“Marrow Asleep or Awake?”: William Aird Simplifies Anemia Diagnosis
William Aird, Professor of Medicine at Harvard Medical School, posted on X:
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RETIC COUNT
When evaluating anemia, I teach my trainees to start with a simple binary:
Is the bone marrow responding appropriately or not?
That answer lies in a single number: the absolute reticulocyte count (ARC).
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- ARC < 0.12 ×10⁶/µL → hypoproliferative (marrow asleep).
- ARC ≥ 0.12 ×10⁶/µL → hyperproliferative (marrow awake).
It’s a light switch, not a dimmer.
Once the marrow’s “on,” the next question isn’t how high, but why.
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Getting granular (0.15 vs 0.20) adds math, not meaning.
At that point, you’ve already learned the key thing:
the marrow can respond, so look for blood loss or hemolysis, not marrow failure.
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Clinical reasoning often starts binary and gets nuanced later (i.e., decision thresholds are categorical).
The ARC is one of those places where simple still works.
Below 0.12 → asleep.
Above 0.12 → awake.
Everything else is physiology, not management.՛՛
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