Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

RKUR
84475
carbohydrates passing through the GI tract.
Useful For: Work-up of a patient with chronic diarrhea
Interpretation: Typically, stool potassium is 20 times serum potassium. A useful formula is 2x (stool
sodium + stool potassium) = stool osmolality + or - 30 mOsm. Fecal potassium concentration and daily
excretion rate are usually below the median level in patients with osmotic diarrhea. Normal fecal sodium
and potassium in the presence of an osmotic gap (>30 mOsm/kg) suggests osmotic diarrhea. Increased
fecal sodium content or daily excretion rate with normal fecal potassium and no osmotic gap indicates
secretory diarrhea. High fecal potassium in association with normal or low fecal sodium suggests
deterioration of the epithelial membrane or a bleeding lesion. High sodium and potassium (3 times
normal) in the absence of an osmotic gap indicate active electrolyte transport in the gastrointestinal (GI)
tract that might be induced by agents such as cholera toxin, hypersecretion of vasointestinal peptide, or
islet cell tumor. For very low stool osmolality, consider factitial diarrhea. The fecal potassium
concentration and excretion rate are increased 2-fold to 3-fold with ulcerative colitis, or bleeding into the
GI tract, when exposed to cholera toxin, with ingestion of mineralocorticoids, in primary aldosteronism,
and due to bacterial metabolism of unabsorbed carbohydrates. The fecal water potassium concentration
and daily excretion rate exceeds 3 times normal in association with islet cell tumors and increased
secretion of vasointestinal peptide.
Reference Values:
0-15 years: not established
> or =16 years: 0-199 mEq/kg
Clinical References: 1. Phillips S, Donaldson L, Geisler K: Stool composition in factitial diarrhea: a
6-year experience with stool analysis. Ann Intern Med 1995;123:97-100 2. Agarwal R, Afzalpurkar R,
Fordtran JS: Pathophysiology of potassium absorption and secretion by the human intestine.
Gastroenterology 1994;107:548-571 3. Ho J, Moyer T, Phillips S: Chronic diarrhea: the role of
magnesium. Mayo Clin Proc 1995;70:1091-1092
Potassium, Random, Urine
Clinical Information: Potassium (K+) is the major intracellular cation. Functions of K+ include
regulation of neuromuscular excitability, heart contractility, intracellular fluid volume, and hydrogen ion
concentration. The physiologic function of K+ requires that the body maintain a low extracellular fluid
(ECF) concentration of the cation; the intracellular K+ concentration is 20 times greater than the
extracellular concentration. Only 2% of total body K+ circulates in the plasma. The kidneys provide the
most important regulation of K+. The proximal tubules reabsorb almost all the filtered K+. Under the
influence of aldosterone, the remaining K+ can then be secreted into the urine in exchange for sodium in
both the collecting ducts and the distal tubules. Thus, the distal nephron is the principal determinant of
urinary K+ excretion. Decreased excretion of K+ in acute renal disease and end-stage renal failure are
common causes of prolonged hyperkalemia. Renal losses of K+ may occur during the diuretic (recovery)
phase of acute tubular necrosis, during administration of nonpotassium sparing diuretic therapy, and
during states of excess mineralo- corticoid or glucocorticoid.
Useful For: Determining the cause for hyper- or hypokalemia
Interpretation: Hypokalemia reflecting true total body deficits of K+ can be classified into renal and
nonrenal losses based on the daily excretion of K+ in the urine. During hypokalemia, if urine excretion of
K+ is 30 mEq/d in a hypokalemia setting is inappropriate and indicates that the kidneys are the
primary source of the lost K+.
Reference Values:
No established reference values
Clinical References: 1. Tietz Textbook of Clinical Chemistry. 3rd edition. Edited by CA Burtis, ER
Ashwood. Philadelphia, WB Saunders Co, 2001 2. Toffaletti J: Electrolytes. In Professional Practice in
Current as of January 3, 2013 2:22 pm CST 800-533-1710 or 507-266-5700 or MayoMedicalLaboratories.com Page 1466