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  • 1. C H A P 42 E R T 3Pharmacology for NursesCalculation of Dosageof Drugs Weights and Measuresimperial systemIt is an ancient system of measurement. It includes avoirdupois and apothecaries’ systems. 1. Avoirdupois system: The basic unit of weight is pound (lb). 1 lb = 16 ounces (oz) 1 ounce (oz) = 437.5 grains (gr) 2. Apothecaries’ system: This system was used to measure weight of solids and volume of liquids before introduction of metric system. It is rarely used today. Weight Volume 1 pound (lb) = 12 troy ounces 1 gallon = 160 fluid ounces 1 ounce (oz) = 480 grains (gr) 1 quart = 2 pints 1 pint = 20 fluid ounces 1 fluid ounce = 480 minimsMetric systemIt is a commonly used system of measurement today. In this system, the primary unit of length is metre, ofweight is gram and of volume is litre (Table 3.1).table 3.1 Metric system of measurement Weight Volume Length unit abbreviation unit abbreviation unit abbreviation nanogram ng microlitre µL millimetre mm microgram mcg millilitre mL centimetre cm milligram mg decilitre dL metre m gram g litre L kilogram kg
  • 2. 43 Calculation of Dosage of Drugshousehold systemThis system of measurement is not very accurate. It involves the use of cups, spoons, glasses, etc. Volume Length Drops (gtt) Inch (in) Teaspoon (tsp) Tablespoon (tbsp)Conversion from One system to another Weight Volume 1 kilogram (kg) = 1000 grams (g) 1 litre = 1000 millilitre (mL) 1 gram (g) = 1000 milligrams (mg) 1 decilitre (dL) = 100 millilitre (mL) 1 milligram (mg) = 1000 micrograms (mcg) 1 millilitre (mL) = 1000 microlitre (µL) 1 microgram (mcg) = 1000 nanograms (ng) 1 ounce (oz) = 30 millilitre (mL) 1 kilogram (kg) = 2.2 pounds (lb) 1 mL = 16 drops 1 grain (gr) = 60 milligrams (mg) 1 pint = 480 mL (16 oz) 1 teaspoonful = 5 mL 1 tablespoonful = 15 mL 1 teacupful = 150 mL 1 tumblerful = 250 mLCaLCuLatiOn Of drug dOsageThe following formulae are useful in calculating the drug dosage: 1. For children: l Young’s formula Age (years) Child dose = × Adult dose Age + 12 l Clark’s formula Weight (pounds) Child dose = × Adult dose 150 2. Based on body weight, drug dose for lean or obese individuals and children is calculated by the following formula: Body weight (kg) Individual dose = × Average adult dose 70
  • 3. Pharmacology for Nurses 44 3. Based on body surface area (BSA): 2 Body surface area (m ) Individual dose = × Average adult dose 1.7 BSA can be calculated by the following formula: Body surface area (BSA) = Body weight (kg)0.426 × Height (cm)0.725 × 0.007184 BSA can also be obtained from a nomogram. Calculation of drug dosage by using BSA is cumbersome. It is mainly used to calculate the dose of anticancer drugs. 4. Based on kidney function: In patients with renal failure, dosage of certain drugs have to be calculated based upon creatinine clearance. For example, dose of aminoglycosides, amphotericin B, etc. should be modified in patients with impaired renal function.CaLCuLatiOn Of dOsage Of OraLLy adMinistered drugssolid dosage formProblem 1: The doctor has prescribed 500 mg of amoxicillin. The amoxicillin dose reads 250 mg per Required stripcapsule. How many capsules will you give? Number of capsules = Available dose in hand Required dose 500 mg Number of capsules = = =2 Available dose in hand 250 mg 500 mgYou will give two capsules (each 250 mg) to=the patient to get the required 500 mg as prescribed. =2 250 mgProblem 2: The doctor has prescribed 250 mg of paracetamol to a child. The tablet is available as 500 mg. Required dose Number of tablets =How will you administer the required dose? Available dose in hand Required dose 250 mg 1 Number of tablets = = = 1/2 tablet Available dose in hand 500 mg 2 250 mg 1 =Half a tablet is to be administered (Figure 3.1). = tablet 500 mg 2 1/2 1/4 1/2 (a) (a) Scored tablet (b) (b) ½ tablets (c) (c) ¼ tablets Figure 3.1 (a) Scored tablet (b) 1/2 Tablets (c) 1/4 Tablets.
  • 4. 45 Calculation of Dosage of DrugsNote: Only scored tablets can be broken into 1/2 or 1/4. Capsules and coated tablets should not bedivided.Problem 3: The doctor has prescribed drug A in a dose of 0.250 mg. The tablet is available as 62.5 mcg.How many tablets should be administered to the patient?Step 1:Convert drug dosage to same units. That is, convert 0.250 mg to mcg (1 mg = 1000 mcg).Therefore, 0.250 mg = 0.250 × 1000 mcg = 250 mcgStep 2: Required dose 250 mcg Number of tablets = = =4 Available dose in hand 62.5 mcgNote: unit of required dose and available dose must be the same.Liquid dosage formsProblem 1: Doctor has prescribed 75 mg of paracetamol syrup for a child. It is available as 125 mg/5 mLsyrup. How much of the syrup is to be administered to the child?125 mg is contained in 5 mL. Therefore, 75 mg is contained in 5 × 75 = 3 125So, 3 mL of the syrup is to be administered to the child.Liquid dosage forms can be administered orally with a medicine cup, dropper, teaspoon or tablespoon.Medicine cups are calibrated in millilitre or ounce. Droppers are calibrated in millilitre. The dose of theliquids should always be measured at the lowest point of the meniscus (Figure 3.2). 60 50 40 30 20 10 ML (a) (b) Figure 3.2 - (a) Measuring cup (b) Dropper.
  • 5. Pharmacology for Nurses 46Note: dose of 5 mL or multiples of 5 can be measured in a medicine cup. dose of less than 5 mLshould be measured in a syringe for accuracy.reading of Label for Orally administered drugBefore administering a drug, the label should be read carefully and compared with the doctor’s instructions(Figure 3.3). iii, iv xi ii iii i xii x vii v vi ix viii Figure 3.3 Label of an orally administered drug (Courtesy: J.B. Chemicals & Pharmaceuticals Ltd.).The following information are obtained from a label of tablet/syrup: (i) Trade name, e.g. Nicardia Retard 20 (ii) Generic name, e.g. nifedipine (iii) Dose, total number of tablets/total volume of liquid, e.g. dose of each tablet—20 mg; total number of tablets per strip is 10; pharmacopoeia, e.g. USP. (iv) Form of the medication (tablet/capsule/syrup), e.g. tablet nifedipine (v) Date of manufacture (vi) Date of expiry (vii) Name of the manufacturer, e.g. J.B. Chemicals & Pharmaceuticals Ltd. (viii) Direction for storage and precaution, e.g. protect from light (ix) Direction for administration, e.g. shake well before use
  • 6. 47 Calculation of Dosage of Drugs (x) Batch number (xi) Others: DS (double strength), DT (dispersible tablet), SR (sustained release), retard, etc. (xii) PriceCaLCuLatiOn Of dOsage Of ParenteraLLy adMinistered fLuidsstrength of solutionConcentration of solutions can be expressed as percent or ratios. The percentage concentration of solutionscan be expressed as given below: 1. Percent weight in volume (w/v), i.e. ‘w’ g (weight) of solute in ‘v’ mL (volume) of solution, e.g. 1% w/v solution contains 1 g of solute in 100 mL of solution. 2. Percent weight in weight (w/w) solution, i.e. ‘w’ g (weight) of solute in ‘w’ g (weight) of solution, e.g. 1% w/w solution is 1 g of solute in 100 g of solution. 3. Percent volume in volume (v/v) solution, i.e. ‘v’ mL (volume) of solute in ‘v’ mL of solution, e.g. 1% v/v solution is 1 mL of solute in 100 mL of solution.Commonly, solutions are expressed as w/v percent, for example, 1. 0.9% normal saline (0.9% w/v solution of sodium chloride in water)—0.9 g of sodium chloride in 100 mL of solution. 2. 5% dextrose (5% w/v solution)—5 g of dextrose in 100 mL of solution. 3. 50% dextrose (50% w/v solution)—50 g of dextrose in 100 mL of solution. 4. 20% mannitol (20% w/v solution)—20 g of mannitol in 100 mL of solution.Solution can be expressed as v/v percent, e.g. 70% alcohol (70% v/v solution of alcohol)—70 mL ofabsolute alcohol in 100 mL of its aqueous solution.ratio solutions: w/v solutions can also be expressed in ratios, for example, 1. 1:100 is 1 g of solute in 100 mL of solution. 2. 1:1000 is 1 g of solute in 1000 mL of solution, e.g. adrenaline 1:1000 solution means 1 g of adrenaline is present in 1000 mL of its solution.Intravenous fluids (i.v. fluids): The commonly used i.v. solutions are mentioned below: Intravenous fluid abbreviation Normal saline NS Half-strength saline 0.45% NS 5% Dextrose 5% D 10% Dextrose 10% D 50% Dextrose 50% D Dextrose normal saline DNS Ringer lactate RL
  • 7. Pharmacology for Nurses 48Intravenous fluid rate should be calculated as drops/min or mL/h. drop factor is the number of dropsrequired to deliver 1 mL(gtt/mL) and is used in calculating the number of drops per minute. The drop factoris mentioned on the infusion set. A macrodrip set usually delivers 10, 15, 16 or 20 drops/mL. A microdripset delivers 60 drops/mL. drip rate of a solution is the number of drops per minute (drops/min, gtt/min). flow rate of a solution isthe number of millilitre per minute or per hour. In microdrip set, the drip rate is the same as flow rate (e.g.if flow rate is 150 mL/h, then drip rate is 150 drops/min). Total volume (mL) × Drop factor (gtt/mL) Drip rate, i.e. drops/min (gtt/min) = Time (min)Problem 1: Infuse 1000 mL dextrose normal saline over 5 h. Calculate the flow rate and drip rate. The dropfactor for the infusion set is 15 gtt/mL.Step 1: Total volume (mL) 1000 mL Flow rate = = =200 mL/h Total number of hours 5hTo express as mL/min, convert 1 h to 60 min.Therefore, 200 mL can be given in 60 min.That is, in 60 min the amount of fluid to be infused is 200 mL.So, in 1 min the amount of fluid to be infused is 200/60 mL = 3.33 mL.Therefore, the flow rate is 200 mL/h or 3.33 mL/min.Step 2: Total volume (mL) × drop factor (gtt/min) 200×15 Drip rate = = =10 gtt/min Time (min) 60 × 5The drip rate is 10 gtt/min.CaLCuLatiOn Of dOsage Of ParenteraLLy adMinistered drugsLabel of Parenterally administered drugThe label should be read carefully before administering a drug (Figure 3.4).The following information are obtained from the label: (i) Brand name, e.g. Taxim (ii) Generic name, e.g. cefotaxime sodium
  • 8. 49 Calculation of Dosage of Drugs ii vi iii xi i ix vi vii viii iv v x Figure 3.4 Label of a parenterally administered drug (Courtesy: Alkem). (iii) Amount of the drug, e.g. 1 g; total volume of solution (iv) Date of manufacture (v) Date of expiry (vi) Special instructions like reconstitution of the drug if in powder form; storage conditions, e.g. dissolve the content in 4 mL of sterile water for injection; store in a cool place (vii) Approved routes of administration, e.g. IM/IV (viii) Batch number (ix) Name of the company, e.g. Alkem (x) Price (xi) Pharmacopoeia, e.g. IPProblem 1: A doctor has prescribed 50 mg of drug X to be administered. The ampoule contains 100 mg/2 mL.What amount of the drug will you administer? Required dose 50 mg × Volume = × 2 mL = 1 mL Available dose 100 mgOne millilitre of the drug X is to be administered to the patient to get the required dose of 50 mg asprescribed by the doctor.Problem 2: A patient has been prescribed 10,000 units of heparin subcutaneously. The vial contains5000 units/mL of heparin. What volume of heparin should the patient receive? Required dose 10, 000 units × Volume = × 1 mL = 2 mL Available dose 5000 unitsTherefore, 2 mL of heparin is to be injected to the patient.
  • 9. Pharmacology for Nurses 50insulin syringes: Insulin syringe is calibrated in units (10 units = 0.1 mL). Hence, there is no need forcalculating the volume (mL) of insulin to be administered. Therefore, draw the required number of units ofinsulin into the syringe. The syringes are available in 0.4 mL size that measures up to 40 units, 0.5 mL sizethat measures 50 units and 1 mL size that measures 100 units (Figure 3.5a). Figure 3.5 (a) Insulin syringes.Problem 3: A patient has been prescribed 20 units of regular insulin subcutaneously. The vial contains100 units/mL of regular insulin. You have a 1 mL insulin syringe. How much insulin will you withdraw?Indicate in the Figure 3.5b. Draw up to this level Draw up to this level Figure 3.5 (b) Insulin syringe (1 mL).Problem 4: A doctor has prescribed injection ampicillin 125 mg intramuscularly. It is available as 500 mgin powder form in a multiple dose vial. The direction on the label for reconstitution of the powdered drugis to add 1.8 mL of sterile diluent to obtain 250 mg/mL of ampicillin.
  • 10. 51 Calculation of Dosage of DrugsStep 1:Add 1.8 mL of sterile diluent to the powder in the multiple dose vial. Shake the vial to dissolve the drug.The resulting solution contains 250 mg/mL of ampicillin.Step 2:There is 250 mg of ampicillin in 1 mL of solution.Therefore, 125 mg of ampicillin is present in 1/250 × 125 mL of solution = 0.5 mL of solutionTherefore, 0.5 mL of the reconstituted drug is drawn into a syringe and administered intramuscularly/intravenously.Note: The label on the vial or the package insert will contain directions for reconstitution of the drug andfor how long the reconstituted drug can be stored. For a single-dose vial, the reconstituted drug is usedimmediately and the vial is discarded.Adding drugs to intravenous fluids and calculating the flow and drip rates:The doctor has ordered to mix 200 mg of dopamine in 500 mL of 5% dextrose and infuse at the rate of400 mcg/min. What is the required flow rate and drip rate? The drop factor for the set is 16 gtt/mL.Step 1:Add 200 mg of dopamine to 500 mL of 5% dextrose.Therefore, 200 mg of dopamine is present in 500 mL of solution or 200 × 1000 mcg of dopamine is presentin 500 mL of solution. 500 × 400Therefore, 400 mcg of dopamine is present in mL of solution 200 × 1000 = 1 mL of solution.Therefore, 400 mcg of dopamine is present in 1 mL of solution.Step 2:Required rate of infusion is 400 mcg/min = 1 mL of solution/min.Therefore, flow rate is 1 mL/min.Step 3:Drop factor is 16 gtt/mL, i.e. the set delivers 16 drops in 1 mL.Required flow rate is 1 mL/min.Therefore, drip rate is 16 drops/min.So, dopamine has to be infused at the rate of 16 drops/min to deliver the prescribed dose.
  • 11. Pharmacology for Nurses 52 Key POints l Dosage forms of a medication could be solid, liquid or semisolid. l Scored tablets can be divided to get the required dose. l Capsules, enteric-coated and sustained-release formulations should not be divided or crushed. l Controlled release formulations have the suffix like CR, SR, SA, retard, etc. l Concentration of a solution can be expressed as percent or ratio. l Read the label of the drug carefully before administering a drug to the patient. l Drop factor is the number of drops required to deliver 1 mL of fluid during i.v. infusion. l Drip rate of a solution is expressed as drops/min. l Flow rate of a solution is expressed as mL/min or mL/h. l A microdrip set delivers 60 drops/mL. l A macrodrip set delivers 10, 15, 16 or 20 drops/mL.exerCises 1. Calculate the amount of adrenaline present in 1 mL of the following solutions: a. 1:1000 Answer: a. 1 mg/mL b. 1:10,000 b. 0.1 mg/mL c. 1:1,00,000 c. 0.01 mg/mL 2. Calculate the amount of adrenaline present in 0.5 mL of 1:1000 adrenaline preparation. Answer: 0.5 mg 3. A patient has been prescribed 600 mg of chloroquine base to be taken stat. The available tablet of chloroquine phosphate (500 mg) contains 300 mg of chloroquine base. How many tablets of chloroquine will you administer to the patient? Answer: 2 tablets 4. Calculate the amount of sodium chloride required to prepare 500 mL of normal saline. Answer: 4.5 g of sodium chloride 5. Calculate the amount of dextrose in 5% dextrose solution. Answer: 5 g of dextrose
  • 12. 53 Calculation of Dosage of DrugsCOMMOn abbreViatiOns used in drug theraPyabbreviation Meaningad. lib. as much as you pleasea.c. before mealsp.c. after mealsaq. waterCaps. capsuleTab. tabletgtt. dropsInj. injectionLinct. linctusUng. ointmentCrem. creamo.d. once dailyb.i.d. twice a dayt.i.d. three times a dayq.i.d. four times a dayqh. every hourq4h. every 4 hq6h. every 6 hh.s. at bed timeq.s. quantity sufficients.o.s. as and when requiredstat. immediately, at oncep.o. by mouthn.p.o. nothing by mouth