The gap formula, more specifically the anion gap formula, calculates the difference between measured cations (positively charged ions) and measured anions (negatively charged ions) in the blood. It helps to identify imbalances that may indicate various medical conditions.
Here's the primary anion gap formula:
(Na+ + K+) – (Cl- + HCO3-) = Anion Gap
Where:
- Na+ = Sodium
- K+ = Potassium
- Cl- = Chloride
- HCO3- = Bicarbonate
However, a more commonly used and simplified version omits Potassium:
Na+ – (Cl- + HCO3-) = Anion Gap
The reference ranges for the anion gap vary slightly between laboratories. Typically, using the first formula (including Potassium), the normal range is 8-16 mEq/L. Using the second, simplified formula (omitting Potassium) the normal range is 3-10 mEq/L. Note that these reference ranges are approximate and can depend on the specific lab and method used.
What does the Anion Gap tell us?
An increased anion gap suggests that there are unmeasured anions in the blood. This can be caused by a variety of conditions, including:
- Ketoacidosis: Occurs in uncontrolled diabetes, starvation, or alcohol abuse.
- Lactic acidosis: Caused by inadequate oxygen delivery to tissues.
- Renal failure: The kidneys are unable to excrete acids.
- Toxins: Such as methanol, ethylene glycol (antifreeze), and salicylate poisoning.
A decreased anion gap is less common and can be caused by:
- Hypoalbuminemia: Low levels of albumin in the blood.
- Multiple myeloma: A type of cancer that produces abnormal proteins.
- Bromide intoxication: Exposure to bromide-containing substances.
Important Considerations:
- Always interpret the anion gap in the context of the patient's clinical presentation and other laboratory results.
- Different laboratories may use different methods for measuring electrolytes, which can affect the calculated anion gap.
