Dose Calculation

1. Dose Calculation Dr. Hayder B Sahib PhD., M. Sc., D. Sc., B. Sc Pharm.

2. Calculations of dose • Dose: is the quantitative amount administered or taken by patient for the intended medicinal effect. • Single dose: The amount taken at one time • Daily dose or total dose: The amount taken during the course of therapy. • Divided dose: Two or more times per day • Drugs doses vary among drug substance some of them have small dose other have large.

3. The dose of the drug is based on • A) Biochemical and Pharmacologic activity • B) Physical and chemical properties • C) Dosage form used • D) Rout of administration • E) patient’s factors such as age, weight, BSA, general physical health and liver and kidney function

4. Calculation • No. Of Doses = total quantity / size of dose • All must be in the same unit of measure • The dose response of individual is vary and may required dose adjustment in a given patients for certain conditions, such as with chemotherapy treatment and in drugs combinations

5. The usual dosage range for a drug indicates the quantitative range or amounts of the drug that may be prescribed within the guidelines of usual medical practice. • Drug use and dose information is provided in the package labeling and inserts that accompany manufacturers’ pharmaceutical products, as well as in a variety of references, such as Drug Facts and Comparisons,1 Physicians’ Desk Reference,2 Paediatric Dosage Handbook,3 Geriatric Dosage Handbook,4 and Drug Information Handbook.

6. The usual adult dose of a drug is the amount that ordinarily produces the medicinal effect intended in the adult patient. • The usual pediatric dose is similarly defined for the infant or child patient. • The ‘‘usual’’ adult and pediatric doses of a drug serve as a guide to physicians who may select to prescribe that dose initially or vary depending on the assessed requirements of the particular patient

8. Median effective dose: is the amount that produces the desired intensity of effect in 50% of the individual tested • Median toxic dose is the amount that produces toxic effect in 50% of the individual tested • Drugs intended to produce systemic effect must be absorbed or placed directly into the circulation and distributed in adequate concentration to the site of action.

9. There is direct correlation between drug dosage, the drug’s blood serum concentration after administration and degree of drug effects. • Minimum concentration expected to produce pharmacological activity called minimum effective concentration (MEC) • Ideally appropriate drug dosage should result in blood serum drug concentration that is above the MEC and less than MTC

10. In manufacture industry they made the drugs in large scale for distribution to the medical institutions, the pharmacist use these prefabricate dosage in filling prescription this called compounding • The fabrication of a pharmaceutical product from individual ingredients weight, measure and mix. • If the pharmacist mixed two or more different compounds to prepare the desired formula this called combination products

11. Rout of Drugs Administration • Oral: Tablet, capsule, lozenges, solution, drops, syrup and suspention Sublingual tablet • Parental Solution and suspension • Epicutaneous/ transdermal skin Ointment, cream, powder, lotion aerosol and patches • Conjunctivel Solution, suspension and ointment

12. Intranasal: Spray, solution and ointment Intra respiratory: Aerosol, inhalation solution • Rectal Ointment , cream, suppositories, solution and suspension • Vaginal Ointment, cream, suppositories, gels, solution, and emulsion • Urethral Solution and suppositories

13. Dose measurements • In the institutional setting doses are measured and administered by professional and para professional personnel. • A variety of measuring devise may be use including calibrated cups for oral liquid and syringes and intravenous sets for parenteral medication in the home setting the adult patient or child parents generally measures and administered medication • Teaspoon and Tablespoon • In calculating doses pharmacist and physician accept a capacity of 5ml for the teaspoonful, and 15ml for the tablespoonful. It should be noted that the capacity of house hold teaspoons may vary from 3-7ml and those of tablespoons may vary from 15-22ml

14. Factors affecting dose measurements • Like • 1- viscosity • 2- surface tension of a given liquid • 3- technique of the person measuring the liquid can influence the actual volume held by house hold spoon

15. The drops as unit of measures • Occasionally, the drop (abbreviated gtt) is used as a measure for small volume of liquid medication. A drop does not represent a definite quantity, because drops of different liquid vary greatly. • The united state pharmacopoeia defined the official medicine dropper as being constructed at the delivery end, to around opening with an external diameter of about 3 mm. the dropper when held vertically deliver water in drops each of which weight 45 and 55 mg, accordingly the official dropper is calibrated to deliver a proximately 20 drops of water per ml. • it should be kept in mind that few medical liquid have the same surface and flow characteristics as water and therefore the size of drops varies materially from one liquid to another.

16. Case in points • A physician asked a pharmacist to calculate the dose of cough syrup so that it may be safely administered drop wise to a child, the cough syrup contains the active ingredient, Dextromethorphan HBr 30 mg/15ml in a 120ml Bottle, based on the child weight and literature, the pharmacist determines the dose of dextromethorphan HBr to be 1.5mg for the child. The medicine dropper to be dispensed with the medication is calibrated by the pharmacist and shown to deliver 20 drops of the cough syrup per one ml • Calculate the dose in drops for the child. • Solution • 30/15= 2mg/ml • 2mg ml • X= 0.75ml • x • 20 drop ml • X 0.75ml X = 15 drop

17. General Dose Calculation • Number of doses = Total amount / size of dose • Example if the dose of a drug is 200 mg, how many doses are contained in 10g? • 10g= 10000mg • 10000/200= 50 dose • Size of Dose • Size of dose= Total amount / Number of doses • How many teaspoonful would be prescribed in each dose of an elixir if 180ml contained 18 drops. • Solution 180/18=10 ml= 2 tsp • How many drops would be prescribed in each dose of a liquid medicine if 15ml contained 60 doses? The dispensing dropper calibrates 32 drops/ml. • 15mlx 32drops= 480drops • 480/60=8 drops • Total quantity of the product • Total amount = number of doses x size of dose • Examples of dose calculation please refer to your text book

18. Dosing options • The administration of doses that are much smaller or higher than the usual dose of a drug is referred to as low –dose or high dose therapy respectively. This differences based on variations in patients age, weight, renal function or other specific parameter. • Examples like aspirin 81mg which is used as a prophylaxis of heart attack as the usual dose is 325mg. other example in treat the postmenopausal sign and symptoms the hormone used her 50% less than the usual dose. • Examples in the text book Fixed dose combination products • Many medical and non medical products are available containing two or more therapeutic agents in fixed dose the advantages of this combination are • may be more convenient • enhance patients compliance • less expensive • act as a synergistic • Disadvantages • Inflexibility in dosing • Splitting tablets • A number of tablets are scored or grooved to allow braking into approximately equal pieces. This allow dosage flexibility and to reduce the price of treatment.

19. Example: A patient attempted to split in half 20-mg un-scored tablets of a drug, resulting in half tablets differing by 1.5mg in drug content. Assuming a whole tablets was uniform in drug content, calculate the amount of drug in each half tablet. • If L= large half S= smaller half • L+S=20 mg • L-S=1.5 mg • 2L= 21.5mg • L= 10.75mg • S=20-10.75= 9.25mg • Special Dosing Regimes • Certain drugs have unique dosing regimes. Among them are chemotherapeutic agents and oral contraceptives in the case of the later the prescribed regimens based on the 28 day cycle of 21 consecutive days of tablets containing a combination of estrogen and progesterone drugs followed by 7 consecutive days of tablet s containing non drug material. One tablet is taken daily preferably at approximately the same time. The tablets generally are color coded and packaged in special dispenser to facilitate compliance

20. Dose Calculation • Dose: is the amount that ordinarily produces the desired therapeutic response • Dosage range: is the range of dosage determined to be safe and effective. • This provides the prscriber with dosing guideline in initially selecting a drug dose for a particular patient and flexibility to change that dose as the patients clinical response warrants. However, for certain drugs and certain patient’s drugs dosage id determined on the basis of specific patient’s parameters. These parameters include age, weight, BSA, Nutritional and functional status. • Among patients requiring individualize are • Neonates and pediatric patients • Elderly patients with damaged biologic functions • Individual age groups with compromised liver and/or kidney function • Critically ill patients and patients treated with chemotherapeutic agents or toxic compounds. • Certain drugs with a narrow therapeutic windows such as Digoxin at blood level 0.9-2 ng /ml is consider therapeutic but above 2 ng/ml is toxic. • Pediatric patients • It’s the branch of medicine that deals with disease in children from birth through adolescence because of the variation in age and physical development the inclusive groups are defined further as follow • Neonate 1 month • Infant 1 month- 1 year • Early childhood 1-5 years • Late childhood 6-12 years • Adolescence 13-17 years • A neonate is consider premature if born at less than 37 weeks gestation

21. Factors affecting on pediatric patients in drug dosing • patients age • weight • over all health state • the condition of such biologic function as respiration and circulation and the stage of developments of body system for drugs metabolism. • In neonates the kidney function developed over the span of the first 2 years of life so we have to be very careful when the anti microbial and other drugs which are determined by the kidney. • If drugs elimination is not probably Drugs accumulation Toxicity • That is why the use of pharmacokinetics data (rate and extent of drugs absorption, distribution, metabolism and elimination together with the individual patients drugs handling characteristics.