Bone-marrow aspiration and biopsy principles — branching viva

Branching viva — bone marrow aspiration and biopsy

The examiner releases the stem and then branches into four probes. A strong candidate names the two samples first, defends the site and the technique, and interprets the dry tap and the bleeding risk without prompting. [1] [2]

Opening (examiner)

"A six-year-old is referred with a haemoglobin of 60, a platelet count of 20, and blasts on the peripheral film. You plan a bone marrow aspirate and trephine. What samples will you take, and why both?" [1]

Branch 1 — The two samples (expected answer)

The aspirate is liquid marrow drawn from the medullary sinusoids and answers questions about cells as individuals: morphology, flow cytometry, cytogenetics, molecular and culture. The trephine is a core of bone that preserves the architecture and is the only sample that shows overall cellularity, fibrosis, granulomas and infiltration. Both are taken at the same sitting so the child is not brought back twice, and each fails where the other succeeds. [2]

Probe. "Why is the trephine essential in aplastic anaemia?" — Because cellularity under 25 per cent defines severe aplastic anaemia, and a liquid aspirate taken from a surviving pocket of marrow can look deceptively normal. [1] [10]

Branch 2 — Site and technique (expected answer)

Site: the posterior superior iliac spine, one to two centimetres below and lateral to the spine, where the bone is broadest and flattest and far from any vital structure. The sternum is avoided in young children because the thin cortex admits the needle into the mediastinum. Aspirate before the trephine, because the trephine disrupts the marrow and dilutes the aspirate; take the trephine from the same skin puncture but a slightly different track. [1] [2]