Electrolyte and Osmolality Panel, Feces
NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.
Workup of cases of chronic diarrhea
Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea)
Profile Information A profile is a group of laboratory tests that are ordered and performed together under a single Mayo Test ID. Profile information lists the test performed, inclusive of the test fee, when a profile is ordered and includes reporting names and individual availability.
|Test ID||Reporting Name||Available Separately||Always Performed|
|OG_F||Osmotic Gap, F||No||Yes|
Osmotic Gap: Calculation
Sodium, Potassium, and Chloride: Indirect Ion-Selective Electrode (ISE) Potentiometry
Osmolality: Freezing Point Depression
Phosphorus: Photometric, Ammonium Molybdate
Magnesium: Colorimetric Titration
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Electrolyte and Osmolality Panel, F
Specimen Type Describes the specimen type needed for testing
Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.
Collection Container/Tube: Stool container (Supply T291)
Specimen Volume: 10 g
1. Collect a very liquid stool specimen.
1. Do not send formed stool. In the event a formed stool is submitted, the test will not be performed. The report will indicate "A formed stool specimen was submitted for analysis. This test was not performed because it only has clinical value if performed on a watery stool specimen."
2. Osmolality results will be reported as mOsm/kg regardless of collection duration.
3. Sodium, chloride, and potassium will be reported as mmol/L
4. Magnesium and phosphorus will be reported as mg/dL
Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
|Fecal||Frozen (preferred)||14 days|
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
The concentration of electrolytes in fecal water and their rate of excretion are dependent upon 3 factors:
-Normal daily dietary intake of electrolytes
-Passive transport from serum and other vascular spaces to equilibrate fecal osmotic pressure with vascular osmotic pressure
-Electrolyte transport into fecal water due to exogenous substances and rare toxins (eg, cholera toxin)
Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major affector of this equilibrium. Fecal osmolality is normally 2 x (sodium + potassium) unless there are exogenous factors inducing a change in composition, such as the presence of other osmotic agents (magnesium sulfate, saccharides) or drugs inducing secretions, such as phenolphthalein or bisacodyl.
Osmotic diarrhea is caused by ingestion of poorly absorbed ions or sugars and can be characterized by the following:
-Stool volume typically decreased by fasting
-Fecal fluid usually has an elevated osmotic gap
-Osmotic agents such as magnesium, sorbitol, or polyethylene glycol may be the cause through the intentional or inadvertent use of laxatives
-Carbohydrate malabsorption due most commonly to lactose intolerance
-Carbohydrate malabsorption can be differentiated from other osmotic causes by a low stool pH (<6)
Secretory diarrhea is caused by disruption of epithelial electrolyte transport and can be characterized by the following:
-Stool volume is usually unaffected by fasting
-Fecal fluid usually has elevated electrolytes (primarily sodium and chloride) and a low osmotic gap (<50 mOsm/kg)
-Common causes include bile acid malabsorption, inflammatory bowel disease, endocrine tumors, and neoplasia
-Secretory agents such as anthraquinones, phenolphthalein, bisacodyl, or cholera toxin should also be considered
-Infection is a common secretory process; however, it does not typically cause chronic diarrhea (defined as symptoms >4 weeks)
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
No established reference values
-Osmotic gap is calculated as 290 mOsm/kg-(2[Na]+2[K]). Typically, stool osmolality is similar to serum since the gastrointestinal (GI) tract does not secrete water.(1)
-An osmotic gap >50 mOsm/kg is suggestive of an osmotic component contributing to the symptoms of diarrhea.(1-3)
-Magnesium-induced diarrhea should be considered if the osmotic gap is >75 mOsm/kg and is likely if the magnesium concentration is >110 mg/dL.(1)
-An osmotic gap < or =50 mOsm/kg is suggestive of secretory causes of diarrhea.(1-3)
-A highly negative osmotic gap or a fecal sodium concentration greater than plasma or serum suggests the possibility of either sodium phosphate or sodium sulfate ingestion by the patient.(4)
-Phosphorus elevation >102 mg/dL is suggestive of phosphate-induced diarrhea.(4)
-Sodium is typically found at lower concentrations (mean 30 +/- 5 mmol/L) in patients with osmotic diarrhea caused by magnesium-containing laxatives, while typically at higher concentrations (mean 104 +/- 5 mmol/L) in patients known to be taking secretory laxatives.(5)
-Stool osmolality <220 mOsm/kg indicates dilution with a hypotonic fluid.(1)
-Stool osmolality >330 mOsm/kg in the absence of increased serum osmolality indicates improper storage.(1)
Sodium and Potassium:
-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.(1)
-Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.(6)
-Fecal chloride may be elevated (>35 mmol/L) in phenolphthalein- or phenolphthalein plus magnesium hydroxide-induced diarrhea.(3)
-Fecal chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(3)
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Steffer KJ, Santa Ana CA, Cole JA, Fordtran JS: The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea. Gastroenterol Clin North Am 2012;41:539-560
2. Sweetser S: Evaluating the patient with diarrhea: A case-based approach. Mayo Clin Proc 2012;87:596-602
3. Eherer AJ, Fordtran JS: Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology 1992;103:545-551
4. Fine KD, Ogunji F, Florio R, et al: Investigation and diagnosis of diarrhea caused by sodium phosphate. Dig Dis Sci 1998;43(12):2708-2714
5. 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
6. Casprary WF: Diarrhea associated with carbohydrate malabsorption. Clin Gastroenterol 1986;15:631-655
7. Ho J, Moyer TP, Phillips SF: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092
8. Fine KD, Santa Ana CA, Fordtran JS: Diagnosis of magnesium-induced diarrhea. N Engl J Med 1991;324:1012-1017
Method Description Describes how the test is performed and provides a method-specific reference
Calculated result = 290 mOsm/kg - 2 (stool Na [mmol/L] + stool K [mmol/L]).
The depression of the freezing point of serum or other fluid is used to measure osmolality using an Advanced Instruments osmometer. The extent of lowering below 0 degrees C (the freezing point of water) is a function of the concentration of substances dissolved in the serum. By definition, 1 milliosmole per kilogram lowers the freezing point 0.001858 degrees C.(Murphy JE, Henry JB: Evaluation of renal function, and water, and electrolyte, and acid base balance. In Todd-Sanford-Davidsohn Clinical Diagnosis and Management by Laboratory Methods. 16th edition. Edited by JB Henry. Philadelphia, WB Saunders Company, 1979, pp 135-152)
Sodium, Potassium, and Chloride:
The Roche Cobas c 501 analyzer dilutes samples 1:31 and makes use of the unique properties of certain membrane materials to develop an electrical potential (electromotive force: EMF) for the measurements of ions in solution. The electrode has a selective membrane in contact with both the test solution and an internal filling solution. The internal filling solution contains the test ion at a fixed concentration. The membrane EMF is determined by the difference in concentration of the test ion in the test solution and the internal filling solution. The EMF develops according to the Nernst equation for a specific ion in solution.(Package insert: Roche ISE reagent. Roche Diagnostics Corp, 2009-09, V8)
In the presence of sulfuric acid, inorganic phosphate and ammonium molybdate form an ammonium phosphomolybdate complex. The concentration of phosphomolybdate formed is measured photometrically and is directly proportional to the inorganic phosphate concentration.(Package insert: Roche Phosphorus reagent. Roche Diagnostics Corp., Indianapolis, IN, 2010-10, V5)
In an alkaline solution, magnesium forms a purple complex with xylidyl blue, a diazonium salt. The magnesium concentration is measured photometrically via the decrease in the xylidyl blue absorbance.(Package insert: Magnesium reagent. Roche Diagnostics Corp., Indianapolis, IN, 2012-04, V2)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Monday through Friday; evening
Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.
Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result
Specimen Retention Time Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded
Performing Laboratory Location The location of the laboratory that performs the test
CPT Code Information Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.
84999 x 2-Osmolality, Potassium
LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.
|Result ID||Reporting Name||LOINC Code|
|OG_F||Osmotic Gap, F||73571-2|
|POU_F||Phosphorus, F||In Process|