SU9.1 | Selection and Interpretation of Surgical Investigations — Summary & Reflection

KEY TAKEAWAYS

Selecting and interpreting investigations is a reasoning skill, not a reflex. Start from a specific clinical question and an estimate of pretest probability, and order a test only if its result could change management — both over- and under-investigation harm. Test behaviour is described by sensitivity (a negative rules out, SnNOut), specificity (a positive rules in, SpPIn) and the predictive values PPV and NPV, which shift with pretest probability/prevalence — the reason a marker performs well in a high-risk patient and poorly as a screen. Choose by modality — biochemical (including tumour markers CEA, CA 19-9, AFP, PSA, beta-hCG, CA-125), microbiological, pathological (cytology/FNAC versus definitive histopathology and frozen section) and imaging (USG, X-ray, CT, MRI, PET-CT, endoscopy) — and by purpose (diagnostic, staging T/N/M, or baseline/fitness). Tumour markers are for monitoring and supporting diagnosis, never for general-population screening. Finally, interpret in context: weigh the result against pretest probability, prefer trends to single values, suspect false results that contradict the clinical picture, and obtain tissue when a diagnosis must be definitive.

REFLECT

Think back to the last patient whose investigations you saw ordered — on a ward, in clinic or in a simulated case. For each test, could you now state the clinical question it was meant to answer, and whether it actually changed the plan? Were any tests ordered 'to be safe' that, on reflection, could not have altered management? When you next receive a surprising result — a mildly raised marker, an unexpected positive in a low-risk patient — notice your instinct: do you act on the number, or do you pause to weigh it against the pretest probability and the clinical picture? Choose one habit to build now: always naming the clinical question before ordering a test, or always reading a tumour marker as a trend in context rather than a single screening number, so that deliberate, question-led investigation becomes second nature before you carry it to real surgical patients.