Diagnostic test selection and Bayesian reasoning in paediatrics — branching viva

Station map

Branch A — High-risk neonate, reassuring marker

Examiner: A 14-day-old is poorly feeding and mottled. CRP is normal. The night team wants discharge because “the bloods are fine.” ^{[7] [9]}

Strong answer should include:

• Pre-test risk is high from age and physiology. ^{[7] [9]}
• Sensitivity/NPV language: a normal marker is not automatic rule-out. ^{[1] [2]}
• Post-test probability may remain above treatment or observation thresholds. ^{[2] [4]}
• Residual-risk plan: treat/observe, senior review, timed reassessment, family explanation. ^{[4] [9]}

Trap: equating one normal number with zero disease probability. ^{[2] [9]}

Branch B — Stable bronchiolitis and “just in case” imaging

Examiner: Classic bronchiolitis, stable. Registrar requests chest radiograph and full panel. ^{[5] [8]}

Strong answer should include:

• Clinical question already answered by history/examination for typical disease. ^[5]
• Routine radiograph often low value and can drive unnecessary treatment. ^{[5] [8]}
• Reopen testing if the question changes (focal signs, severe course, mismatch). ^[5]
• Stewardship without denying needed tests for true residual risk. ^[8]

Trap: protocol stacking as a substitute for Bayesian indication. ^{[5] [8]}

Branch C — Incidental adult-range laboratory flag

Examiner: Asymptomatic school-age child; adult reference interval flags a value high. ^[6]

Strong answer should include:

• Reinterpret with age-appropriate paediatric intervals. ^[6]
• Avoid cascade testing for a software-labelled “abnormal” that fits childhood physiology. ^{[6] [8]}
• Only investigate further if independent clinical concern exists. ^{[8] [9]}

Trap: treating laboratory flags as diagnoses. ^{[6] [8]}

Branch D — Appraisal probe

Examiner: A paper claims 99% sensitivity from ICU cases versus healthy controls. Can you use that number tonight? ^[10]

Strong answer should include:

• Spectrum bias and applicability concerns. ^[10]
• Need for STARD/QUADAS-style appraisal of selection, reference standard and flow. ^[10]
• Bedside use requires match to your patient spectrum and decision thresholds. ^{[2] [3]}