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]

SAQ 2 — Dry tap and bleeding risk in the thrombocytopenic child (12 marks, 12 minutes)

A four-year-old with suspected severe aplastic anaemia has a platelet count of 12 and an INR of 2.1. During the aspirate, no marrow can be withdrawn despite a correctly placed needle. [1] [11]

Question. Interpret the dry tap, state the bleeding-risk management before the trephine, and describe the complications to prevent and how.

Model answer

Dry tap (3 marks). A dry tap is not a failed procedure; it is a sign of marrow fibrosis, a marrow packed with cells, or in this case severe marrow failure. The correct response is a trephine biopsy. The trephine cellularity under 25 per cent defines severe aplastic anaemia; the aspirate alone is misleading because surviving pockets of marrow can look normal. [1] [10]

Bleeding risk (4 marks). The trephine core bleeds more than the aspirate, and the thrombocytopenic, coagulopathic child needs blood-product cover first. Give a platelet transfusion to lift the count above 20 to 50, and above 50 when the child is also coagulopathic, and correct the clotting before the trephine. Have blood and a reversal agent to hand. A CT-guided biopsy series confirms the procedure is safe when the count and clotting are corrected. [11]

Complications and prevention (3 marks). Name and prevent: pain (local anaesthetic to the periosteum plus sedation); bleeding (platelet and clotting-factor cover, firm pressure, site observation); infection — cellulitis, osteomyelitis, septic arthritis — (sterile technique, risk rises with neutropenia and lines); and retropneumoperitoneum from over-penetration at the posterior iliac crest (correct perpendicular angle, stop at the give of the cortex). [10]

Disposition (2 marks). Observe until sedation has worn off and the site is dry, document the procedure and the samples, and give the family a safety-net for bleeding, fever or worsening pain. The outcome is driven by the underlying diagnosis, but the marrow examination secures it and the correct cellularity. [1] [10]