The dose of cephalexin using Dilling's formula for a 2 year old child when the adult dose is 500 mg TDS is 50 mg.

1. PH2.4
DEMONSTRATE THE CORRECT METHOD
OF CALCULATION OF DRUG DOSAGE IN
PATIENTS INCLUDING THOSE USED IN
SPECIAL SITUATIONS
Dr. Mani Bharti
S.R. Pharmacology
North DMC,Medical College,Delhi

2. Competency: PH 2.4
• Demonstrate the correct method of calculation of drug
dosage in patients including those used in special
situations
Domain
S
Level
S H
Core
(Y)
Suggested
Teaching
Learning method
DOAP sessions
Suggested
Assessment
method
Skills
assessment
Vertical
Integration
Pediatrics,
General
Medicine

3. Learning Objectives
At the end of the teaching learning session, the M.B.B.S Phase II
student shall be able to
• Know the general methods for drug dose calculation.
• Calculate the number of tablets/ quantities of solution or
drip rate for a given condition
• Calculate the doses for various drugs in pediatric
population using appropriate formulae.
• Calculate the doses for various drugs in special situations
(renal/hepatic/heart diseases/obesity) using appropriate
formulae.
• Calculate the loading & maintenance doses.

4. GENERAL METHODS FOR DRUG
CALCULATION
1. BASIC FORMULA
2. RATIO& PROPORTION
3. FRACTIONAL EQUATION
4. BODY WEIGHT
5. BODY SURFACE AREA

5. 1. Basic Formula
• A=D/H X V
• D-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 patient.

6. Examples:
• Avandia 2mg OD. How much tablet will you give?
A=D/HXV
• Solution: 2mg/4 x 1 tablet 4mg = .5 tablet

7. Example
• Paracetamol 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?
• D-500 mg, V- 5 ml,H-250mg
• A=D/H X V
A=500/250 X 5= 10 ml

8. 2. Ratio and Proportion
• H : V = D : X
• Where: H = drug on hand (available)
• V = vehicle/drug form (capsule/tablet/liquid)
• D = desired dose (as ordered)
• x = unknown amount to give

9. Example
• Order: Cefdinir (3rd gen cephalosporin) 100 mg PO q.i.d
*How many mL should the patient receive?
• H : V = D : X
• 250:5=100:X
• X=DV/H
• X=100X5/250
• X=2 ML

10. Fractional Equation
• • Same as R & P except it is written as a fraction.
• H/V = D/X
• Example
• Order:Ciprofloxacin 250 mg PO q12h*how many tablets
should the client receive?
• H/V = D/X
• 750/1=250/X
• 1/3 Tab

11. 4. Body Weight (BW)
• Allows individualization of the drug dose
• Involves 3 steps:
1. Convert pounds to kg – 1 Kg = 2.2 lbs
2. 2. Determine drug dose per BW – Drug dose x body
weight = patients dose per day
3. 3. Follow basic formula, R & P

12. Example
• Order: 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 = 60kg
• 2. mg x kg = patient’s dose: 12 x 60 = 720mg/day
• Answer: fluorouracil 720 mg/day

13. 5. Body Surface Area (BSA)
Most accurate to calculate drug dose for:
– Infants
– Children
– Older adults
– patients who are on antineoplastic agents
– low body weight
BSA is expressed as square meters (m2).
It can be determined using: -
Equations (e.g., Mosteller Method, Du Bois Method) –
Nomograms
- An average-sized adult has a BSA of 1.73 m2

14. Mosteller Equations:

15. Example

16. nomogram
• A nomogram has three columns:
• - Height (expressed in centimeters and in inches)
• - Body surface area (expressed in square meters)
• - Weight (expressed in kilograms and in pounds)
• - To use a nomogram, the height and weight of a patient
are found on the nomogram and then a straight line is
drawn connecting the two values.
• - The BSA for that patient is found where the line
intersects the BSA column.

17. Example
• Order:Cyclophosphamide (Cytoxan) 100 mg/m2/day,
• IV; available dosage is 200mg; patient’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

18. Nomogram for estimation of BSA

19. Estimation of BSA

20. IV Drip Rate Calculation:
Drop factor of tubing:
• Macrodrip = 10, 15, or 20 gtt/mL
• Microdrip = 60 gtt/mL
• The drop factor of the tubing is found on the tubing box.
For calculations, it must be stated in the problem.

21. Example
A patient is to receive 1 L of Hartmann’s solution over the
next 12 hours. What is the rate of infusion in drops per
minute (dpm), if the drop factor is 60 drops per mL .
Solution: Volume = 1 000 mL, since 1 L = 1 000 mL.
Time = 12 hours.
Drop Factor = 60 drops per mL.
Substituting these values into the drip rate formula gives:
Drip Rate (dpm) = 1 000ml/ 12 hr × 60 drops/ml/60 min/ h
= 83.833 ≈ 84 drops/minutes .

22. Drug dosing in special populations
• A special population are persons displaying one or more
of the following characteristics:
• a. prone to under or over responding to usual dosing
regimens
• b. least able to tolerate, recognize, or communicate drug
effects
• c. accidentally frequently mis‐dose

23. Patient conditions that may altered the
dosing of most drugs
• • Renal or hepatic disease,.
• • Dialysis procedures
• • Heart failure
• • Obesity
• • Age.

24. Renal Disease
Two important equatation are there for calculation of drug
dose in renal disease patients
Cockcroft-gault
MDRD(eGFR)
• The most common method of estimating glomerular
filtration for the purpose of drug dosing is to measure
/estimate Creatinine Clearance CrCl(ml/min)

25. MDRD equation
eGFR is estimated GFR calculated by the abbreviated MDRD equation :
186 x (Creatinine/88.4)-1.154 x (Age)-0.203 x (0.742 if female) x (1.210 if black).

27. Modifying Doses for patients with renal
impairment
• It is possible to decrease the drug dose
and retain the usual dosage interval, or
• Retain the usual dose and increase the
dosage interval, or
• Both decrease the dosage and prolong
the dosage interval
• The choice was made depend on the
route of drug administration, the dosage
forms available

28. St
Stageg GFR* Description
1 90+
Normal kidney function but urine findings or structural
abnormalities or genetic trait point to kidney disease
2 60-89
Mildly reduced kidney function, and other findings (as for
stage 1) point to kidney disease
3A
3B
45-59
30-44
Moderately reduced kidney function
4 15-29 Severely reduced kidney function
5
<15 or on
dialysis Very severe, or end-stage kidney failure

29. For drugs with narrow therapeutic index
• Measured or estimated CrCl may be used to
estimate pharmacokinetic parameters for a patient
based on prior studies conducted in other patients
with renal dysfunction
• Estimated pharmacokinetic parameters are then
used in pharmacokinetic dosing equation to
compute initial dose

30. Hepatic Disorders
• Most lipid soluble drugs are metabolized to some
degree by the liver
Two major types of liver disease
1. Hepatitis ‐ Acute hepatitis: mild, transient
decreases in drug metabolism required no or minor
changes in drug dosing ‐ Chronic hepatitis:
irreversible hepatocytes damage required drug
dosage changes. Patients with long term
hepatocytes damage can progress to hepatic
cirrhosis
2. Cirrhosis; a permanent lost of functional
hepatocytes. Drug dosage schedules usually need
to be modified

32. Heart Failure
• Is accompanied by a decrease in cardiac output
results in lower liver and renal blood flow
• Decreased drug bioavailability has been
reported, due to collection of edema fluid in the GI
tract difficult absorption and decreased blood flow
to GI tract

33. Obesity
• Estimating the optimal dose for obese patients
is difficult and, in many cases, ill defined.
• Basing maintenance doses on total body
weight is unlikely to result in a comparable
drug response across different body sizes and
generally increases the risk of adverse events.
Individualised
• dosing based on the patient’s lean body
weight is recommended, with accompanying
therapeutic drug monitoring and monitoring of
the patient’s clinical response
• .

34. Body size descriptors commonly used in
drug dosing
Name Formula
Total body weight (kg) –
Ideal body weight (kg)
13
45.4 + 0.89 x (height (cm) – 152.4) +
(4.5 if male)
Adjusted body weight (kg) Correction factor* x (TBW – IBW) +
IBW
Lean body weight (kg)
males 9270 x TBW (kg)6680+216 x BMI (kg/
m2)
Females 9270 x TBW (kg)6680+244 x BMI (kg/
m2)

35. Age
Children are not small adults but rather distinct
individuals who have different absorption,
distribution, metabolism, and excretion rates of
medications than adults.

36. Pediatric considerations
• Fried’s rule –
• applies to the child younger than 1 year of age.
• Infant dose=
• infant age (months) X adult average dose/ 150

37. Pediatric considerations
• Young’s Rule-
• applies to children 1-12 yr.

38. Clark’s Rule
• One of the most popular methods for determining
medication dosages for children.
• Based upon a child’s body weight and the assumption that
the average normal adult weighs 150 pounds.
• Ratio proportions are used in this method.
• Clark’s rule should be used if no other formula is
specified.

40. Loading doses
Loading doses are useful for drugs that are eliminated from the
body relatively slowly
• Such drugs need only a low maintenance dose in order to
keep the
amount of the drug in the body at the target level
• Without loading, it would take longer for the amount of the
drug to
reach target level
• Loading doses typically are adjusted based on Vd and are
not adjusted
for renal failure
•If extracellular volume depletion is present, Vd may be
reduced and
reductions in loading dose should occur

41. Three variables are used to calculate the loading dose:
• Cp = desired peak concentration of drug
• Vd = volume of distribution of drug in body
• F = bioavailability
•The required loading dose may then be calculated as:
Loading dose = Cp x Vd x Ideal body weight/F
•For an intravenously administered drug, the bioavailability F
will equal 1, since the drug is directly introduced to the
bloodstream.
If the patient requires an oral dose, bioavailability will be less
than 1 (depending upon absorption, first-pass metabolism,
etc.),
requiring a larger loading dose

42. Drug Dosing–Maintenance Dose
• Maintenance doses ensure steady-state blood
concentrations and lessen the likelihood of sub-therapeutic
regimens or overdoses
•In the absence of a loading dose, maintenance doses will
achieve 90% of their steady-state level in 3–4½ lives
•Two options for ESRD
• Reduce the dose
• Lengthen the interval between doses (more useful for a
drug with a wide therapeutic range and long half-life)
• Maintenance dose can be calculated the same way as
loading dose

43. Conversions:
• 1 liter (L) = 1000 ml (milliters)
• 1 gram (g) = 1000 mgs (milligrams)
• 1 mg (milligrams) = 1000 mcgs (micrograms)
• 1 gram (g) = 15 grain (gr)
• 1 grain (gr) = 60 mg (milligrams)
• 1 dram (dr) =4 ml (milliters)
• 1 ounce (oz) = 30 ml (milliters)
• 1 tsp (teaspoon) = 5 ml (milliters)
• 1 tbs (tablespoon) = 15 ml (milliters)
• 1 kg (kilogram) = 2.2 lbs (pounds)
• 1 inch = 2.54 cm (centimeters)
• 16 ounces (ozs) = 1 lb (pound)
• 1 cup = 8 ounces (ozs)
1 ml (milliter) = 15 minims
• 1 tsp (teaspoon) = 5 ml (milliters)
• 1 tbs (tablespoon) = 15 ml (milliters)
• 1 drop (gtt) = 15 minims

44. Q. The normal adult doses of medication
is 150 mg you have a child that weighs
30 kg and is 1.2 meter in height. How
much medication will you give to the
child?

45. Answer
• Calculate BSA= 30kg x120cm/3600 =1 , now child
dosage= BSA x ADULT DOSE / 1.7M2 = 88.24mg

46. Q. A pediatric patient is to receive
Atropine 0.02 mg/kg,the patient weight
is 10 kg,what is the Desired Dose?

47. • Desired Dose = .02mg/kg*10kg=.2mg
Answer

48. Q. Calculate the dose of Amoxicillin for 8 yr old girl using
young’s formula ,when the adult dose of capsule
Amoxicillin is 250 mg TDS ?

49. • Youngs Formula = (Age/Age+12 )*Adult Dose so
(8/8+12)*250 = 100mg
Answer

50. Q. Calculate the dose of Amoxicillin for 8 yr old girl using
young’s formula ,when the adult dose of capsule
Amoxicillin is 250 mg TDS ?

51. • Youngs Formula = (Age/Age+12 )*Adult Dose so
(8/8+12)*250 = 100mg
Answer

52. Q. Find out the dose of cephalexin
using Dilling’s formula for a 2 yr
old child when the adult dose is
500 mg TDS?

53. • Dillings formula ={( Age / 20) *Adult Dose } , So 2/20 *500
= 50 mg
Answer

54. •Find out the dose of a drug X for a
3 yr old child by Dilling formula as
well as young formula when adult
dose of that drug is 650 mg TDS.

55. • Dilling formula = Age/20 *Adult dose , So here Dose =
4/20*500mg=100mg
Answer

56. • Calculate the daily dose of Amikacin for 75 years old
female obese patient weight 90 kg having S. creatinine –
2.5 mg/dl suffering from renal parenchyma disease.

57. • Creatinine clearance in renal insufficiency = ( 140 - Age ) *
Weight ( kg ) multipy 0.85 ( If females) /72 * S.
Creatinine ( mg / ml )
• 140-75 *90*0.85*100/72*2.5 =2.763gm.

58. •Thank you