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Test Code EFPO Electrolyte and Osmolality Panel, Feces

Reporting Name

Electrolyte and Osmolality Panel, F

Useful For

Workup of cases of chronic diarrhea


Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea)

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Specimen Type


Advisory Information

This test is only clinically valid if performed on watery specimens. In the event a formed fecal specimen is submitted, the test will not be performed.

Specimen Required

Patient Preparation: No barium, laxatives, or enemas may be used for 96 hours prior to start of, or during, collection.

Supplies: Stool containers - 24, 48, 72 Hour Kit (T291)

Container/Tube: Stool container

Specimen Volume: 10 g

Collection Instructions: Collect a very liquid stool specimen.

Specimen Minimum Volume

5 g

Specimen Stability Information

Specimen Type Temperature Time Special Container
Fecal Frozen (preferred) 14 days
  Refrigerated  7 days
  Ambient  48 hours

Reference Values

An interpretive report will be provided

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information





84999 x 2-Osmolality, Potassium

LOINC Code Information

Test ID Test Order Name Order LOINC Value
EFPO Electrolyte and Osmolality Panel, F 88697-8


Result ID Test Result Name Result LOINC Value
NA_F Sodium, F 15207-4
K_F Potassium, F 15202-5
CL_F Chloride, F 15158-9
MG_F Magnesium, F 29911-5
OG_F Osmotic Gap, F 73571-2
POU_F Phosphorus, F 88713-3
OSMOF Osmolality, F 2693-0

Clinical Reference

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. Ho J, Moyer TP, Phillips SF: Chronic diarrhea: the role of magnesium. Mayo Clin Proc. 1995;70:1091-1092

3. Fine KD, Santa Ana CA, Fordtran JS: Diagnosis of magnesium-induced diarrhea. N Engl J Med. 1991;324:1012-1017

4. Fine KD, Ogunji F, Florio R, Porter J, Ana CS: Investigation and diagnosis of diarrhea caused by sodium phosphate. Dig Dis Sci. 1998;43(12):2708-2714

5. Eherer AJ, Fordtran JS: Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103:545-551

6. Casprary WF: Diarrhea associated with carbohydrate malabsorption. Clin Gastroenterol. 1986;15:631-655

7. Sweetser S: Evaluating the patient with diarrhea: A case-based approach. Mayo Clin Proc. 2012;87:596-602

8. Phillips S, Donaldson L, Geisler K, Pera A, Kochar R: Stool composition in factitial diarrhea: a 6-year experience with stool analysis. Ann Intern Med. 1995;123:97-100

9. Makela S, Kere J, Holmberg C, Hoglund P: SLC26A3 mutations in congenital chloride diarrhea. Hum Mutat. 2002 Dec;20(6):425-438. doi: 10.1002/humu.10139

10. Ali OM, Shealy C, Saklayen M: Acute pre-renal failure: acquired chloride diarrhea after bowel resection. Clin Kidney J. 2012;5(4):356-358. doi: 10.1093/ckj/sfs082

Method Description

Osmotic Gap:

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 in most osmometers. The extent of lowering below 0° 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° C.(Schnidler EI, Brown SM, Scott MG: Electrolytes and Blood Gases. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:610-612)


Sodium, Potassium, and Chloride:

The Roche cobas c 501 analyzer 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 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; V14 02/2018)



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; V9.0 09/2019)



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; V8.0 01/2020)

Reject Due To

All specimens will be evaluated at Mayo Clinic Laboratories for test suitability.

Method Name

OG_F: Calculation

NA_F, K_F, CL_F: Indirect Ion-Selective Electrode (ISE) Potentiometry

OSMOF: Freezing Point Depression

POU_F: Photometric, Ammonium Molybdate

MG_F: Colorimetric Titration


If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen.

Profile Information

Test ID Reporting Name Available Separately Always Performed
NA_F Sodium, F No Yes
K_F Potassium, F No Yes
CL_F Chloride, F Yes Yes
OSMOF Osmolality, F Yes Yes
MG_F Magnesium, F Yes Yes
OG_F Osmotic Gap, F No Yes
POU_F Phosphorus, F Yes Yes

Secondary ID