Ion-selective electrode and anion gap range: What should the anion gap be?

Area of Science:

• Clinical Chemistry
• Medical Diagnostics
• Renal Medicine

Background:

• Historically, flame photometry determined the normal anion gap (AG) to be 12 ± 4 meq/L.
• The introduction of autoanalyzers utilizing ion-selective electrodes (ISE) has led to a decline in measured AG values.

Purpose of the Study:

• To determine the current normal anion gap (AG) values in US veterans.
• To compare AG levels in subjects with normal renal function and albumin versus those with lactic acidosis or end-stage renal disease on dialysis.

Main Methods:

• Retrospective study of US veterans from a single medical center.
• Analysis of anion gap (AG) in patients with normal renal function (eGFR ≥60 mL/min/1.73 m² and serum albumin ≥4 g/dL).
• Comparison of AG in patients with lactic acidosis (lactate ≥4 meq/L) and end-stage renal disease on dialysis.

Main Results:

• In subjects with normal renal function and albumin, the mean AG was 7.2 ± 2 meq/L (range 3-11).
• Patients with lactic acidosis and end-stage renal disease on dialysis showed higher mean AG values of 12.5 meq/L and 12.4 meq/L, respectively.
• An AG <2 meq/L may indicate drug intoxication or paraproteinemic disorders.

Conclusions:

• Modern ISE technology has lowered the typical anion gap (AG) values.
• Physicians and laboratories must be aware of and establish institution-specific AG reference ranges based on current instrumentation.
• Accurate AG interpretation requires knowledge of contemporary normal values.