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SI Units

SI (le Système International d’Unités) units are used in many countries to express clinical laboratory and serum drug concentration data*. Instead of employing units of mass (such as micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mol (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to μmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L).

Micromoles per Liter (μmol/L)



An equivalent weight of a substance is that weight which will combine with or replace 1 g of hydrogen; a milliequivalent is 1/1000 of an equivalent weight.

Milliequivalents per Liter (mEq/L)


*This appendix contains information from Appendices 1 and 2 of Anderson PO, Knoben JE, Troutman WG, et al, eds. Handbook of Clinical Drug Data. 10th ed. New York, NY: McGraw-Hill; 2002:1053–1058, with permission.

Approximate Milliequivalents

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Weights of Selected Ions
Salt mEq/g Salt mg Salt/mEq
Calcium carbonate (CaCO3) 20.0 50.0
Calcium chloride (CaCl2 ⋅ 2H2O) 13.6 73.5
Calcium gluceptate (Ca[C7H13O8]2) 4.1 245.2
Calcium gluconate (Ca[C6H11O7]2 ⋅ H2O) 4.5 224.1
Calcium lactate (Ca[C3H5O3]2 ⋅ 5H2O) 6.5 154.1
Magnesium gluconate (Mg[C6H11O7]2 ⋅ H2O) 4.6 216.3
Magnesium oxide (MgO) 49.6 20.2
Magnesium sulfate (MgSO4) 16.6 60.2
Magnesium sulfate (MgSO4 ⋅ 7H2O) 8.1 123.2
Potassium acetate (K[C2H3O2]) 10.2 98.1
Potassium chloride (KCl) 13.4 74.6
Potassium citrate (K3[C6H5O7] ⋅ H2O) 9.2 108.1
Potassium iodide (KI) 6.0 166.0
Sodium acetate (Na[C2H3O2]) 12.2 82.0
Sodium acetate (Na[C2H3O2] ⋅ 3H2O) 7.3 136.1
Sodium bicarbonate (NaHCO3) 11.9 84.0
Sodium chloride (NaCl) 17.1 58.4
Sodium citrate (Na3[C6H5O7] ⋅ 2H2O) 10.2 98.0
Sodium iodide (NaI) 6.7 149.9
Sodium lactate (Na[C3H5O3]) 8.9 112.1
Zinc sulfate (ZnSO4 ⋅ 7H2O) 7.0 143.8

Valences and Atomic Weights of Selected Ions

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