Module 8: Pharmaceutical Calculations & Techniques (PDF)

MODULE 8: Calculations of Measure of Activity or Potency  The potencies of some antibiotics, endocrine products, vitamins, products derived through biotechnology, and biologics (e.g., vaccines) are based on their activity and are expressed in terms of units (of activity), in micro...

MODULE 8: Calculations of Measure of Activity or Potency  The potencies of some antibiotics, endocrine products, vitamins, products derived through biotechnology, and biologics (e.g., vaccines) are based on their activity and are expressed in terms of units (of activity), in micrograms per milligram (mcg/mg), or in other standardized terms of measurement. In general, they conform also to international standards defined by the World Health Organization and termed International Units (IU).  potency is a measure of drug activity expressed in terms of the amount required to produce an effect of given intensity refer to the quantity of drug necessary to produce a given effect, or it may refer to the maximum response which can be achieved with the drug  Degrees of activity are determined by comparison against a suitable working standard, generally a USP Reference Standard. Reference standards are authentic specimens used as comparison standards in compendial tests and assays. Ex. The activity or potency of antibiotics is determined by their inhibitory effect on microorganisms. No relationship exists between the unit of potency of one drug and the unit of potency of another drug  Dispensing errors can occur when a medication order abbreviates the term units with a U and places it after the number of units, since a poorly written U can be mistaken for a zero; for example, ‘‘100U’’ could be mistaken for 1000 units. Thus, it is good practice to spell out the term units when used following the designated number; for example, 100 units or 100 Units.  Biologics are preparations produced from a living source. They include vaccines, toxoids, and immune sera, used for the development of immunity, or resistance to disease; certain antitoxins and antivenins, used as treatment against specific antigens; and, toxins and skin antigens, used as diagnostic aids. are prepared from human serum (e.g., immune globulin), horse serum (e.g., tetanus antitoxin), chick cell culture (e.g., measles virus vaccine), and other such animate media.  Strengths of biologic products are expressed as: a bacterial vaccine commonly is expressed in terms of micrograms (mcg) or units of antigen per milliliter viral vaccine is expressed most commonly in terms of the tissue culture infectious dose (TCID50), which is the quantity of virus estimated to infect 50% of inoculated cultures. a toxoid is generally expressed in terms of flocculating units (Lf Unit), with 1 Lf Unit having the capacity to flocculate or precipitate one unit of standard antitoxin. Example Calculations of Measures of Activity or Potency Units of Activity 1. How many mililiters of U-100 insulin should be used to obtain 40 units of insulin? U-100 insulin contains 100 units/ml. 100units 40units = 1ml x = 0.4 ml Dimensional Analysis: 1ml 40unitsx = 0.4ml 100units  2. How many mililiters of a heparin sodium injection containing 200,000 units in 10 ml should be used to obtain 5,000 Heparin Sodium Units that are to be added to an intravenous dextrose solution? 200,000units 5000units = 10ml x = 0.25 ml Dimensional Analysis: 10ml 5000unitsx = 0.25ml 200,000units Activity Based on Weight 1. If a neomycin sulfate has a potency of 600 mcg of neomycin per miligram, how many miligrams of neomycin sulfate would be equivalent in potency to 1 mg of neomycin? 1 mg = 1000 mcg neomycin 600mcg (neomycin) 1000mcg (neomycin) = 1mg (neomycinsulfate) x = 1.67 mg Dimensional Analysis 1000mcg 1mg 1mgx x = 1.67mg 1mg 600mcg Dose or Antigen Content of a Biologic Based on Potency 1. A biologic contains 50 Lf Units of diptheria toxoid in each 2.5 ml of product. If a pediatric patient is to receive 10 Lf Units, how many mililiters of product should be administered? 50 Lf 10 Lf = 2.5ml x = 0.5 ml Dimensional Analysis 2.5ml 10 Lfx = 0.5ml 50 Lf  Electrolyte Solutions: Miliequivalents, Milimoles, and Miliosmoles Ions- Any atom or molecule with a net charge, either positive or negative Non-electrolytes- substances that are not dissociated in solution Ex. Urea and dextrose Electrolytes- those with varying degrees of dissociation Ex. Sodium Chloride in body fluids Cations- Ions that are positively charged because they have more protons than electrons Anions-Ions that are negatively charged because they have more electrons than protons Electrolyte preparations are used in the treatment of disturbances of the electrolyte and fluid balance in the body. Form: oral solutions and syrups, dry granules, oral tablets and capsules, intravenous infusions  Milliequivalents (mEq) it is a unit of measurement of the amount of chemical activity of an electrolyte For a given chemical compound, the milliequivalents of cation equals the milliequivalent of anion equals the milliequivalents of the chemical compound. Example Calculations of Milliequivalents  1. What is the concentration, in milligrams per ml, of a solution containing 2 mEq of potassium chloride per ml? Molecular weight of KCl= 74.5 Equivalent weight of KCl= 74.5  Millimoles and Micromoles Mole - the molecular weight of a substance in grams Millimole - one thousandth of a mole micromole - is one millionth of a mole Micromole Keep in Mind: Eq wt of a substance= MW/valence Moles of a substance =MW  Sample Problem How many millimoles of monosodium phosphate (MW= 138) are present in 100 g of a substance? 1mole x = 138 g 100 g = 0.725 moles = 725 mmol (milliosmole) OSMOLARITY- is the milliosmoles of solute per liter of solution mOsmol/L of solution OSMOLALITY - is the milliosmoles of solute per kilogram of solvent mOsmol/kg of solvent OSMETERS- used in laboratory to measure osmolality  What is the ideal osmolarity of 0.9% sodium chloride injection?  MORE EXAMPLES If U-100 Insulin contains 100 units of insulin in each milliliter of suspension, how many milliliters must be administered to give a patient 60 units?  Tetanus antitoxin is an intramuscular or subcutaneous injection, given in a single dose ranging between 1500-10,000 units, as prophylaxis against tetanus. How many milliliters of tetanus antitoxin (1500 units/ml) must be injected to give the patient 2500 units? How many mEq of calcium are present in a liter of a solution containing 150mg/dL of calcium?  A patient is to receive 10 mEq of potassium gluconate foru times a day for three days. The dose is a teaspoonful. What is the volume in ml, to be dispensed to provide the prescribed dosage regimen?