Part 3: Metrology & Pharmaceutical Calculations References: 1- Remington 2006, Chapter 11, Meterology & Pharmaceutical Calculations. 2- Ansel H.C., Mitchell J.S., 11th Edt., Pharmaceutical Calculations, 2001

Roman numbers ss = 1/2 I or i = 1 II or ii = 2 V or v = 5 X or x = 10 L or l = 50 C or c = 100 D or d = 500 M or m = 1000 LX = 60 XL = 40

2- Avoirdupois system The basic unit of weight in the avoirdupois system is the grain (gr). The larger units are the ounce (oz) and the pound (lb).

WEIGHT: AVOIRDUPOIS SYSTEM a. You must know the following relationships between the avoirdupois units: 1 lb = 16 oz =7000 gr b. To convert from the avoirdupois system to the metric system, you must know these conversion factors: 1 kg = 2.2 lb 1 lb = 454 g

3- Apothecary system Apothecary weight units 1 pound (lb) = 12 oz 1 pound (lb) = 5760 grain = 373 g 1 oz Apot = 31.1 g The units of volume in the apothecary system are Minim, fluid dram (fζ), fluid ounce (fξ), pint (pt), quart (qt), and gallon (gal).

VOLUME: APOTHECARY SYSTEM Know the following relationships between apothecary units: 1 gal = 4 qt 1 qt = 2 pt 1 pt = 128 fζ (fl dram) 1 fl dram = 60 minim To convert from the apothecary system to the metric system, you must know the following factors: 1 gal = 3785 ml

Length Uinits Inch Foot 1ft = 12 inch =30 cm 1ft = 0.30480m

Length units 1 meter 39.37 inch 1 inch 2.54 cm 1 foot 0.305 m

Different Kinds of Balances Single-beam equal arm balance Unequal arm balances Preferred in laboratory works when large amounts are to be weighed Electronic Balances Single pan balances, much more sensitive than the traditional ones, easier & quicker to use, but usually more expensive.

Single-beam equal arm balance

Different Kinds of Balances Single-beam equal arm balance Unequal arm balances Preferred in laboratory works when large amounts are to be weighed Electronic Balances Single pan balances, much more sensitive than the traditional ones, easier & quicker to use, but usually more expensive.

Unequal arm balances

Prescription Balance

Electronic Balances

NIST (National Institute of Standards and Technology) Weight & Measure Division: Discuss about the specifications and standards used primarily in weights and measures and commercial applications.

Sensitivity (readability) of a balance: Minimum weight that causes the indicator of balance change one unit. If addition of 6mg weight to balance causes one unit movement of indicator , sensitivity of balance is 6mg. Class 3 balances: Minimum sensitivity 6mg up to 60g. Electronic divices have usually a sensitivity less than 10mg.

Accuracy of a balance if a known calibration standard weight of 100.000 grams was placed on a balance and the display shows 100.002 grams we could say the accuracy of the balance is 0.002 grams or 2 milligrams.  Accuracy tells how close a balance gets to the real value. 

Different terms related to balances Capacity (maximum weight can be weight by balance) Readability minimum weight can be weight by the balance Repeatability ( variation in different measurements of one object) Are usually mentioned by the manufacturer for electronic devices. E.g. Capacity 60g, readability 6mg, repeatability ±1mg

Protection & Maintenance of a Balance Position chosen for balance should be flat. Should be far from dampness, dust & corrosive vapor Analytical balances are kept in glass cases. Some balances can be locked & protect from vibration when they are not in use. Should be cleaned just by a clean clothe. Should be checked & calibrated periodically.

Protection & Maintenance of a Balance For compounding, accurate sets of weights from 10mg to 10g should be kept precisely in a closed box. For analytical purposes, weights from 1mg to 100g should be available. The weight should not be handled by finger.

Torsion Prescription Balance

Checking a balance periodically Calibration: Weighing a standard mass on balance to see how much it is different from the real weight. It tells us accuracy (how much the balance work accurate).

Periodically checking of a prescription balance Sensitivity requirement Arm Ratio test Shift test

Minimum Weighable Quantity (MWQ) The USP allows a maximum error of 5% in a single weighing operation The percent of the error depend on the amount of drug weighed & sensitivity of balance according to the formula below: The percent of error will increase as the amount of drug decreases

Examples

Percent of Error

Aliquot Method for weighing & measuring A torsion prescription method has a sensitivity requirement of 6 milligram. Explain how you would weight 4 milligrams of atropine sulfate with an accuracy of ± 5%, using lactose as the diluent?

Measuring (for liquids) In pharmacy, measuring usually refers to the exact determination of a definite volume of liquid. Reading should be made at the bottom of this meniscus.

Measuring (for liquids) Glassware are preferred. Metal devices some times are used but should be preferably stainless steel or copper not iron. If there are materials susceptible to oxidation in the formulation. Copper should not be used. There are two types of Graduated glassware for measuring: Conical (less accurate) Cylindrical (more accurate) For volumes less than 1.5 mL, a graduated pipette, burette or dropper should be used.

Volume of drop can be differ by many factors, density, temperature, viscosity, surface tension, size & shape of orifice. When drugs are potent, a calibrated dropper should be used.

Unmarked portion of graduated glassware To improve accuracy, the lower portion of graduates should not be used.

Alligation Alternate Method

Potency of a Drug The Potency of some antibiotics, hormones, vitamins & Biologics (Vaccines) is based on their activity & expressed in terms of Units or Micrograms per milliliters (µg/mL). Insulin & Penicillin antibiotics These preparation should obey USP standard reference, Penicillin G 1590 Units/mg are USP reference standard 1500 U/mg < Penicillin G <1750 U/mg is acceptable

محاسبه دوز دارو در کودکان: روش مساحت سطح بدن (Normogram method) روش یانگ روش کلارک روش فراید

روش مساحت سطح بدن (Normogram method)

روش مساحت سطح بدن (Normogram method) The nomogram method is the best method: because it is formulated on the patient's size. It takes into consideration the person's body surface area in meters square. 1.73 M2 is the surface area of the average adult (150-154 lbs.).

روش مساحت سطح بدن (Normogram method) Child's surface area in m2 ----------------------------------- × Adult dose = Child's dose 1.73 m2

فرمول محاسبه مساحت سطح بدن BSA BSA= W0.425 x H0.725 x 71.84 W; kg H; cm BSA; Square Centimeter

Fried's Rule Child's age in months × Adult dose Child's dose = ------------------------------------------------- 150

c. Young's Rule: Child's age in years × Adult dose Child's dose = --------------------------------------------- (Child's age in years + 12) d. Clark's Rule: Child's weight in lbs × Adult dose Child's dose = ------------------------------------------ 150

Examples

Answers

Aliquot Method for weighing & measuring A torsion prescription method has a sensitivity requirement of 6 milligram. Explain how you would weight 4 milligrams of atropine sulfate with an accuracy of ± 5%, using lactose as the diluent?

Aliquot Method for weighing & measuring A prescription calls for 0.2 mL of olive oil, using a 5 mL graduate calibrated in units of 0.5 mL, how would you obtain the required amount of olive oil using the aliquot method & alcohol, as the diluent?