Calculating the<unk>  is clinically useful because it helps in the differential diagnosis of a number of disease states. The total number of cations (positive ions) should be equal to the total number of<unk> s (negative ions), so that the overall electrical charge is neutral. However, routine tests do not measure all types of ions. The<unk>  is representative of how many ions are not accounted for by the lab measurements used in the calculation. These "unmeasured" ions are mostly<unk> s, which is why the value is called the<unk> ." By definition, only the cations sodium (Na+) and potassium (K+) and the<unk> s chloride (Cl−) and bicarbonate (HCO) are used to calculate the<unk> . (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<unk> .<unk> s 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<unk>  phosphate (PO) specifically, but it isn't used to calculate that "gap," even if it is measured. Commonly 'unmeasured'<unk> s include sulfates and a number of serum proteins. In normal health there are more measurable cations than measurable<unk> s in the serum; therefore, the<unk>  is usually positive. Because we know that plasma is electro-neutral (uncharged), we can conclude that the<unk>  calculation represents the concentration of unmeasured<unk> s. The<unk>  varies in response to changes in the concentrations of the above-mentioned serum components that contribute to the acid-base balance.
Anion gap
Osmol gap
Lactate threshold
Respiratory quotient
Oxygen equivalent
CO2 content
Sucrose gap
NC ratio