clarks formula, body surface area, dubois formula, infusion pump, Cockcroft gault formula

1. CALCULATION OF DOSAGE OF DRUGS USING APPROPRIATE
FORMULA FOR INDIVIDUAL PATIENT INCLUDING
CHILDREN,ELDERLY & PATIENT WITH RENAL DYSFUNCTION
Dr Zoya Khan
JUNIOR RESIDENT
Department of Pharmacology & Therapeutics

2. At the end of session, Student must be able to:
1)Enumerate general methods for drug calculations
2)Calculate dose for individual
3)Calculate dose in pediatric population
4)Dose adjustment in renal dysfunction

3. DOSE OF A DRUG
• Quantitative amount taken by patient
Expressed as:
 A single dose: Amount is to be taken 1 time
 A daily dose
 A total dose: the amount taken during time-course of therapy.
DOSAGE REGIMEN=Schedule of dosing.
Eg: Tab Augmentin 625mg TDS for 5 days

4. DOSE OF DRUG BASED ON:
 Biochemical & Pharmacological activity
 Physical & Chemical Properties
 Dosage form used
 Route of administration
 Various patient factors

5. DOSE FOR PARTICULAR PT DETERMINED ON
BASIS OF:
 Patients age
 Weight
 Body surface area
 General physical health,liver & kidney function
 Severity of illness

6. DOSE CALCULATION FOR AN INDIVIDUAL
• ON WEIGHT Basis[Clark’s formula]
Individual dose={Adult dose x Body weight[kg]}/70
• ON BODY SURFACE AREA(BSA) Basis:
Individual dose=[BSA(in m^2) x Adult dose]/1.7
for calculating BSA, either of these 2 formulas used:
1)Mostellers formula
2)Dubois formula

8. Calculation for Intravenous Fluids Infusion
Flow Rate:
Drop factor
• Macro drip=15/ml
• Micro drip=60/ml
• Example : NS 1L for 12 hrs the drop factor is 15 drops/ml. compute
the no of drops/min.
• Solution = 1000(ml) x 15 drops / min =20.8drops/min
12 x 60

9. DEVICES USED FOR CONTROLLING INFUSION
RATE OF MEDICINES:

11. DOSE CALCULATION IN CHILDREN:
• Pharmacokinetics & response to drugs differs in young because
metabolic & physiological processes are immature at birth.
ABSORPTION: Slow rate of gastric emptying;
Intestinal transit
DISTRIBUTION: Blood brain barrier remains immature at birth,
causing CNS toxicity.
Neonates have low body fat content & high total body water
So water soluble drugs have higher volume of distribution.

12. • METABOLISM: Enzyme systems are immature in neonatal liver,1st
pass
metabolism & hepatic drug clearances are low.
• ELIMINATION:-Renal function less developed in neonates & infants.
-Tubular secretion increases to adult value until age of
6-12 months
-Due to low GFR & immature tubular transport,half life of
drugs excreted by glomerular filtration(Gentamycin) &
Tubular secretion(Penicillin) are prolonged.

13. 1) FRIED’S RULE FOR INFANTS:
Dose for Infant=[AGE(month) x Adult dose]/150
2)FOR CHILDREN UPTO 2 YRS:
On weight basis(Clark’s formula):
Pediatric dose=[Adult dose x Body weight(in kg)]/70
3)FOR CHILDREN>2yrs :
YOUNG’S FORMULA=[Adult dose x Age in yrs]/[Age in yrs +12]
On Body surface area(BSA) basis:
Individual dose=[BSA(in m^2) x Adult dose]/[Age in yrs +12]

14. DOSE CALCULATIONS IN ELDERLY PATIENTS:
• METABOLISM:
-Size of liver & its blood flow decreases.
-Reduction in hepatic enzyme activity.
-Increase in oral bioavailability & reduction in Systemic Clearance, which
raises drug concentrations.
• RENAL ELIMINATION:- Creatinine Clearance reduces in elderly;
-COCKCROFT-GAULT formula used for dosage adjustments

15. DOSE CALCULATION IN RENAL DYSFUNCTION:
• KIDNEY=main organ of elimination of majority of drugs
-Elimination requires reduction in doses,related to Creatinine Clearance.
Calculated by COCKCROFT GAULT’S FORMULA:
In males :
In females

16. • DOSE ADJUSTMENT IN RENAL DYSFUNCTION:
-Loading dose doesnot change(except in edema)
-Maintenance dose depends on Clearance
Maintenance dose=Target plasma concentration x Clearance on iv
administration
DOSE IN RENAL DYSFUNCTION = NORMAL DOSE X CLrd
CL
CL=Normal Clearance
CLrd=Clearance in renal dysfunction
• Normal creatinine clearance is 120ml/min & 75% reduction shows renal
dysfunction requiring dose reduction.

17. • Reduced GFR(75% at 50 yrs & 50% at 75yrs)delays excretion of drug &
requires dose reduction.

18. RANGES OF NORMALAND DECREASED CREATININE
CLEARANCE (SI UNITS)
Category Value
Men (Normal) 95–145 mL/min (1.58–2.42 mL/s)
Women (Normal) 75–115 mL/min (1.25–1.92 mL/s)
Mild renal insufficiency* 50–70 mL/min (0.83–1.17 mL/s)
Moderate renal insufficiency* 25–50 mL/min (0.42–0.83 mL/s)
Severe renal insufficiency* < 25 mL/min (< 0.42 mL/s)

19. FORMATIVE ASSESSMENT
• Q1)A 3yr old female child was brought to pediatrics OPD by her
mother,child had fever,diarrhea,weakness.She was diagnosed as case of
enteric fever.You decide to prescribe syp cefixime along with other
medications.Calculate dose of cefixime?[Adult dose=200mgBD]
• Q2)A 75yrs old male suffering from gout was taking allopurinol for last
5yrs.On routine annual checkup his serum creatinine was reported to be
3mg/dl.His weight is 55kg.Calculate new dose for Allopurinol?[Adult
dose300mgBD]

20. THANK YOU