Appendix A - Part 1: Conversion Factors | Pharmacotherapy Principles and Practice Study Guide, 6e | Pharmacotherapy Principles & Practice

CONVERSION FACTORS

SI Units

SI (le Systéme International d’Unités) units are used in many countries to express clinical laboratory and serum drug concentration data.^* Instead of using units of mass (eg, micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mole (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

Micromoles per liter (μ mol / L) = \frac{drug concentration (mcg / mL) \times 1000}{molecular weight of drug (g / mol)}

Milliequivalents

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

Milliequivalents per Liter

\begin{matrix} Milliequivalents per liter (mEq / L) = \frac{weight of salt (g) \times valence of ion \times 1000}{molecular weight of salt} \\ Weight of salt (g) = \frac{mEq / L \times molecular weight of salt}{valence of ion \times 1000} \end{matrix}

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Approximate Milliequivalents: Weight Conversions for 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

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Valences and Atomic/Molecular Weights of Selected Ions

Substance

Electrolyte

Valence

Atomic/Molecular Weight

Calcium

Ca²⁺

40.1

Chloride

Cl^–

35.5

Magnesium

Mg²⁺

24.3

Phosphate

HPO₄^2–(80%)

1.8

96.0^a

(pH = 7.4)

H₂PO₄^–(20%)

97.0

Potassium

K⁺

39.1

Sodium

Na⁺

23.0

Sulfate

SO₄^–

96.0^a

^a The atomic/molecular weight of phosphorus is only 31; that of sulfur is only 32.1.

Anion Gap

The anion gap is the concentration of plasma anions not routinely measured by laboratory screening. It is useful in the evaluation of acid–base disorders. The anion gap is greater with increased plasma concentrations of endogenous species (eg, phosphate, sulfate, lactate, and ketoacids) or exogenous species (eg, salicylate, penicillin, ethylene glycol, ethanol, and methanol). The formulas for calculating the anion gap are as follows:

Anion gap = ({Na}^{+} + K^{+}) - ({Cl}^{-} + {HCO}_{3}^{-})

Anion gap = {Na}^{+} - ({Cl}^{-} + {HCO}_{3}^{-})

where the expected normal value for the first equation is 11 to 20 mmol/L and for the second equation is 7 to 16 mmol/L. Note that there is a variation in the upper and lower limits of the normal range.

Temperature

Fahrenheit to Celsius: (°F – 32) × 5/9 = °C

Celsius to Fahrenheit: (°C × 9/5) + 32 = °F

Celsius to Kelvin: °C + 273 = °K

Weights and Measures

Metric Weight Equivalents

1 kilogram (kg) = 1000 grams

1 gram (g) = 1000 milligrams

1 milligram (mg) = 0.001 gram

1 microgram (mcg, μg) = 0.001 milligram

1 nanogram (ng) = 0.001 microgram

1 picogram (pg) = 0.001 nanogram

1 femtogram (fg) = 0.001 picogram

Metric Volume Equivalents

1 liter (L) = 1000 milliliters

1 deciliter (dL) = 100 milliliters

1 milliliter (mL) = 0.001 liter

1 microliter (μL) = 0.001 milliliter

1 nanoliter (nL) = 0.001 microliter

1 picoliter (pL) = 0.001 nanoliter

1 femtoliter (fL) = 0.001 picoliter

Apothecary Weight Equivalents

1 scruple (∋) = 20 grains (gr)

60 grains (gr) = 1 dram ()

8 drams () = 1 ounce ()

1 ounce () = 480 grains (gr)

12 ounces () = 1 pound (lb)

Apothecary Volume Equivalents

60 minims (m) = 1 fluidram (fl )

8 fluidrams (fl ) = 1 fluid ounce (fl )

1 fluid ounce (fl ) = 480 minims (m)

16 fluid ounces (fl ) = 1 pint (pt)

Avoirdupois Equivalents

1 ounce (oz) = 437.5 grains

16 ounces (oz) = 1 pound (lb)

Weight/Volume Equivalents

1 mg/dL = 10 mcg/mL

1 mg/dL = 1 mg%

1 ppm = 1 mg/L

Conversion Equivalents

1 gram (g) = 15.43 grains (gr)

1 grain (gr) = 64.8 milligrams (mg)

1 ounce () = 31.1 grams (g)

1 ounce (oz) = 28.35 grams (g)

1 pound (lb) = 453.6 grams (g)

1 kilogram (kg) = 2.2 pounds (lb)

1 milliliter (mL) = 16.23 minims (m)

1 minim (m) = 0.06 milliliter (mL)

1 fluid ounce (fl oz) = 29.57 milliliters (mL)

1 pint (pt) = 473.2 milliliters (mL)

1 US gallon = 3.78 liters (L)

1 Canadian gallon = 4.55 liters (L)

0.1 milligram = 1/650 grain

0.12 milligram = 1/540 grain

0.15 milligram = 1/430 grain

0.2 milligram = 1/320 grain

0.3 milligram = 1/220 grain

0.4 milligram = 1/160 grain

0.5 milligram = 1/130 grain

0.6 milligram = 1/110 grain

0.8 milligram = 1/80 grain

1 milligram = 1/65 grain

Metric Length Conversion Equivalents

2.54 cm = 1 inch

30.48 cm = 1 foot

1 m = 3.28 feet

1.6 km = 1 mile

Appendix A - Part 1: Conversion Factors

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