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9 Cards in this Set
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What is the formula to measure Estimated creatinine clearance? |
The Cockcroft and Gault formula is the preferred method for estimating renal function or calculating drug doses in patients with renal impairment who are elderly or at extremes of muscle mass (see below); it provides an estimate of CrCl (which is not equivalent to eGFR, see Important: dosage adjustment advice in the BNF above). Estimated Creatinine Clearance in mL/minute = ((140 - Age) x Weight x Constant)/Serum creatinine Age in years Weight in kilograms (use ideal body weight where fat is likely to be the major contributor to body mass) Serum creatinine in micromol/litre Constant = 1.23 for men; 1.04 for women |
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In pharmacy, how are the concentrations of substances expressed? |
The concentration of substances can be expressed in many different ways, for example: grams per litre (g/l) milligrams per litre (mg/l) milligrams per millilitre (mg/ml) grams per millilitre (g/ml) moles per litre (mol/l) parts (e.g. 1 in 200) percentages (e.g. 1%) In general, the concentrations of drugs you will see in the dispensary are expressed as mg/l, mg/ml or as a percentage. For some injections the concentration may also be expressed as mol/l (or mmol/ml). |
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How would you convert units from g to mg for example or l to ml? |
Example 1 Convert 250micrograms into milligrams. From the units of mass table above we know there are 1000micrograms in 1mg (and therefore the answer is going to be less than 1mg as we do not have 1000micrograms). To work out the answer we must do the following calculation: 250 / 1000 = 0.25 The answer is that 250micrograms = 0.25mg Example 2 Convert 0.6g into milligrams From the units of mass table above we know that there are 1000milligrams in 1g (and therefore, as we have over half a gram, we know our answer must be over 500milligrams). To work out the answer we must do the following calculation: 0.6 × 1000 = 600 The answer is that 0.6g = 600mg Example 3 Convert 0.05g into micrograms First convert 0.05g into milligrams by multiplying by 1000: 0.05 × 1000 = 50 So 0.05g = 50mg Then convert 50mg into micrograms by multiplying by 1000: 50 × 1000 = 50000 The answer is that 0.05g = 50000micrograms Alternatively, this could have been completed in one step by multiplying 0.05 by 1000000 Example 4 Convert 0.76L into millilitres As we are going from litres to millilitres we need to multiply by 1000 0.76 × 1000 = 760 The answer is that 0.76L = 760mL |
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How are percentages expressed? |
Percentage strength is the amount of drug in grams per 100g . This means that a 1% preparation contains 1g of the drug per 100g of the preparation. There are different ways to express percentage strengths depending on whether the solvent and solute are solid or liquid. Solids are measured by weight, whereas liquids are measured by volume. 1% w/v means 1g in 100ml 1% w/w means 1g in 100g 1% v/v means 1ml in 100ml Example 1 1% Hydrocortisone Cream This cream contains 1% hydrocortisone. Percent means "out of every hundred". In this example, our cream contains 1 part hydrocortisone cream in 100 parts or 1g of hydrocortisone in 100g. How much hydrocortisone is there in a 30g tube of 1% hydrocortisone cream? We know: 1% = 1g in 100g If we divide by 10 this tells us there is 0.1g of hydrocortisone (or 100mg) in 10g of the cream 1/ 10 = 0.1g In our case we have 30g so we need to multiply the amount in 10g by 3: 0.1 × 3 = 0.3g The answer is therefore that there is 0.3g hydrocortisone in 30g of cream Alternatively, this could be expressed in the following formula, where 1 is the percentage of hydrocortisone, 30 is the total amount of cream in grams and 0.3 is the number of grams of hydrocortisone in 30g: 1/100 × 30 = 0.3 Example 2 100mg of drug X was mixed with Aqueous Cream BP until it was evenly dispersed. The cream was then made up to a total weight of 50g with more Aqueous Cream BP. What is the percentage strength? We know that percentage strength is the amount of drug in grams per 100g. We used 100mg of the drug. We can convert this figure to grams by dividing by 1000 100 / 1000 = 0.1g So we have: 0.1g of drug X in 50g To convert this to the amount in 100g we need to multiply by 2: 0.1 × 2 = 0.2g We have calculated that there is 0.2g of drug X in 100g so the percentage strength is 0.2% Or, using the same formula as before: ?/ 100 × 50 = 0.1 Where ? is the percentage of drug X that we want to calculate, 50 is the total number of grams of cream and 0.1 is the total amount of drug X in grams. This can be rearranged to give: (0.1 × 100) / 50 = 0.2 Example 3 A 30g tube of ointment contains 30mg of mometasone furoate. What is the percentage strength? To determine the percentage strength of the ointment, we need to calculate the amount of mometasone furoate per 100g of ointment. Here's how we can do that: Convert 30mg to grams: 30mg ÷ 1000 = 0.03g Calculate the amount of mometasone furoate per 100g of ointment: (0.03g / 30g) x 100% = 0.1% Therefore, the percentage strength of the ointment is 0.1% |
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How to calculate a dilution? |
Example You are required to make 10ml of a 0.5% solution of sodium chloride and the stock (concentrated) solution is 5%. How much of the stock solution will be required for the dilution? |
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How to convert parts into percentages? |
1 in 400 means 1g in 400ml. To convert parts into a percentage, divide the number of grams by the number of millitres then multiply by 100. Number of grams × 100 / Number of millilitres = 1 × 100 / 400 = 0.25%
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How to convert ratios into percentages? |
1:4 means 1 part to 4 parts, so there is a total of 5 parts. To convert parts into a percentage, divide the number of parts by the total number of parts then multiply by 100. 1 part × 100 / 5 parts = 20%
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How to work out g/l to percentage? |
Remember that: 1% w/v means 1g in 100ml 1% w/w means 1g in 100g 1% v/v means 1ml in 100ml You should start by converting the units of mass and volumes into grams and millilitres to make the units consistent and make your calculations easier. |
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How to work out mg/ml to percentage? |
Remember that: 1% w/v means 1g in 100ml 1% w/w means 1g in 100g 1% v/v means 1ml in 100ml You should start by converting the units of mass and volumes into grams and millilitres to make the units consistent and make your calculations easier. The key is to remember that 1%w/v means 1g/100ml, and to convert to the correct units before starting the calculation. |
