DEFINITIONS • Posology – the pharmacological study of drug dosage. – The study of dosage quantity and prescription.
Usual Recommended dose – the amount of drug that will ordinarily produce the effect for what is intended.
•Minimum dose - the smallest dose of drug that produces therapeutic effect. •Maximum dose -the largest dose that can safely administered. Ex. Acetaminophen 10 to 15 mg/kg/dose
Toxic dose •amount of drug that cause harmful effect. Lethal dose •amount of substance that will cause death.
Single dose - to be taken at one time.Daily dose - amount to be taken in 24 hour period.
Maintenance dose - amount to be taken to maintain the therapeutic effective dose.Loading dose - first dose given to achieve maintenance drug level quickly.
System of Measurement1. Metric system2. Apothecary system3. Household system
System of Measurement1. Metric System – Most widely used system of measurement – A decimal system based on the power of ten – Units: • Gram (weight) • Liter (volume) • Meter (length)
System of Measurement2. Apothecary System – Uses Roman numerals to express quantity – Old system of measurement – Uses: • minim = liquid • Grains = solid
System of Measurement3. Household System – Not as accurate as metric system due to lack of standardization of spoons, cups and glasses – Teaspoon (tsp) = liquid – Pound (lb) = solid
System of Measurement ml- millilitercc- cubic centimetergm- grammg- milligramgr- grainmEq- milliequivalentmcg- microgram
GENERAL METHODS FOR DRUG CALCULATION1. BASIC FORMULA2. RATIO& PROPORTION3. FRACTIONAL EQUATION4. BODY WEIGHT5. BODY SURFACE AREA
1. Basic Formula: DxV=AHD- desired dose (drug dose ordered by health care provider)H- on-hand (stock on hand/on label of container)V- vehicle ( Q- quantity/drug form in which drug comes)A- Amount calculated to be given to client
Examples:1. Order:Avandia 2mg OD. How much tablet will you give? DxV=A HSolution: 2mg x 1 tablet 4mg = .5 tablet
Paracemol 500mg. Q 4 hour Round the Clock (RTC) p.o. The stock on hand is 250mg/5ml in 60ml bottle. How many ml should be administered? DxV=A H 500 mg x 5 ml = 250 mg 10 ml
2. Dilute Terramycin to 50ml of sterile water. Give 5000mg po Q12.
3. Banophen 50mg. TID p.o. a. How many tablet will you give in 1 dose? b. How many tablets will you give in a day?
2. Ratio and Proportion H : V = D : xWhere: H = drug on hand (available) V = vehicle/drug form (capsule/tablet/liquid) D = desired dose (as ordered) x = unknown amount to give
ExampleOrder: Cefdinir (3rd gen cephalosporin) 100 mg PO q.i.d *How many mL should the client receive?
Order: Cefdinir (3rd gen cephalosporin) 100 mg PO q.i.d *How many mL should the client receive?Solution: H : V = D : X 250mg : 5mL = 100mg : xmL
• Your drug order is for morphine sulfate 5 mg, IV, q3h, PRN. The drug is available as morphine sulfate 10 mg/mL. Your instructions are: infuse morphine sulfate 5 mg; not to exceed 10 mg/4mins – How many mL equal morphine sulfate 5 mg? – What is the number of minutes to administer morphine sulfate?Solution:• a. 0.5 ml• b. 2 min
3. Fractional Equation• Same as R & P except it is written as a fraction. H = D V X
ExampleOrder:Ciprofloxacin 500 mg PO q12h*how many tablets should the client receive? H = D V X
Order:Ciprofloxacin 500 mg PO q12h*how many tablets should the client receive?Solution: H=D 750mg = 500mg V X 1 tablet x 750(x) = 500 x = 0.67 tablets
4. Body Weight (BW)• Allows individualization of the drug dose• Involves 3 steps:1. Convert pounds to kg – 1 Kg = 2.2 lbs2. Determine drug dose per BW – Drug dose x body weight = clients dose per day3. Follow basic formula, R & P
ExampleOrder:Fluorouracil (5-FU), 12 mg/kg/day IV, not to exceed 800 mg/day. The adult weighs 132 lb.1. Convert pounds to Kg : 132/2.2 = 60kg2. mg x kg = client’s dose: 12 x 60 = 720 mg/kg/dayAnswer: fluorouracil 720 mg/kg/day
5. Body Surface Area (BSA)• Most accurate to calculate drug dose for: – Infants – Children – Older adults – clients who are on antineoplastic agents – low body weight• In m2, determined by where the person’s height and weight intersect the nomogram scale.• To calculate the drug dosage using the method, multiply the drug dose by # of square meters.
ExampleOrder:Cyclophosphamide (Cytoxan) 100 mg/m2/day, IV; available dosage is 200mg; client’s height is 70 inches, weight is 160 lbs.1. 70 inches and 160 lbs intersect the nomogram scale at 1.97 m2 (BSA).2. 100 mg x 1.97 = 197mgAnswer:Administer cyclophosphamide 197 mg/day
ExampleThe doctor has ordered an antibiotic whose average adult dose is 250 mg per day. What would the dosage for this medication be on a child who has a length of 120 cm and weight of 40 kg?Determine the BSA.
The doctor has ordered an antibiotic whose average adult dose is 250 mg per day. What would the dosage for this medication be on a child who has a length of 120 cm and weight of 40 kg?Child dose : surface area in sq m (m2) x ave adult dose (mg) 1.73Solution: 1.2 m2 x 250 mg 1.73 = 173 mg of medication would be given.
Pediatric considerationsFried’s rule – applies to the child younger than 1 year of age.Child’s dose: infant age (months) X adult average dose 150 months
Pediatric considerationsYoung’s Rule- applies to children 1-12 y.oChild’s dose: child’s age (years) X adult average dose child’s age (yrs) +12
Pediatric considerationsClark’s Rule – Uses the child’s weight to calculate the appropriate dose and assumes that the adult dose is base on 150 lb person.Child’s dose: child’s weight (pounds) X adult average dose 150 lbs (pounds)
Calculation of Intravenous Fluid – 3 different methods – gtt/minMETHOD I: Three-StepMETHOD II: Two-StepMETHOD III: One-Step
Calculation of Intravenous Fluid• METHOD I: Three-Step Step 1: Amt of soln = mL/hr hrs o administer Step 2: mL per hr = ml/min 60 minutes Step 3: Ml/min x gtt/ml = gtt/min
Calculation of Intravenous Fluid• METHOD II: Two-Step Step 1: Amnt of fluid = ml/hr Hrs to administer Step 2: mL per hr x drops per mL = gtt/min 60 mins
Calculation of Intravenous Fluid• METHOD III: One-Step Amnt fluid x gtt/mL = gtt/min Hrs to admin x min/hr
Drop factors:Macro drip – 15/60Micro drip – 60/60No. of drops/min= no. of fluids (cc) X drop factor no. of hours X 60No of hours= no. of fluids (cc) X drop factor no. of drops/min X 60
• Your drug order is for 1000mL of D5/0.5NSS to run for 8 hours. – Would you used macrodrip or microdrip IV set? – Calculate the drops per minute (gtt/min) using the 2-step method
Example:No. of drops/min= no. of fluids (cc) X drop factor no. of hours X 601. D5LR 1L for 12 hours. The drop factor is 15gtts/ml. Compute for the no. of drops/min.gtts/min= 1000cc X 15 12 hour X 6020gtts/min.
2.No. of HourNo of hours= no. of fluids (cc) X drop factor no.of drops/min X 60D5LR 1L at 2ogtts/min. The IV set delivers at 15gtts/ml. Compute for the no. of hours consumed.No. of hour= 1000cc X 15 20gtts/min X 6012 hours = answer
Fahrenheit And Centigrade conversionsC= F- 32 x 5 9F= C X 9 +32 5
Frequency• OD = once a day • PRN = as needed• BID = twice a day • H.S = hour of Sleep• TID = Thrice a day • STAT = immediately• QID = Four times a day • ASAP = as soon as possible• Q4hr = every 4 hours • QD = everyday• Q6hr = Every 6 hours • QOD = every other day• Q8hr – Every 8 hours • D• Q12hr = every 12 hour