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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # |
Osmotic Gap, Feces
Test CodeCPT Codes
84302, 84311
Includes
Potassium, Feces
Fecal Osmotic Gap
Preferred Specimen
Minimum Volume
Instructions
Note: Specimen must be shipped frozen to reduce the odor during shipping and to minimize the risk of the container rupturing due to gas accumulation.
This test only has clinical utility if performed on a watery fecal specimen.
Stable up to 3 freeze-thaw cycles
Transport Temperature
Specimen Stability
Refrigerated: 7 days
Frozen: 60 days
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Methodology
Flame Photometric
Setup Schedule
Reference Range
Clinical Significance
This panel measures sodium and potassium in liquid stool and provides an estimated osmotic gap. Measurements of electrolytes and osmotic gap in liquid stool help differentiate between secretory and osmotic diarrhea and, thus, guide further evaluation of chronic diarrhea [1,2].
Depending on how the water content of stool is drawn in the intestine lumen, diarrhea can be categorized as secretory or osmotic. Secretory diarrhea is caused by defective electrolyte absorption that leads to increased stool osmolality. Conditions associated with secretory diarrhea include infection, neuroendocrine tumors, congenital chloridorrhea, intestinal resection, and diffuse mucosal disease [1]. Osmotic diarrhea is caused by the osmotic effect of a substance that retains water in the intestine lumen and leads to decreased stool osmolality. Conditions associated with osmotic diarrhea include ingesting poorly absorbed solutes (eg, sorbitol, mannitol, and laxatives containing magnesium) and enzyme dysfunction (eg, lactose intolerance) [2].
Osmotic gap, calculated as the difference between the stool osmolality and twice the sum of the stool sodium and potassium levels, is useful in assessing the contribution of stool electrolytes to osmolality and differentiating between secretory and osmotic diarrhea [1,2]. A theoretical fecal osmolality, approximately the same as plasma osmolality (290 mOsm/kg), is used in the calculation.
The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.
References
1. Schiller LR. Pract Res Clin Gastroenterol. 2012;26(5):551-562.
2. Block DR, et al. Body fluids. In: Rifai R, et al. eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier Inc; 2022.
Performing Laboratory
Quest Diagnostics Nichols Institute |
14225 Newbrook Drive |
Chantilly, VA 20153 |