Calculations

Question 1

How do you convert stone to kg?

Answer 1

pounds x 6.35 (14 pounds in a stone)

Question 2

How do you convert inches to cm?

Answer 2

inches x 2.54 (12 inches in a foot)

Question 3

Where do you find amount of creams needed (eg both hands for 1 week)

Answer 3

Suitable quantities for prescribing - skin conditions management

Question 4

Where do you find info on COCP?

Answer 4

Combined hormonal contraception

Question 5

What is bioavailability?

Answer 5

Fraction of dose reaching general circulation as active form

Available dose = administered dose x f

Question 6

What is the volume of distribution?

Answer 6

Volume which a drug is distributed in body
5 litres if confined to plasma
10-15L extracellular water
40L total body water
Fixed characteristic of drug, L or L/kg

Question 7

Can you work out dose needed for a desired plasma concentration?

Answer 7

Yes if the vol of distribution (Vd) is known

Plasma conc = dose/Vd

Question 8

What is elimination?

Answer 8

Metabolism + excretion

Question 9

What is zero order elimination?

Answer 9

Fixed rate, linear graph

eg 1 unit alcohol eliminated in 1hr

Question 10

What is 1st order elimination?

Answer 10

Exponential elimination curve
Due to fixed clearance rate through kidney, dependent on flow and conc
Higher drug conc, faster elimination rate

Question 11

What is clearance?

Answer 11

Vol of blood from which the drug is cleared per unit time (ml/min)
NOT amount of drug

Question 12

How can rate of elimination be calculated from clearance?

Answer 12

Rate of elimination (mg/h) = clearance (L/h) x plasma conc (mg/L)
Cancel out the L

Question 13

How can you calculate rate of drug infusion needed?

Answer 13

For steady state:
Rate of drug infusion (mg/h)= Rate of drug elimination (mg/h)
= Clearance (L/h) x steady state plasma conc (mg/L)

Question 14

Is the calculation of maintenance oral doses different from infusions?

Answer 14

Final answer the same (rate of dosing = rate of elimination = clearance x steady state conc)
But equation different due to intermittent dosing
(Oral dose x bioavailability)/time interval between doses = clearance x steady state conc

Question 15

How long does it take to reach the steady state plasma concentration desired?

Answer 15

3-5 x half life

Question 16

How do you work out loading doses?

Answer 16

Vol of distribution x steady state plasma conc

L x mg/L