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Introduction ⬇

Normal colon function involves absorption of fluid and electrolytes.

Stool electrolyte tests are used to assess electrolyte imbalance in patients with diarrhea. Stool electrolytes must be evaluated along with the serum and urine electrolytes as well as clinical findings in the patient. Stool osmolality is used in conjunction with blood serum osmolality to calculate the osmotic gap and to diagnose intestinal disaccharide deficiency.

Normal Findings ⬆ ⬇

  1. Sodium: 5.8–9.8 mEq/24 hr or 5.8–9.8 mmol/d

  2. Chloride: 2.5–3.9 mEq/24 hr or 2.5–3.9 mmol/d

  3. Potassium: 15.7–20.7 mEq/24 hr or 15.7–20.7 mmol/d

  4. Osmolality: 275–295 mOsm/kg

  5. Osmotic gap: less than 50 mOsm/kg (secretory diarrhea); more than 75 mOsm/kg (osmotic diarrhea). Note: Osmotic gap = 290 mOsm/kg − (2 [stool Na + stool K]).

Normal values vary greatly; check with your reference laboratory.

Procedure ⬆ ⬇

  1. Collect a random or 24-hour liquid stool specimen following the procedures given earlier in this chapter. Observe standard precautions.

  2. Keep the specimen covered and refrigerated.

Clinical Implications ⬆ ⬇

  1. Electrolyte abnormalities occur in the following conditions:

    1. Idiopathic proctocolitis: increased sodium (Na) and chloride (Cl); normal potassium (K)

    2. Ileostomy: increased sodium (Na) and chloride (Cl), low potassium (K)

    3. Cholera: increased sodium (Na) and chloride (Cl)

  2. Chloride is greatly increased in stool in the following conditions:

    1. Congenital chloride diarrhea

    2. Acquired chloride diarrhea or secondary chloride diarrhea

    3. Idiopathic proctocolitis

    4. Cholera

  3. Stool osmolality 500 mg/dL per day is suspicious for factitious disorders (e.g., laxative abuse). Higher levels indicate high amounts of reducing substances in the stool. The osmotic gap is increased in osmotic diarrhea caused by the following:

    1. Saline laxatives

    2. Sodium or magnesium citrate

    3. Carbohydrates (lactulose or sorbitol candy)

  4. Osmotic gap more than 75 mOsm/kg (osmotic diarrhea)

    1. Lactose intolerance

    2. Malabsorption

    3. Poorly absorbed sugars (e.g., sorbitol, mannitol)

    4. Magnesium-containing laxatives

  5. Osmotic gap less than 50 mOsm/kg (secretory diarrhea)

    1. Acute (e.g., cholera)

    2. Chronic (e.g., celiac disease or sprue, collagenous colitis, hyperthyroidism)

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for stool collection, and interfering factors.

  2. Advise the patient to avoid barium procedures and laxatives for 1 week before stool specimen collection.

  3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for further testing and treatment. Monitor diarrhea episodes and record findings. Assess the patient for electrolyte imbalances.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Formed stools invalidate the results. Stools must be liquid for electrolyte tests.

  2. The stool cannot be contaminated with urine.

  3. Surreptitious addition of water to the stool specimen considerably lowers the osmolality.

  4. see Appendix E for drugs that cause increased values.