The results of serum levels of sodium, potassium, chloride, and bicarbonate may be used to calculate the anion gap. The anion gap refers to the normal discrepancy between unmeasured (i.e., those not routinely measured) cations and anions in the blood. Unmeasured anions include the negative charges contributed by serum proteins and those of phosphates, sulfates, and other metabolites. Unmeasured anions normally total about 24 mEq/L. Cations not routinely measured include calcium and magnesium, and together they account for about 7 mEq/L. Because there are normally more unmeasured anions than cations, the difference between the two is called the anion gap. This is normally 12 to 18 mEq/L.

The anion gap can be determined by subtracting the sum of routinely measured anions, chloride and bicarbonate, from the sum of routinely measured cations, sodium and potassium (i.e., [Na + K] - [Cl + HCO₃]). The concept of anion gap allows consideration of metabolic derangements without measuring specific metabolites. An increase in the anion gap is seen in acidotic states in which there is no compensatory rise in chloride levels. Examples of anion gap acidosis include diabetic ketoacidosis, lactic acidosis caused by either tissue hypoxia (type A) or renal or hepatic metabolic defect (type B), and excessive alcohol ingestion.⁶¹

• Numerous drugs may alter serum bicarbonate levels (see Table 5-25).

• Routine electrolyte screening in acute and critical illness
• Known or suspected disorder associated with altered acid-base or fluid and electrolyte balance, or both
• Support for diagnosing disorders associated with altered serum bicarbonate levels (see Table 5-25)
• Determination of the degree of compensation in acidotic and alkalotic states (Table 5-26)
• Evaluation of the effects of drug therapy on serum bicarbonate levels

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving collection of a peripheral blood sample (see Appendix I).

• Because many drugs may alter serum bicarbonate levels, a medication history should be obtained. It is recommended that drugs that may alter test results be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any medications withheld before the test.
• Increased levels: Note and report increased levels or base excess in relation to hypokalemia and hypochloremia. Assess for vomiting, presence of gastric suctioning, diuretic therapy, or excessive intake of oral bicarbonate (baking soda, antacids). Assess for respiratory changes, tingling in fingers, or more severe muscular irritability symptoms. Assess for cardiac dysrhythmias if hypokalemia is present. Administer ordered oral or IV electrolyte replacement (potassium, chloride). Administer fluids (juices, broth) to replace electrolytes if decreases are not extreme.
• Decreased levels: Note and report decreased levels or base deficit in relation to other electrolytes. Assess gastrointestinal losses such as vomiting leading to acidosis and diarrhea leading to alkalosis, I&O, metabolic acidosis with change in respirations (Kussmaul's breathing), confusion, or lethargy. Prepare for IV sodium bicarbonate. Monitor I&O closely to prevent fluid and electrolyte imbalance.
• Critical values: Notify the physician at once of levels less than 15 mEq/L or greater than 35 mEq/L.