Calculations

Question 1

Pharmacokinetic calculations
Henderson-Hasselbach equation

Answer 1

Acid: PH= PKa + log(unionised/ionised)
Bases: PH= PKa + log(ionised/unionised)

Question 2

Pharmacokinetic calculations
Absolute bioavailability

Answer 2

F= Dose/ AUC (iv) x AUC (oral)/dose

Question 3

Pharmacokinetic calculations
Rate of absorption, Ka

Answer 3

Ka= ln(2) / T1/2

Question 4

Pharmacokinetic calculations
Half-life

Answer 4

T1/2= 0.693/K

Question 5

Pharmacokinetic calculations
Volume of distribution, Vd

Answer 5

Vd= amount of drug administered/ blood plasma concentration

Units of Vd= L or L/Kg

Question 6

Pharmacokinetic calculations
Loading dose, LD

Answer 6

LD= Vd x Desired plasma concentration/ S x F

S= salt factor, 1
F= Bioavailability

Question 7

Pharmacokinetic calculations
Total clearance, CL

Answer 7

CL (systemic)= CL (renal) + CL (hepatic) + CL (other)

CL= Vd x 0.693/T1/2

CL= ER x Q
ER= Extraction ratio, Q= organ blood flow

Question 8

Pharmacokinetic calculations
Extraction ratio, ER

Answer 8

ER= eliminated drug/ eliminated drug + drug which escaped elimination

Question 9

WBE calculations
Load

Answer 9

Load= concentration x flow x (100/100+ stability) x (100/100 - sorption)

sorption and stability as a %

Question 10

WBE calculations
Consumption

Answer 10

Consumption= Load x (1/Excretion) x (MWpar/MWDTR) x (1000/ population) - OS

Excretion not as a % write as a decimal

Question 11

Alcohol calculations
Conversions between breath, blood and urine alcohol concentrations

Answer 11

Breath –> Blood –> Urine
x2.3 x1.3375

Question 12

Alcohol calculations
Peak alcohol concentration, C

Answer 12

C = a x 100 / p x r

C= mg/100ml
a= mass of alcohol, g
p= weight, Kg
r= gender widmark factor (men=0.68, women= 0.56)

Question 13

Alcohol calculations
Calculating Mls and g of alcohol

Answer 13

ml of drink x (ABV% / 100)= Mls of alcohol
Mls of alcohol x 0.789= g of alcohol

Question 14

Alcohol calculations
Elimination

Answer 14

Low range: 9 x time between incident and sample taken
Medium range: 19 x time between incident and sample taken
High range: 29 x time between incident and sample taken

Question 15

Alcohol calculations
Range of BAC

Answer 15

Low range: Peak alcohol concentration - low elimination
Medium range: Peak alcohol concentration - Medium elimination
High range: Peak alcohol concentration - High elimination

Question 16

Alcohol Concentration
Back calculation

Answer 16

Low range: blood sample result + low elimination value
Medium range: blood sample result + medium elimination value
High range: blood sample result + high elimination value