Anion gap (AG) is a calculated value commonly used in clinical practice. It approximates the difference between the concentration of unmeasured anions (UA) and unmeasured cations (UC) in serum. At present, the reference range of anion gap has been lowered from 8-16 to 3-11 mmol/l because of the changes in technique for measuring electrolyte. However, clinicians and textbooks still refer and use the old reference value of 8-16 mmol/l. This may lead to misinterpretation of the value of anion gap. Our study updated the value of anion gap in clinical diagnosis and laboratory evaluation. Criteria for using anion gap were also suggested. We analyzed serum electrolyte using the Beckman Synchron CX5. The anion gap was calculated from the formula: [Na(+)-(Cl(-)+HCO(3)(-))]. We estimated the reference range using the non-parametric percentile estimation method. The reference range of anion gap obtained from 124 healthy volunteers was 5-12 mmol/l, which was low and confirmed the reports from other studies (3-11 mmol/l) using ion-selective electrode. From the retrospective study on the 6868 sets of serum electrolyte among hospitalized patients, we found the incidences of normal, increased, and decreased anion gaps were 59.5%, 37.6%, and 2.9%, respectively. The mean and central 90% range of increased anion gap were 16 and 13-20 mmol/l, which was lower than those reported in previous study (25 and 19-28 mmol/l). Anion gap exceeding 24 mmol/l was rare. The mean and central 90% range of decreased anion gap were 3 and 2-4 mmol/l, which were lower than those reported in previous study (6 and 3-8 mmol/l). The value of less than 2 mmol/l was rare. The most common causes of increased anion gap (hypertensive disease, chronic renal failure, malignant neoplasms, diabetes mellitus and heart diseases) and decreased anion gap (liver cirrhosis and nephrotic syndrome) in this study were similar to those in previous studies. We found two cases of IgG multiple myeloma with anion gap of 2 mmol/l. In conclusion, clinicians and laboratorians can use the anion gap as clue in quality control. They can check the incidences of increased and decreased anion gap. If one finds high incidence of increased anion gap (>24 mmol/l) or decreased anion gap (<2 mmol/l), one should check the quality control of electrolyte and whether the patients were hypoalbuminemia or hyperglobulinemia. An anion gap exceeding 24 mmol/l will suggest the presence of metabolic acidosis. It is very rare to find anion gap with the negative sign.