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Calculations- NAPLEX Exam Questions and Answers 2024/2025( A+ GRADED 100% VERIFIED).
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Calculations- NAPLEXMeasuring Drugs - ANS Drugs can be measured in different ways:
- As weights, using grams (g), milligrams (mg), micrograms (mcg), and nanograms (ng).
- A liquid volumes, using liters (L), or milliliters (mL)
- As percentage strengths (grams in 100mL, grams in 100g, or mL in 100mL)
- As concentrations of a given amount of a drug in a given volume of liquid (mcg/mL, mg/L).
Very small amounts are measured in ng/mL. BUN and SCr are measured in mg/dL (1 liter is 10
dL, so 1 dL is 100mL or 0.1 Liters).
- As concentrations using milliequivalents (mEq) per Liter (mEq/L).
Common Liquid Volume Conversions - ANS - tsp (t) = 5 mL
- tbsp (T) = 15 mL
- 1 fl oz = ~30 mL, 29.57 mL (actual)
- 1 cup = 8 fl oz, ~240 mL, 236.56 mL (actual)
- 1 pint = 16 fl oz, ~480 mL, 473 mL (actual)
- 1 quart = 2 pints, 32 fl oz, ~960 mL, 946 mL (actual)
- 1 gallon = 4 quarts, 8 pints, 128 fl oz, ~3,840 mL, 3,785 mL (actual)
Common Solid Weight, Height, and Other Conversions - ANS Weight
- 1 kg = 2.2 pounds
- 1 oz = 28.4 grams
- 1 pound = 454 grams
- 1 grain (gr) = ~65 mg, 64.8 mg (actual)
Height
- 1 inch (in) = 2.54 centimeters (cm)
- 1 meter (m) = 100 cm
Other: Milliequivalents and Millimoles
- K, Na, and other MONOvalent ions: 1 mEq = 1 mmol
- Ca, Mg, and other DIvalent ions: 1 mEq = 0.5 mmol
If mEq are provided for potassium, and the answer requires mmols, use the same numbers.
When mEq is provided for calcium, and the answer requires mmols, use half the mEq number.
Percentage Strength - ANS Drug concentrations can be expressed in many ways, but there are
a ratio of the amount of an ingredient to the total amount of product.
A percent is the number of parts in *100*. Percents are often written as decimals or fractions,
such as 0.25 or 25/100. The type of percentage concentrations are as follows:
,1. Percent weight-in-volume (% w/v), which is expressed in *g/100mL*. This is a solid mixed into
a liquid. This % concentration also applies to common IV fluids.
2. Percent volume-in-volume (% v/v), which is expressed as *mL/100mL*. This is a liquid mixed
in a liquid.
3. Percent weight-in-weight (% w/w), which is expressed as *g/100g*. This is a solid mixed into
a solid.
Ratio Strength - ANS The concentration of a weak solution can be expressed as a ratio
strength. It is denoted as *one unit of solute* contained *in the total amount* of the solution or
mixture (e.g. 1:500). Ratio strength is another way of presenting a percentage strength. This
makes sense because percentages are ratios of parts per 100.
However, in clinical practice, ratio strengths have been associated with medication errors. The
FDA now requires removal of ratio strengths from the labeling of injectable drug products with
only 1 active ingredient (e.g. epinephrine, isoproterenol). Ratio strength is still commonly used in
compounding.
Shortcut for Ratio Strength Calculations - ANS Most multi-step calculations will require
converting ratio strength to percentage strength. If a ratio strength is presented in a problem,
convert it to a percentage strength and convert it back if needed.
1. Ratio strength to percentage strength: 100/ratio strength = % strength
2. Percentage strength to ratio strength: 100/% strength = ratio strength
Parts Per Million/Billion and Shortcuts - ANS PPM and PPB are used to express the strength of
very dilute solutions. They are defined as the number of *parts of the drug per 1 million (or 1
billion) parts of the whole*. The same designations are used as for percentage strength (% w/w,
%w/v, % v/v).
The shortcuts for PPM and PPB and converting to percentage strength and back are as follows:
- If PPM to % strength: Move the decimal *to the left 4 places*
- If % strength to PPM: Move the decimal *to the right 4 places*
If the question asks to express something in PPB, divide by 1,000,000,000 (9 zeros).
Specific Gravity - ANS Specific Gravity is the ratio of the density of a substance to the density
of water. SG can be important for calculating doses of IV medications, in compounding, and
interpreting a urinalysis. *Water has a SG of 1, 1g water = 1mL water*. Substances with a SG
<1 are *lighter* than water and those with a SG >1 are *heavier* than water.
, Essentially, specific gravity is mass of the substance over the volume it takes up, in mL. So SG
is:
- SG = grams of substance/mL of substance
*SG is essentially equivalent to density in g/mL*. If asked for the density in the problem, the
specific gravity is what they are looking for.
Q1C1 Calculations - ANS This formula can be used to change the strength of the quantity.
Q1C1 is used specifically when the problem deals with *two concentrations*. Be careful, as *the
units on each side must match*, and one or more may need to be changed, such as mg to
grams, or vice-versa.
Q1 x C1 = Q2 x C2, where Q1 is the old quantity, C1 is the old concentration, and C2 is the new
concentration and Q2 is the new quantity.
0% and 100% Purity - ANS If the prescription calls for an ingredient that is *pure*, the
concentration is 100%.
A diluent, such as petrolatum, lanolin, alcohol, ointment base, inert base, lactose, or Aquafor,
does not contain any drug, so the concentration of the diluent is 0%.
Osmolarity - ANS The total number of particles in a given solution is directly proportional to its
osmotic pressure. The particles are usually measured in milliosmoles. Osmolarity is the
measure of total number of particles, or solutes, per liter of solution, defined as osmoles/liter
(Osmol/L), or more commonly milliosmoles/liter (mOsmol/L).
Solutions can either be *ionic*, such as NaCl, which dissociates into two solutions in solution Na
and Cl, or *non-ionic*, which does NOT dissociate (such as glucose or urea).
Since the volume of water changes according to temperature, the term osmolality is used in
clinical practice (mOsmol/kg). It is independent of temperature. When solute concentrations are
very low, osmolarity and osmolality are similar.
Milliosmole calculation problems differ from osmolarity calculations in that *osmolarity will
always need to be normalized to a volume of 1 liter*. When doing these calculations, the
dissociation particles need to be known for the exam.
Common Compounds and # Dissociation Particles - ANS 1. Dextrose- 1
2 Mannitol- 1
3. Potassium Chloride- 2
4 Sodium Chloride- 2
5. Sodium Acetate- 2
6. Magnesium Sulfate- 2
7. Calcium Chloride- 3
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