Good Prescribing

Question 1

Renal function

Answer 1

Cockcroft-Gault equation
Creatinine clearance = [1.23(140-age) x weight] / serum creatinine
(1.04 in females)

eGFR
>90 normal
60-89 mild impairment
30-59 moderate impairment 
15-29 severe impairment

Question 2

Renal impairment - prescribing precautions

Answer 2

Short acting agents (e.g. Tolbutamide as a choice sulphonylurea)

Increase dosing interval (e.g. Gentamicin)

Choose non renally excreted alternatives
(E.g. Amlodipine in HTN, Gliclazide in t2DM)

Some drugs avoided e.g. Metformin

Some drugs become ineffective as require renal excretion to act (e.g. Thiazides diuretics)

In haemodialysis and CAPD (continuous ambulatory peritoneal) seek specialist advice.

Question 3

Prescribing in pregnancy, pharmacokinetic changes and considerations

Answer 3

Up volume of distribution
Down plasma concentration
Up renal excretion
Up hepatic metabolism

EVERY drug crosses the placenta
EXCEPT heparin, large molecular weight and polarity, does not cross.

Assume every woman of child bearing she is pregnant until proven otherwise
Drug treatment- only when advantages > disadvantages

Question 4

Drugs that cause issues in pregnancy

Answer 4

Anti epileptics
Anticoagulants
Antibiotics
Anti-HTN- labetalol, nifedipine, methyldopa
Anti-diabetics- insulin, metformin, glibenclamide
Antidepressants

Question 5

Drugs in pregnancy

Answer 5

Most pregnancies unexpected
Even “planned” are not detected until 8-10 weeks gestation
Many take vitamin supplements and OTCs

Question 6

Specific pregnancy drug complications

Answer 6

Folic acid taken to avoid NTDs/ spinal bifida/ hydranencephaly/ anencephaly (closed/ open cranium)

Thalidomide - critical exposure window (24-36 days post fertilisation)

Phenytoin

• cranial abnormalities
• hypoplasia of distal phalanges
• growth deficiencies
• mental deficient

Valproate
- associated with NTDs

Carbamazepine
- similar to phenytoin but decreased risk

Cleft lip/palates
- phenolbarbital (anticonvulsant)/ isotretinoin (retinoid)/ methotrexate/ valproic acid

Question 7

Pregnancy anti epileptics

Answer 7

Continued treatment preferable- counselling
Planned discontinuation an option
Carbamazepine WAS preferred WITH 5mg folic acid

NICE 2004
Lamotrigine first line in generalised tonic clonic seizures to avoid teratogenic / interacting drugs

Question 8

OCP Interactions

Answer 8

Inducing agents

Phenytoin/ carbamazepine / phenobarbital/ Rifampicin

Favour non inducing agents or use alternative contraceptive methods

Question 9

Anticoagulants in pregnancy

Answer 9

Pregnancy produces a thrombophilic state

Prevents post partum haemorrhage

Mothers with artificial heart valves

Warfarin- teratogenic

• chondroplasia punctata (altered bone growth)
• optic atrophy
• mental retardation

Avoid warfarin in trimester 1&3
Favour LMWH

Question 10

Prescribing in hepatic impairment

Answer 10

LFTs a poor guide
Considerations:
Hepatic clearance
Protein binding
Sodium retention
Effects on coagulation (maybe increased INR)
Gastric effects
CNS effects
Sedation

Question 11

WHO Good prescribing steps

Answer 11

1. Define patients problem
2. Specify therapeutic objective
3. Verify suitability of treatment (safety and effectiveness)
4. Start the treatment
5. Give info, instructions and warnings
6. Monitor (and stop?) treatment