Anion Gap

We often see anion gap reported on patient charts. The gap defines the type and cause of acidosis. Normal is 8-16 mEq/L. The formula is Na+ + K+ - (Cl- + HCO3-) and luckily the computer generates the reading for us! 😊

Only in select clinical situations, does the reading have significance from a nursing perspective. We will discuss two of the most common in this “Pearls of Knowledge”, utilizing our Memory Hint.

#1 When to give sodium bicarbonate based on the anion gap.

#2 The implication the anion gap has in a patient in diabetic ketoacidosis and insulin administration.

Anion gap and sodium bicarbonate

When a patient presents in metabolic acidosis, nurses are anticipating the administration of sodium bicarbonate. However, this may or may not be appropriate. When the anion gap is normal, sodium bicarbonate is the appropriate treatment. When the anion gap is high, the underlying condition needs to be treated, such as insulin administration in diabetic ketoacidosis.

Look at these two clinical examples. Both are metabolic acidosis, as evidenced by the low pH and low bicarbonate. Sodium bicarbonate would only be indicated in Patient Example #1 because the anion gap is normal. This could be a patient with diarrhea, renal failure, or on diuretic therapy. Patient #2 (with a high anion gap) could be septic or in diabetic ketoacidosis.

Anion gap and diabetic ketoacidosis

In diabetic ketoacidosis, the anion gap will be high, and tells you the degree of ketosis. When ketosis is corrected, the insulin drip may be discontinued. In most situations, an insulin bolus of 0.1 units per kg is given, followed by an infusion at 0.1 units/kg/hr. Insulin administration is aimed at stopping ketone production and treating hyperglycemia.

Even if blood glucose is 100-160 mg/dl, and anion gap is still elevated, the insulin drip is continued and an infusion of D10W at 100 mL/hr. is started. The provider may even order a bolus of 50 mL of D50W prior to starting the D10W infusion. Only when the anion gap is closed, is the insulin drip discontinued.

Exceptions to the administration of sodium bicarbonate with a high anion gap includes patients who are in shock, where bicarbonate will be considered. Providers may also do a calculation which may indicate the need for bicarbonate administration.

Summary

So, the next time you have a patient with an acid-base imbalance, look at the anion gap!

• If it is high, you expect the provider to treat the cause.

• If the anion gap is “Normal”, you expect sodium bicarbonate to be given.

• If the patient is in DKA with a high anion gap, the insulin infusion will not be discontinued until the anion gap is normal. (If the provider is following evidence-based practice.)