Electrolyte imbalances occur when important charged minerals in the blood are too high or too low. This reference covers sodium, potassium, calcium, magnesium, chloride, phosphate, and bicarbonate in a clinical science context. Students need this cheat sheet because electrolyte levels affect nerve signals, muscle contraction, heart rhythm, fluid balance, and acid-base balance.
It helps connect lab values with common causes and warning signs.
The most important skill is comparing a patient value with the normal range and then identifying whether it is low, normal, or high. Sodium is closely linked to water balance, potassium strongly affects heart and muscle function, and calcium affects bones, nerves, and muscle contraction. Bicarbonate and chloride help show acid-base and fluid patterns.
Always interpret electrolytes with symptoms, hydration status, medications, kidney function, and clinical context.
Key Facts
- Normal serum sodium is about 135 to 145 mEq/L, with hyponatremia below 135 mEq/L and hypernatremia above 145 mEq/L.
- Normal serum potassium is about 3.5 to 5.0 mEq/L, and abnormal potassium can cause dangerous cardiac rhythm changes.
- Normal total serum calcium is about 8.5 to 10.5 mg/dL, and low calcium may cause tingling, cramps, or tetany.
- Normal serum magnesium is about 1.7 to 2.2 mg/dL, and magnesium imbalance can affect nerves, muscles, and heart rhythm.
- Normal serum chloride is about 98 to 106 mEq/L, and chloride often shifts with sodium, hydration status, and acid-base changes.
- Normal serum phosphate is about 2.5 to 4.5 mg/dL, and phosphate is important for ATP, bones, and cell function.
- Normal serum bicarbonate is about 22 to 28 mEq/L, and low bicarbonate can suggest metabolic acidosis while high bicarbonate can suggest metabolic alkalosis.
- Anion gap is calculated as Na - (Cl + HCO3), and a typical normal range is about 8 to 12 mEq/L depending on the lab.
Vocabulary
- Electrolyte
- An electrolyte is a charged mineral ion in body fluids that helps control nerve signals, muscle action, hydration, and pH.
- Hyponatremia
- Hyponatremia is a blood sodium level below 135 mEq/L, often related to excess water, fluid loss, or hormone imbalance.
- Hyperkalemia
- Hyperkalemia is a blood potassium level above 5.0 mEq/L that can interfere with normal electrical activity of the heart.
- Tetany
- Tetany is involuntary muscle contraction or spasm that can occur with low calcium or low magnesium levels.
- Bicarbonate
- Bicarbonate is a major blood buffer that helps maintain acid-base balance and is commonly reported as HCO3 or CO2 on lab panels.
- Anion Gap
- The anion gap is a calculated value, Na - (Cl + HCO3), used to help classify causes of metabolic acidosis.
Common Mistakes to Avoid
- Calling a value abnormal without checking the lab's reference range is wrong because normal ranges can vary slightly by laboratory and patient context.
- Treating sodium as only a salt problem is wrong because sodium imbalance is often a water balance problem involving hydration, kidneys, or hormones.
- Ignoring potassium changes is dangerous because both low and high potassium can affect cardiac rhythm even when symptoms seem mild.
- Interpreting calcium without considering albumin can be misleading because much of total calcium is protein-bound and low albumin can lower measured total calcium.
- Looking at one electrolyte in isolation is incomplete because related values such as sodium, chloride, bicarbonate, kidney function, and medications often explain the pattern.
Practice Questions
- 1 A patient's sodium is 128 mEq/L. Is this hyponatremia, normal sodium, or hypernatremia?
- 2 A patient's potassium is 5.8 mEq/L. Is this low, normal, or high, and what body system is at special risk?
- 3 Calculate the anion gap for Na = 140 mEq/L, Cl = 104 mEq/L, and HCO3 = 18 mEq/L.
- 4 A patient has confusion, muscle weakness, and abnormal electrolyte values after severe vomiting. Explain why symptoms and clinical history must be considered along with the numbers.