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Section 2 of 6 · Open sections

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Use two axes before naming a cause

Use two axes: cell size by mean corpuscular volume (MCV) and marrow response by ARC. Red-cell distribution width (RDW) describes size variation and helps expose mixed populations; neither index alone supplies etiology.

A limited ARC supports inadequate compensation only in the stated anemia context. Here ARC 35.5 × 10^9/L is inadequate because hemoglobin is 8.4 g/dL and hematocrit is 27.0% despite a low hemoglobin requiring increased production; it is not called limited merely because it lies near a reference interval.

A reticulocyte production index (RPI) is optional and only valid when its convention is complete: corrected reticulocyte percentage = reticulocyte percentage × patient hematocrit ÷ 45%; RPI = corrected percentage ÷ maturation factor. For this teaching convention, maturation factor is 2.0 at hematocrit 25–35%, and RPI <2 indicates an inadequate response. Do not apply this convention without its timing and local validation assumptions.

Illustrative drawing — this picture was drawn rather than captured.

Grid crossing microcytic, normocytic, and macrocytic MCV patterns with limited or increased reticulocyte response and example follow-up domains.
Figure 1ARC-first anemia grid; it is not a recognition image.

ARC-first anemia organization workflow

  1. Confirm timing and context

    Verify local interval, specimen stability, treatment/transfusion timing, and same-specimen CBC/reticulocyte assumption.

  2. Classify by ARC first

    Interpret ARC against the degree of anemia; calculate RPI only when the stated convention applies.

  3. Use indices and observed smear

    Pair MCV/RDW with actual reviewed morphology under the declared smear convention.

  4. Choose one focused endpoint

    Select the most discriminating next test or state when needed data are missing.

Knowledge checks

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Knowledge check 1

Which observations can expose a mixed red-cell population when MCV is near the local interval? Select all that apply.

Choose at least 3 options.

Knowledge check 2

Which calculation defines ARC from the same sample?

Choose one option.

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