calculate the anion gap. (As discussed above, potassium may or may not be used, depending on the specific lab.) The cations calcium (Ca2+) and magnesium (Mg2+) are also commonly measured, but they aren't used to calculate the anion gap. Anions that are generally considered "unmeasured" include a few normally occurring serum proteins, and some pathological proteins (e.g., paraproteins found in multiple myeloma). Similarly, tests do often measure the anion phosphate (PO) specifically, but it isn't used to calculate that "gap," even if it is measured. Commonly 'unmeasured' anions include sulfates and a number of serum proteins. In normal health there are more measurable cations than measurable anions in the serum; therefore, the anion gap is usually positive. Because we know that plasma is electro-neutral (uncharged), we can conclude that the anion gap calculation represents the concentration of unmeasured anions. The anion gap varies in response to changes in the concentrations of the above-mentioned serum components that contribute to the acid-base balance. Normal value ranges Different labs use different formulae and procedures to calculate the anion gap, so the reference range (or "normal" range) from one lab isn't directly interchangeable with the range from another. The reference range provided by the particular lab that performed the testing should always be used to interpret the results. Also, some healthy people may have values outside of the "normal" range provided by any lab. Modern analyzers use ion-selective electrodes which give a normal anion gap as <11 mEq/L. Therefore, according to the new classification system, a high anion gap is anything above 11 mEq/L. A normal anion gap is often defined as being within the prediction interval of 3–11 mEq/L, with an average estimated at 6 mEq/L. In the past, methods for the measurement of the anion gap consisted of colorimetry for [HCO] and [Cl−] as well as flame photometry for [Na+] and [K+]. Thus normal reference values ranged from 8 to 16 mEq/L plasma when not including [K+] and from 10 to 20 mEq/L plasma when including [K+]. Some specific sources use 15 and 8–16 mEq/L. Interpretation and causes Anion gap can be classified as either high, normal or, in rare cases, low. Laboratory errors need to be ruled out whenever anion gap calculations lead to results that do not fit the clinical picture. Methods used to determine the concentrations of some of the ions used to calculate the anion gap may be