calculations of doses

CALCULATION

OF DOSES

PATIENT PARAMETERS

PREFACE

• For certain drugs and for certain patients, drug dosage is determined on the basis of specific patient parameters:

• Age,

• Weight,

• Body surface area,

• Nutritional and functional status.

• Among patients requiring individualized dosage are:

• Neonates and other pediatric patients,

• Elderly patients with diminished biologic functions,

• Individuals of all age groups with compromised liver and/or kidney function,

• Critically ill patients,

• Patients being treated with highly toxic chemotherapeutic agents.

• Patients being treated with certain drugs with a narrow therapeutic window.

PEDIATRIC PATIENTS

• Pediatrics is the branch of medicine that deals with disease in children from birth through adolescence.

• The inclusive groups are defined further as follows:

• Neonate (newborn), from birth to 1 month;

• Infant, 1 month to 1 year;

• Early childhood, 1 year through 5 years;

• Late childhood, 6 years through 12 years;

• Adolescence, 13 years through 17 years of age.

• A neonate is considered premature if born at less than 37 weeks’ gestation.

PEDIATRIC PATIENTS

• Proper drug dosing of the pediatric patient depends on:

• Age and weight,

• Overall health status,

• The condition of such biologic functions as respiration and circulation,

• The stage of development of body systems for drug metabolism (e.g., liver enzymes) and drug elimination (e.g., renal system).

• In the neonate, these biologic functions and systems are underdeveloped:

• Renal function, for example, develops over the span of the first 2 years of life.

• This fact is particularly important because the most commonly used drugs in neonates, infants, and young children are antimicrobial agents, which are eliminated primarily through the kidneys.

• If the rate of drug elimination is not properly considered, drug accumulation in the body could occur, leading to drug overdosageand toxicity.

PEDIATRIC PATIENTS

GERIATRIC PATIENTS

• Geriatric medicine or geriatrics is the field that encompasses the management of illness in the elderly.

• Functional capacities of most organ systems decline throughout adulthood,

• Important changes in drug response occur with advancing age.

• Most age-related physiologic functions peak before age 30 years, with subsequent gradual linear decline.

• Reductions in physiologic capacity and function are cumulative, becoming more profound with age:

• Kidney function is a major consideration in drug dosing in the elderly because reduced function results in reduced drug elimination.

• Renal blood flow diminishes nearly 1% per year after age 30, making the cumulative decline in most persons 60 to 70 years of age, about 30% to 40%, a value that is even greater in older persons.

• Start with low dose; monitor for need of dose adjustment.

GERIATRIC PATIENTS

• Other common features of medication use in the elderly:

• The long-term use of maintenance drugs,

• The need for multidrug therapy,

• with the attendant increased possibility of drug interactions

and adverse drug effects;

• Difficulties in patient compliance, due to:

• impaired cognition,

• confusion over the various dosing schedules of multiple

medications,

• depression or apathy,

• economic reasons.

DOSAGE FORMS APPLICABLE TO

PEDIATRIC AND GERIATRIC

PATIENTS

• In the general population, solid dosage forms, such as tablets

and capsules, are preferred for the oral administration of drugs.

• because of their convenience, ease of administration, etc..

• However, solid dosage forms are often difficult or impossible

for the pediatric, geriatric, or infirm patient to swallow.

• liquid forms are preferred, such as oral solutions, syrups,

suspensions, and drops.

• liquid forms of medication may be administered by oral

feeding tube.

• the dose can easily be adjusted by changing the volume of

liquid administered.

DOSAGE FORMS APPLICABLE TO

PEDIATRIC AND GERIATRIC

PATIENTS

• Many options for individuals unable or unwilling to

swallow whole tablets:

• Compound an oral liquid from a counterpart solid dosage form

when a liquid product is not available.

• Chewable tablets and solid gel forms that disintegrate or

dissolve in the mouth.

• Tablet splitting and tablet crushing.

• For systemic effects, injections may be used rather than the

oral route of administration when needed for pediatric and

elderly patients.

DRUG DOSAGE BASED ON AGE

• Before the physiologic differences between adult and pediatric patients were

clarified, the latter were treated with drugs as if they were merely miniature adults.

• Various rules of dosage in which the pediatric dose was a fraction of the adult dose,

based on relative age, were created for youngsters.


• Today these rules are not in general use, because age alone is no longer

considered a singularly valid criterion in the determination of accurate dosage

for a child.

• Currently, when age is considered in determining dosage of a potent

therapeutic agent, it is used generally in conjunction with another factor, such

as weight.

• Example; the dose of digoxin is determined by a combination of the patient’s

age and weight.

EXAMPLE CALCULATIONS OF

DOSE BASED ON AGE

• An over-the-counter cough remedy contains 120 mg of

dextromethorphan in a 60-mL bottle of product. The label

states the dose as 11⁄2 teaspoonfuls for a child 6 years of age.

How many milligrams of dextromethorphan are contained in

the child’s dose?

• From the data in Table 8.1, calculate the dosage range for

digoxin for a 20-month-old infant weighing 6.8 kg.


• Some OTC non potent medications include labeling

instructions that provide guidelines for safe and effective

dosing.

• For pediatric use, doses generally are based on age

groupings; e.g.

• 2 years of age or younger “consult your physician”

• 2 to 6 years old give X1 mg

• 6 to 12 years old give X2 mg

• over 12 years of age give X3 mg

DRUG DOSAGE BASED ON

BODY WEIGHT

** The patient’s weight is an important factor in dosing since the size of the

body influences the drug’s concentration in the body fluids and at its site of

action.


DOSE BASED ON BODY WEIGHT

• The usual initial dose of chlorambucil is 150 mcg/kg of body

weight. How many milligrams should be administered to a

person weighing 154 lb.?

• The usual dose of sulfisoxazole for infants over 2 months of

age and children is 60 to 75 mg/kg of body weight. What would

be the usual range for a child weighing 44 lb.?



• Using Table 8.2 and a daily dose of 0.5 mg/kg, how many 20-

mg capsules of the drug product should be dispensed to a

patient weighing 176 lb. if the dosage regimen calls for 15

weeks of therapy?


BODY SURFACE AREA

• The Body Surface Area (BSA) method of calculating drug

doses is widely used for two types of patient groups:

• Cancer patients receiving chemotherapy,

• Pediatric patients,

• with the general exception of neonates, who are usually

dosed on a weight basis with consideration of other factors.

• The average adult is considered to have a BSA of 1.73 m2.


BODY SURFACE AREA

EXAMPLE CALCULATIONS OF DOSE

BASED ON BODY SURFACE AREA

• If the adult dose of a drug is 100 mg, calculate the approximate

dose for a child with a BSA of 0.83 m2, using (a) the equation

and (b) Table 8.3.

• Using Table 8.4, find the dose of the hypothetical drug at a

dose level of 300 mg/m2 for a child determined to have a BSA

of 1.25 m2. Calculate to verify.

EXAMPLE CALCULATIONS OF DOSE

BASED ON BODY SURFACE AREA

• If the adult dose of a drug is 75 mg, what would be the dose for a child weighing 40 lb. and measuring 32 in. in height?

• The usual pediatric dose of a drug is stated as 25 mg/m2. Calculate the dose for a child weighing 18 kg and measuring 82 cm in height.

• If the usual adult dose of a drug is 120 mg, what would be the dose based on BSA for a person measuring 6 ft. tall and weighing 200 lb.?

• If the dose of a drug is 5 mg/m2, what would be the dose for a patient with a BSA of 1.9 m2?

DOSAGE BASED ON THE MEDICAL

CONDITION TO BE TREATED

• By using Table 8.5, calculate the IV drug dose for a 3-pound 3-

ounce neonate.

SPECIAL DOSING CONSIDERATIONS

IN CANCER CHEMOTHERAPY

• The term chemotherapy applies to the treatment of disease

with chemical drugs or chemotherapeutic agents.

• Chemotherapy is primarily associated with the treatment of

cancer patients, and is considered the mainstay of such

treatment in that it is effective in widespread or metastatic

cancer, whereas treatments such as surgery and radiation

therapy are limited to specific body sites.

• Often combination therapy is used, with more than a single

treatment modality included in a patient’s treatment plan (e.g.

radiation plus chemotherapy).



• Chemotherapeutic agents most often are administered:

• orally,

• by intravenous injection,

• by continuous intravenous infusion;

• Other routes of administration may be used:

• intraarterial (artery),

• intrathecal (around spinal column endings),

• intramuscular,

• subcutaneous injection.

• Administration to a specific site:

• the lung (intrapleural),

• the abdomen (intraperitoneal),

• the skin (topical), etc..



• Although a single anticancer drug may be used in a patient’s treatment plan, combination chemotherapy perhaps is more usual.

• By using combinations of drugs having different mechanisms of action against the target cancer cells:

• the effectiveness of treatment may be enhanced,

• lower doses used,

• side effects reduced.

• The combination chemotherapy plans often include:

• two-agent regimens,

• three-agent regimens,

• four-agent regimens.



Cancer chemotherapy is unique in the following ways:

• It may involve single or multiple drugs of well-established drug therapy regimens or protocols, or it may involve the use of investigational drugs as a part of a clinical trial.

• Combinations of drugs may be given by the same or different routes of administration, most often oral and/or intravenous.

• The drugs may be administered concomitantly or alternately on the same or different days during a prescribed treatment cycle (e.g., 28 days).

• The days of treatment generally follow a prescribed format of written instructions, with D for ‘‘day,’’ followed by the day(s) of treatment during a cycle, with a dash (–) meaning ‘‘to’’ and a comma (,) meaning ‘‘and.’’

• Thus, D 1–4 means ‘‘days 1 to 4,’’ and D1,4 means ‘‘days 1 and 4.’



• The drugs used in combination chemotherapy often fit into a standard drug/dosage regimen identified by abbreviations or acronyms.

For example;

• a treatment for bladder cancer referred to as MVAC consists of methotrexate + vinblastine + doxorubicin (or actinomycin) + cisplatin;

• a treatment for colorectal cancer called FU/LU consists of fluorouracil + leucovorin;

• a treatment for lung cancer called PC consists of paclitaxel + carboplatin;

• a treatment for ovarian cancer called CHAD consists of cyclophosphamide + hexamethylmelamine + adriamycin + diamminedichloroplatinum (cisplatin).



• In addition to the use of abbreviations for the drug therapy

regimens, the drugs themselves are commonly abbreviated in

medication orders, such as:

• MTX for ‘‘methotrexate”, DOX for ‘‘doxirubicin’’, VLB for

‘‘vinblastine’’, and CDDP for ‘‘cisplatin.’’

• For systemic action, chemotherapeutic agents are usually dosed

based either on body weight or on body surface area.

• Often, the drug doses stated in standard regimens must be

reduced, based on a particular patient’s diminished kidney or

liver function.

• Doses may also be reduced based on patient’s CBC

(platelets & WBC count).



• To help prevent errors in chemotherapy, pharmacists

must:

• Correctly interpret medication orders for the

chemotherapeutic agents prescribed,

• Follow the individualized dosing regimens,

• Calculate the doses of each medication prescribed,

• Dispense the appropriate dosage forms and

quantities/strengths required.


CHEMOTHERAPY DOSAGE REGIMENS

• Regimen: VC

Cycle: 28 d; repeat for 2–8 cycles

Vinorelbine, 25 mg/m2, IV, D 1,8,15,22

Cisplatin, 100 mg/m2, IV, D 1.

For each of vinorelbine and cisplatin, calculate the total

intravenous dose per cycle for a patient measuring 5 ft. 11 in. in

height and weighing 175 lb.



• Regimen: CMF

Cycle: 28 d

Cyclophosphamide, 100 mg/m2/d po, D 1–14.

Methotrexate, 40 mg/m2, IV, D 2,8.

Fluorouracil, 600 mg/m2, IV, D 1,8.

Calculate the total cycle dose for cyclophosphamide,

methotrexate, and fluorouracil for a patient having a BSA of 1.5

m2.

calculations of doses

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