chloride (NaCl), which is nearly completely absorbed from the gastrointestinal
tract. The body requires only 1 to 2 mmol/d, and the excess is excreted by the kidneys, which are the
ultimate regulators of the amount of Na+ (and thus water) in the body. Na+ is freely filtered by the
glomeruli. Approximately 70-80% of the filtered Na+ is actively reabsorbed in the proximal tubules with
chloride and water passively following in an iso-osmotic and electrically neutral manner. Another 20% to
25% is reabsorbed in the loop of Henle along with chloride and more water. In the distal tubules,
interaction of the adrenocortical hormone aldosterone with the coupled sodium-potassium and sodiumhydrogen
exchange systems directly results in the reabsorption of Na+ and indirectly of chloride from the
remaining 5% to 10% of the filtered load. It is the regulation of this latter fraction of filtered Na+ that
determines the amount of Na+ excreted in the urine.
Useful For: Assessing acid-base balance, water balance, water intoxication, and dehydration
Interpretation: Urinary Na+ excretion varies with dietary intake, and there is a large diurnal variation
with the rate of Na+ excretion during the night being only 20% of the peak rate during the day. Na+ may
be lost in the kidneys as a result of diuretic therapy, salt-losing nephropathies, or adrenal insufficiency,
with the urinary Na+ concentration usually more than 20 mEq/L. In these hypovolemic states, urine Na+
values