Laboratory Utilization and Analytical Validation of Fecal Electrolyte Tests

Area of Science:

• Clinical Chemistry
• Gastroenterology
• Diagnostic Laboratory Medicine

Background:

• Chronic diarrhea is classified as fatty, watery, or inflammatory.
• Watery diarrhea subtypes (secretory vs. osmotic) are differentiated by fecal electrolytes and osmotic gap.
• Clinical utilization of these measurements is uncertain despite endoscopic advancements.

Purpose of the Study:

• To determine the clinical utility of stool electrolytes in chronic diarrhea.
• To validate the performance characteristics of stool electrolyte assays on the Beckman Coulter AU680.
• To address practical laboratory concerns regarding stool specimen validation for electrolyte analysis.

Main Methods:

• Literature review and historical data to establish clinically relevant ranges for stool electrolytes (Na+, Cl-, K+, phosphate, Mg2+).
• Medical chart review of 44 patients to assess clinical utility in chronic diarrhea work-up.
• Analytical validation including linearity, precision, stability, and accuracy (comparison with Roche Cobas 6000 c501) on the AU680.

Main Results:

• Stool electrolytes and osmotic gap were found to be valuable in chronic diarrhea diagnosis.
• Assay imprecision ranged from 0% to 5.9%, with good linearity.
• Accuracy showed minimal bias (-0.48 to 2.39) compared to a reference instrument.
• Stability: Na+, Cl-, K+ stable refrigerated for 5 days; Phosphate and Mg2+ stable for 48 hours.

Conclusions:

• Stool osmotic gap measurement is clinically valuable for chronic diarrhea evaluation.
• Electrolyte concentrations measured on the Beckman Coulter AU680 can be reliably used to calculate the stool osmotic gap.
• The validated performance characteristics support the clinical use of stool electrolytes for diagnosing chronic diarrhea.