Two samples, one site — bone marrow aspiration and biopsy

Structured clinical encounter — procedural leadership and consent

This station tests whether the candidate leads a team through the right sampling decision, consents the family, handles the samples correctly, and manages the bleeding risk. Marks reward the two-samples-one-site principle, the correct site, the periosteal lidocaine dose, the first-pull rule, and the dry-tap interpretation.

Stem

A six-year-old is referred after a routine count shows a haemoglobin of 60, a platelet count of 20, and blasts on the peripheral film. The child is pale and tired but haemodynamically stable, with no active bleeding. The haematology team plans a bone marrow aspirate and trephine to confirm acute leukaemia and set the risk group. The team looks to you. [1] [4]

Candidate tasks

1. Consent and preparation (2 minutes). Explain the procedure, the risks (pain, bleeding, infection, dry tap) and the sedation plan. Confirm fasting status and monitoring, and check the count and clotting before the trephine. [1]
2. Direct the site and the two samples (3 minutes). Choose the posterior superior iliac spine, one to two centimetres below and lateral to the spine; explain why both the aspirate and the trephine are taken, and why the sternum is avoided. [1] [2]
3. Handle the first aspirate pull correctly (2 minutes). Reserve the first 0.5 to 2 mL for morphology and cytogenetics, because it is the only pull not diluted by peripheral blood and it drives the leukaemia risk group; send later pulls to flow, molecular and culture. [4]
4. Manage analgesia and sedation (2 minutes). Infiltrate lidocaine one per cent to the skin, subcutaneous tissue and periosteum at 3 mg per kilogram plain or 7 mg per kilogram with adrenaline; add procedural sedation with monitoring and a reversal agent. [7]
5. Interpret a dry tap and manage bleeding risk (3 minutes). If no marrow is aspirated, take a trephine — a dry tap is a sign, not a failure. Give platelet and clotting-factor cover before the trephine because it bleeds more than the aspirate; plan pressure, site observation, and the family safety-net. [10] [11]

Examiners' discussion points

• Why both samples? Because the aspirate answers questions about cells as individuals and the trephine answers questions about the marrow as a tissue; each fails where the other succeeds. [2]
• Defend the site. The posterior superior iliac spine is broad and flat and far from vital structures; the sternum is avoided in young children because the thin cortex admits the needle into the mediastinum. [1]
• The aspirate is dry. It points to fibrosis or a packed marrow; the trephine shows the cause and often delivers the diagnosis the aspirate could not. [10]
• The platelet count is 20. Give platelet cover before the trephine, which bleeds more than the aspirate; most centres transfuse to keep the count above 20 to 50. [11]

Marking grid (out of 20)