Diagnostic test selection and Bayesian reasoning in paediatrics — formative SAQs

SAQ 1 — Metrics and residual risk (10 marks)

A colleague says a blood test with high sensitivity “rules out sepsis” in a mottled 12-day-old because the result is negative. [2] [7]

Questions

1. Define sensitivity and negative predictive value, and state which depends on prevalence. (3 marks) [1]
2. Using Bayesian language, explain why a negative result may not make discharge safe in this neonate. (4 marks) [2] [3] [7]
3. State the residual-risk sentence you would use with the team and family. (3 marks) [4] [9]

Model answer

Definitions (3). Sensitivity is the proportion of diseased children who test positive (true-positive rate). NPV is the proportion of negative tests that are true negatives. NPV depends on prevalence/pre-test probability; sensitivity does not in the basic formula. [1]

Why not safe discharge (4). Pre-test probability in a mottled neonate is high. A negative result multiplies pre-test odds by LR−; if LR− is not extremely small, post-test probability can remain above a safe observation or discharge threshold. Early markers can also be falsely reassuring while disease evolves. Residual must-not-miss risk therefore continues to drive treatment, observation or escalation. [2] [3] [7] [9]

Residual-risk sentence (3). “Before the test I was highly concerned for serious infection. The negative result lowers concern somewhat but does not make the risk low enough for home. We will treat/observe for residual sepsis risk, reassess trajectory, and explain return triggers.” [4] [9]

SAQ 2 — Choosing and avoiding tests (10 marks)

An 8-month-old has typical bronchiolitis and is stable. Another registrar wants a “full septic screen and chest radiograph just in case.” Separately, a bag urine culture from a different toddler is mixed growth. [5] [8]

Questions

1. Justify selective testing (or no routine radiograph) in typical stable bronchiolitis. (3 marks) [5] [6] [10]
2. Name two harms of protocol-stacked over-testing in this setting. (3 marks) [10] [9]
3. How should the mixed bag-urine culture be interpreted before treatment momentum? (4 marks) [8] [1]

Model answer

Bronchiolitis selectivity (3). When history and examination fit bronchiolitis and the child is stable, routine chest radiography often fails to improve care and can increase unnecessary interventions. Reopen imaging if the clinical question changes (focal signs, severe course, alternative scripts). “Just in case” is not a Bayesian indication. [5] [6] [10]

Harms (3). False positives, cascade procedures, antibiotics or admission without benefit, radiation/sample burden, family anxiety, and distraction from residual clinical risk that actually needs watching. [10] [9]

Bag urine (4). Collection method alters contamination rates. Mixed growth from a bag sample often reflects contamination rather than definite UTI. Reassess pre-test likelihood, urinalysis context and symptoms; consider a better collection method before treating the culture as disease. Do not let a contaminated positive create false diagnostic momentum. [8] [1] [9]