7 - drug calculations Flashcards
what SI prefixes are used in clinical practice?
what is 1kg in grams (g)?
1kg = 1000g
what is 1 gram (g) in milligrams (mg)?
1g = 1000mg
what is 1mg in micrograms?
1mg = 1000 micrograms
what is 1 microgram in nanograms?
1 micrgram = 1000 nanograms
what is one L in millilitres (ml)?
1L = 1000ml
Convert 0.0125 g into mg
12.5mg
Convert 0.000078 mg into nanograms
78 nanograms
Convert 0.65 mg into micrograms
650 micrograms
Convert 25 000 micrograms into mg
25mg
Convert 0.075 mg into micrograms
75 micrograms
Convert 0.002 g into mg
2mg
Convert 0.001 mg into micrograms
1 microgram
rules for writing prescriptions
Quantities of 1 gram or more should be written as 1 g, 1.5 g etc.
Quantities less than 1 gram should be written in milligrams, e.g. 500 mg, not 0.5 g.
Quantities less than 1 mg should be written in micrograms, e.g. 100 micrograms, not 0.1 mg.
The unnecessary use of decimal points should be avoided; e.g. 3 mg, not 3.0 mg.
When decimals are unavoidable, a zero should be written in front of the decimal point where there is no other figure; e.g. 0.5 mL, not .5 mL.
Use of the decimal point is acceptable to express a range, e.g. 0.5 to 1 g.
‘Micrograms’ and ‘nanograms’ should not be abbreviated. Similarly ‘units’ when used to represent activity of a drug should not be abbreviated.
The term ‘millilitre’ (ml or mL) is used in medicine and pharmacy, and cubic centimetre, c.c., or cm3 should not be used. (The use of capital ‘L’ in mL is a printing convention throughout the BNF; both ‘mL’ and ‘ml’ are recognised SI abbreviations).
what are the different ways drug concentrations can be expressed?
as weight per unit volume, for example, 1 mg in 1 millilitre (1 mg/mL)
as the number of parts (by weight, g) of the active ingredient (drug) contained in a given volume (mL). For example, a 1:1 000 solution of drug X means that there is 1 g of drug X in 1 000 mL.
as a percentage (%) weight (in grams, g) of drug in volume (100 mL). For example, a 2.5% solution of drug X means that there are 2.5 g of drug X in every 100 mL volume. This is written as 2.5% (w/v) where w/v represents weight/volume.
for some medicines obtained from natural biological sources, e.g. heparin, oxytocin, insulin; the strength is expressed in units of activity, e.g.100 units/mL.
