Diagnostic Imaging and Laboratory Diagnostics
This domain tests test selection rather than image reading: which modality answers the clinical question with the least harm, and how to avoid the errors that occur before the sample or image ever reaches the specialist.
Core rules
- Justify every exposure (ALARA). Ultrasound and magnetic resonance imaging are preferred in pregnancy and in children where they answer the question.
- Chest radiography is the first-line study for suspected pneumonia and for free air (erect film); computed tomography is more sensitive but is not the initial test.
- Contrast safety: check creatinine and estimated glomerular filtration rate, ask about previous reactions and metformin, and hydrate high-risk patients.
- Focused assessment with sonography for trauma is the bedside test for free fluid in an unstable trauma patient; computed tomography is for the stable patient.
- Computed tomography without contrast excludes haemorrhage in acute stroke before thrombolysis.
- Laboratory: most errors are pre-analytical - prolonged tourniquet time, haemolysis, wrong tube, non-fasting sample. Correct potassium for haemolysis before acting.
- Test performance: sensitivity, specificity and predictive values; positive predictive value falls sharply when prevalence is low.
Worked example
A 70-year-old with pleuritic chest pain and tachycardia has an eGFR of 35. Reasoning: pulmonary embolism is likely and computed tomographic pulmonary angiography is the usual test, but the low eGFR raises contrast-nephropathy risk; the alternatives are ventilation–perfusion scanning or a documented risk–benefit decision with hydration. The answer credits recognition of the contraindication, not blind ordering.
Traps
- Ordering computed tomography in a pregnant patient with right lower quadrant pain instead of ultrasound.
- Interpreting a positive test without considering prevalence.
- Accepting a haemolysed potassium result as true hyperkalaemia.
- Forgetting to ask about previous contrast reactions.