Potassium (K⁺) Level

A potassium (K⁺) level is obtained to measure the amount of potassium in the body. Potassium plays an important role in nerve conduction, muscle function, acid–base balance, and osmotic pressure. Along with calcium and magnesium, potassium controls the rate and force of contraction of the heart and, thus, the cardiac output.

A potassium level also helps to diagnose acid–base and water imbalances. The level of potassium is not an absolute value; it varies with circulatory volume and other factors. Because a totally unsuspected potassium imbalance can suddenly prove lethal, its development must be anticipated. Thus, it is important to check the potassium level in severe cases of Addison disease, uremic coma, intestinal obstruction, acute renal failure, GI loss in the administration of diuretics, steroid therapy, and cardiac patients on digitalis. Potassium levels should be monitored during treatment of acidosis, including ketoacidosis of diabetes.

Potassium is the principal electrolyte (cation) of intracellular fluid and the primary buffer within the cell itself. Ninety percent of potassium is concentrated within the cell; only small amounts are contained in bone and blood. Damaged cells release potassium into the blood.

The body is adapted for efficient potassium excretion. Normally, 80%–90% of the cells’ potassium is excreted in the urine by the glomeruli of the kidneys; the remainder is excreted in sweat and in the stool. Even when no potassium is taken into the body (as in a fasting state), 40–50 mEq is still excreted daily in the urine. The kidneys do not conserve potassium, and when an adequate amount of potassium is not ingested, a severe deficiency will occur. Potassium balance is maintained in adults on an average dietary intake of 80–200 mEq/d (80–200 mmol/d). Normal intake, minimal needs, and maximum tolerance for potassium are almost the same as those for sodium.

Potassium and sodium ions are particularly important in the kidney’s regulation of acid–base balance because hydrogen ions are substituted for sodium and potassium ions in the renal tubule. Potassium is more important than sodium because potassium bicarbonate is the primary intracellular inorganic buffer. With potassium deficiency, there is a relative deficiency of intracellular potassium bicarbonate, and the pH is relatively acid. The respiratory center responds to the intracellular acidosis by lowering PCO₂ through the mechanism of hyperventilation. The potassium concentration is greatly affected by the adrenal hormones. Potassium deficiency will cause a significant reduction in protein synthesis. Evidence of a potassium deficit can be noted on an ECG by flattening of the T wave and the presence of a U wave. Alternatively, an ECG may show a flattened P wave and a widened QRS complex with elevated potassium.

1. Collect a 5-mL venous blood sample using serum or heparinized Vacutainer tube (see Chapter 2, for venous blood collection). Observe standard/universal precautions. Avoid hemolysis in obtaining the sample.

2. Label specimen with the patient’s name, date, and test(s) ordered. Deliver the sample to the laboratory and centrifuge immediately to separate cells from serum. Potassium leaks out of the cell, and levels in the sample will be falsely elevated later than 4 hours after collection.

1. Hypokalemia is associated with shifting of K⁺ into cells, K⁺ loss from GI and biliary tracts, kidney K⁺ excretion, and reduced K⁺ intake, as can occur in the following conditions:

   1. Diarrhea, vomiting, sweating

   2. Starvation, malabsorption

   3. Bartter syndrome (autosomal recessive renal tubular disorders)

   4. Draining wounds

   5. CF

   6. Severe burns

   7. Primary aldosteronism

   8. Alcoholism, chronic

   9. Osmotic hyperglycemia

   10. Respiratory alkalosis

   11. Renal tubular acidosis

   12. Diuretic, antibiotic, and mineralocorticoid administration

   13. Barium chloride poisoning

   14. Treatment of megaloblastic anemia with vitamin B₁₂ or folic acid

1. Potassium levels of 3.5 mEq/L (3.5 mmol/L) are more commonly associated with deficiency than with normality. A falling trend (0.1–0.2 mEq/d or 0.1–0.2 mmol/d) is indicative of a developing potassium deficiency.

   1. The most frequent cause of potassium deficiency is GI loss.

   2. The most frequent cause of potassium depletion is IV fluid administration without adequate potassium supplements.

2. Hyperkalemia occurs when K⁺ shifts from cells to intracellular fluid with inadequate kidney excretion and with excessive K⁺ intake, as can occur in the following conditions:

   1. Kidney disease, acute kidney injury, dehydration, obstruction, and trauma

   2. Cell damage, as in burns, injuries, surgery, chemotherapy, disseminated intravascular coagulation (damaged cells release potassium into the blood)

   3. Metabolic acidosis (drives potassium out of the cells), diabetic ketoacidosis

   4. Addison disease

   5. Pseudohypoaldosteronism

   6. Uncontrolled diabetes, decreased insulin

   7. Primary acquired hyperkalemia, such as in SLE, sickle cell disease, interstitial nephritis, and tubular disorders

   8. Kidney transplant rejection

Pretest Patient Care

1. Explain test purpose and blood-drawing procedure. Do not have patient open and close fist while drawing blood.

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

1. Hemolyzed blood elevates potassium levels as much as 50% over normal with moderate hemolysis. Opening and closing the fist 10 times with a tourniquet in place results in an increase in potassium level by 10%–20%. For this reason, it is recommended that the blood sample be obtained without a tourniquet or that the tourniquet be released after the needle has entered the vein.

2. Drug usage:

   1. IV administration of potassium penicillin may cause hyperkalemia; penicillin sodium may cause increased excretion of potassium.

   2. Glucose administered during tolerance testing or the ingestion and administration of large amounts of glucose in patients with heart disease may cause a decrease of as much as 0.4 mEq/L (0.4 mmol/L) in potassium blood levels.

   3. A number of drugs raise potassium levels, especially potassium-sparing diuretics and nonsteroidal anti-inflammatory drugs, especially in the presence of renal disease.

   4. Excessive intake of licorice decreases potassium levels.

3. Leukocytosis, as occurs in leukemia, raises potassium levels.

4. Patients who have thrombocytosis due to polycythemia vera or a myeloproliferative disease may have spuriously high potassium levels. This falsely elevated level is caused by a high number of platelets, which release potassium during coagulation. Therefore, heparinized samples, rather than clotted serum samples, should be used in these patients.

Normal

• Adults: 3.5–5.2 mEq/L (3.5–5.2 mmol/L)

• Children (1–18 years): 3.4–4.7 mEq/L (3.4–4.7 mmol/L)

• Infants (7 days–1 year): 4.1–5.3 mEq/L (4.1–5.3 mmol/L)

• Neonates (0–7 days): 3.7–5.9 mEq/L (3.7–5.9 mmol/L)