Lyte Reading: Electrolytes Lab Testing

One of the basic parts of running patient labs in the electrolyte levels. They are part of every basis chemistry panel such as the BMP or CMP, or sometimes just ordered as "lytes" Other electrolytes beside the basics (Potassium, Sodium, Bicarbonate, Calcium) can be ordered as well, such as Phosphorus (in the form of phosphate) and Magnesium.

How are they measured? Usually this is done with ion-selective electrodes (ISE) that measure electric currents across a salt solution. It is relatively simple, easy, and quick. However, measurement can be affected by many factors such as dirty or corroded surfaces as well as contamination of water supply or poor collection of samples. For instance, samples containing interstitial fluid will have different electrolyte levels than the blood, so pediatric collections where blood is squeezed into the tube can be incorrect. Also, tube to tube contamination of additives can cause wildly erroneous results, as is the case when blood from the lavender top tube (with K2EDTA), normally used for CBCs gets into the chemistry tubes, binding the calcium and introducing potassium.

What is an electrolyte?

Electrolytes, also known as ions or charged particles, are essential minerals found in the blood and other body fluids that carry an electric charge and play critical roles in maintaining homeostasis. They regulate nerve and muscle function, hydrate the body, balance blood pH, and help rebuild damaged tissue. The most commonly measured electrolytes in clinical laboratory testing include sodium, potassium, chloride, bicarbonate, calcium, magnesium, and phosphate. Each of these electrolytes has a predominant compartment—either intracellular or extracellular—and characteristic clinical implications when their levels are too low or too high.

Let's start with intracellular:

Potassium (K+) is the major intracellular cation, with only a small fraction present in the extracellular fluid. The normal serum range is about 3.5 to 5.0 mmol/L. Hypokalemia, or low potassium, may result from diuretic use, vomiting, diarrhea, or insulin administration and can lead to muscle weakness, cardiac arrhythmias, or ileus. Hyperkalemia, or high potassium, is seen in renal failure, hemolysis, tissue breakdown, or with potassium-sparing diuretics and may cause life-threatening cardiac conduction abnormalities. In fact, the most common method of lethal injection is potassium chloride that introduces so much potassium that the heart (a muscle controlled by electric impulses) stops working.

Magnesium (Mg++) is predominantly an intracellular cation and is vital for enzyme function, neuromuscular conduction, and cardiovascular stability. Its normal serum range is 1.5 to 2.5 mg/dL. Hypomagnesemia may be seen in chronic alcoholism, malabsorption, or diuretic use and can contribute to neuromuscular irritability, seizures, or refractory hypokalemia and hypocalcemia. Hypermagnesemia is less common but can occur in renal failure or with excessive supplementation, causing lethargy, hypotension, and respiratory depression at high levels.

Phosphate or phosphorus (in the form of HPO4--) is mainly an intracellular anion and is important for energy storage and metabolism through its role in ATP. The normal serum phosphate range is 2.5 to 4.5 mg/dL. Hypophosphatemia may arise from malnutrition, refeeding syndrome, or respiratory alkalosis and may lead to muscle weakness, hemolysis, or impaired cardiac function. Hyperphosphatemia is typically seen in chronic kidney disease or excessive intake and can result in secondary hypocalcemia, soft tissue calcification, and bone abnormalities.

Calcium (Ca++) exists in both extracellular and intracellular compartments, with about half of the serum calcium bound to proteins. The normal total serum calcium range is 8.5 to 10.5 mg/dL. Hypocalcemia may result from hypoparathyroidism, vitamin D deficiency, renal disease, or acute pancreatitis and can cause tetany, paresthesias, and cardiac arrhythmias. Hypercalcemia is commonly due to hyperparathyroidism, malignancy, or excessive vitamin D intake and may manifest as nephrolithiasis, abdominal pain, mental status changes, and cardiac arrhythmias.

Other electrolytes found in the intracellular fluid are charged proteins, sugars, and nucleic acids.

Now the extracellular electrolytes:

Sodium (Na+) is primarily an extracellular cation and is the most abundant ion in the extracellular fluid. Its normal range is typically 135 to 145 mmol/L. Hyponatremia, or low sodium levels, may be indicative of conditions such as heart failure, liver cirrhosis, syndrome of inappropriate antidiuretic hormone secretion (SIADH), or excessive fluid intake. Hypernatremia, or high sodium levels, often reflects dehydration, diabetes insipidus, or hyperaldosteronism. Sodium imbalances affect fluid distribution and neurological status, with severe disturbances leading to seizures or coma.

Chloride (Cl-) is another predominant extracellular anion and closely follows sodium to maintain electroneutrality and osmotic balance. Its normal range is approximately 98 to 107 mmol/L. Hypochloremia can be associated with metabolic alkalosis, prolonged vomiting, or diuretic therapy. Hyperchloremia often accompanies metabolic acidosis, dehydration, or excessive saline infusion. Chloride disturbances are typically interpreted in conjunction with other electrolytes and acid-base parameters.

Bicarbonate (in the form of HCO3-), a measurement of total CO₂ content, serves as a buffer and is mainly found in the extracellular fluid. The normal range is about 22 to 28 mmol/L. Low bicarbonate levels suggest metabolic acidosis, as seen in diabetic ketoacidosis, renal failure, or lactic acidosis. Elevated bicarbonate levels suggest metabolic alkalosis, which may occur with prolonged vomiting, diuretic use, or hypokalemia. Because bicarbonate reflects the buffering capacity of blood, its value is central to assessing acid-base disorders.