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“Marrow Asleep or Awake?”: William Aird Simplifies Anemia Diagnosis
William Aird/X
Oct 30, 2025, 07:55

“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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Aabsolute Reticulocyte Count anemia ARC benign hematology Harvard Medical School Health Hematology Hemostasis Hemostasis Today Medicine thrombosis William Aird
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