PLAB 1 Medical Ethics, Consent & Evidence-Based Medicine 2

Question 1

A clinical trial reports that a new drug reduces the relative risk of myocardial infarction by 50% compared to placebo. The absolute event rates are 2% (treatment) and 4% (control). What is the Number Needed to Treat (NNT)?

Accepted Answer

50

Answer

NNT = 1 / Absolute Risk Reduction (ARR). ARR = 4% - 2% = 2% = 0.02. NNT = 1/0.02 = 50. The NNT of 50 means you need to treat 50 patients to prevent one MI. While the relative risk reduction (50%) sounds impressive, the NNT provides more clinically meaningful information.

Question 2

What is a 'double-blind randomised controlled trial' (RCT) and why is it considered the gold standard for establishing therapeutic efficacy?

Accepted Answer

A trial where neither participants nor investigators know which group receives treatment or placebo, eliminating allocation bias and expectation bias — the best evidence for causality

Answer

A double-blind RCT: random allocation prevents selection bias and balances confounders; blinding participants prevents placebo effect; blinding investigators prevents observer bias. The combination of randomisation and blinding provides the most robust evidence for causality in therapeutic research.

Question 3

What is 'sensitivity' of a diagnostic test?

Accepted Answer

The proportion of people with the disease who test positive (true positive rate) — a sensitive test rarely misses disease

Answer

Sensitivity = True Positives / (True Positives + False Negatives). A highly sensitive test has a low false-negative rate — it rarely misses cases of the disease. Mnemonic: SnNout — a Sensitive test, when Negative, rules OUT disease. Useful for screening tests.

Question 4

What is 'specificity' of a diagnostic test?

Accepted Answer

The proportion of disease-free individuals who test negative (true negative rate) — a specific test rarely gives false positives

Answer

Specificity = True Negatives / (True Negatives + False Positives). A highly specific test rarely gives a positive result in people without disease. Mnemonic: SpPin — a Specific test, when Positive, rules IN disease. Useful for confirmatory tests.

Question 5

What does 'intention-to-treat' (ITT) analysis mean in clinical trials?

Accepted Answer

Analysis that includes all participants as originally allocated, regardless of whether they completed or received the allocated treatment — preserving the integrity of randomisation

Answer

ITT analysis maintains all participants in the groups to which they were originally randomised, regardless of non-compliance, withdrawal, or crossover. This preserves the prognostic balance of randomisation and gives a real-world estimate of treatment effect. Per-protocol analysis (completers only) may overestimate treatment benefit.

PLAB 1 Practice Test

PLAB 1 Practice Test

PLAB 1 Medical Ethics, Consent & Evidence-Based Medicine 2