Bone-marrow aspiration and biopsy principles — formative SAQs

SAQ 1 — Aspirate and trephine at the posterior superior iliac spine (15 marks, 15 minutes)

A six-year-old is referred with a haemoglobin of sixty, a platelet count of twenty, and blasts on the peripheral film. A bone marrow aspirate and trephine are planned to confirm acute leukaemia and set the risk group. ^{[1] [4]}

Question. Describe the standard paediatric site and why it is chosen, the two samples taken and the question each answers, the handling of the first aspirate pull, and the local anaesthetic regimen to the periosteum. Explain your reasoning.

Model answer

Site (3 marks). The standard site is the posterior superior iliac spine, marked one to two centimetres below and lateral to the spine on the broadest, flattest part of the bone. It is chosen because the bone is broad and flat and the point is far from any vital structure. The sternum is avoided in young children because the thin cortex admits the needle into the mediastinum. ^{[1] [2]}

Two samples (4 marks). The aspirate is liquid marrow drawn from the medullary sinusoids; it answers questions about cells as individuals — morphology, flow cytometry, cytogenetics, molecular testing and culture. The trephine is a core of bone preserving the architecture; it is the only sample that shows overall cellularity, fibrosis, granulomas and the pattern of infiltration. Both are taken at the same sitting so the child is not brought back twice. ^[2]

First pull (3 marks). The first 0.5 to 2 mL aspirate is reserved for morphology and cytogenetics because it is the only pull not diluted by peripheral blood. A paediatric ALL study showed aspiration technique changes the measured blast percentage and may shift therapy stratification, so later pulls go to flow, molecular and culture. ^[4]

Local anaesthetic (3 marks). Infiltrate lidocaine one per cent into the skin, subcutaneous tissue and the periosteum (the painful layer), at a maximum of 3 mg per kilogram plain or 7 mg per kilogram with adrenaline. Wait for onset before advancing the needle. Add procedural sedation (ketamine with midazolam, or an alfentanil–remifentanil–midazolam regimen) with monitoring and a reversal agent.

Reasoning (2 marks). The aspirate is taken before the trephine because the trephine disrupts the marrow and dilutes the aspirate; the trephine is taken from the same skin puncture but a slightly different track. The combination secures the diagnosis and the risk group in one procedure and respects the child's pain and anxiety. ^{[1] [2]}