Test ID: UOSMF
Osmolality, Feces
Secondary ID 
A test code used for billing and in test definitions created prior to November 2011
NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.
Useful For Suggests clinical disorders or settings where the test may be helpful
In conjunction with a serum osmolality to calculate an osmotic gap
Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea)
Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test
Method Name A short description of the method used to perform the test
Freezing Point Depression
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Aliases Lists additional common names for a test, as an aid in searching
Lytes, Feces
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.
Container/Tube: Stool container (Supply T291)
Specimen Volume: Entire collection
Collection Instructions:
1. Collect a very liquid stool specimen.
2. Random and timed specimens are acceptable.
3. See Stool Collection Information Sheet in Special Instructions.
Additional Information:
1. Results will be reported as mOsm/kg regardless of collection duration.
2. 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."
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.
Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected
| Hemolysis | NA |
| Lipemia | NA |
| Icterus | NA |
| Other | Formed stool |
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
| Specimen Type | Temperature | Time |
|---|---|---|
| Fecal | Frozen (preferred) | 14 days |
| Refrigerated | 48 hours |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major affecter of this equilibrium.
Stool osmolality is normally 2x (stool sodium + stool potassium) unless there are exogenous factors inducing a change in this ratio (demonstrated by the presence of an osmotic gap), such as the presence of other osmotic agents (eg, magnesium, sulfate, saccharides), or drugs (eg, bisacodyl, phenolphthalein) inducing secretions.
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.
0-15 years: not established
> or =16 years: 220-280 mOsm/kg
Interpretation Provides information to assist in interpretation of the test results
Typically, stool osmolality is similar to serum since the gastrointestinal tract does not secrete water. A useful formula is 2x (stool sodium + stool potassium) = stool osmolality + or - 30 mOsm.
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.
An osmotic gap >100 mOsm/kg indicates factitial diarrhea, likely due to magnesium or phenolphthalein consumption.
For very low stool osmolality, consider factitial diarrhea.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test should be performed on watery stools (diarrhea).
In the event a soft or formed stool is submitted, the test will not be performed, and 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."
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
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
Method Description Describes how the test is performed and provides a method-specific reference
The depression of the freezing point of the stool preparation is used to measure osmolality. The extent of lowering below 0 degrees C (the freezing point of water) is a function of the concentration of substances dissolved in the stool. 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)
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
Test Classification Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.
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 (see CPT Coding in Special Instructions)
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 |
|---|---|---|
| 9258 | Osmolality, F | 2693-0 |
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