# RDW-CV to RDW-SD Mapping - Why?

**URL:** <https://forum.loinc.org/t/rdw-cv-to-rdw-sd-mapping-why/2905>\
**Category:** General\
**Created:** [October 9, 2025, 9:30pm UTC](https://forum.loinc.org/t/rdw-cv-to-rdw-sd-mapping-why/2905 "2025-10-09T21:30:30Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![mwilsonhunting](https://forum.loinc.org/letter_avatar_proxy/v4/letter/m/d78d45/32.png) [@mwilsonhunting](https://forum.loinc.org/u/mwilsonhunting)\
**Post date:** [October 9, 2025, 9:30pm UTC](https://forum.loinc.org/t/rdw-cv-to-rdw-sd-mapping-why/2905/1 "2025-10-09T21:30:30Z")

</div>

I’m looking for more guidance on the mapping between RDW-CV and RDW-SD. The standard code description for 21000-5 RDW-SD (fL) and 788-0 RDW-CV (%). Per LONIC search, 21000-5 is deprecated and maps to 788-0, however, their mapping guidance states, “Erythrocyte distribution width (EDW) can be reported with units of % or volume (fL). For units reported as %, use LOINC 788-0. For EDW reported in volume units, use LOINC 21000-5.” I’m trying to understand how a code based on volume (RDW-SD) is equivalent to a percentage (RDW-CV). Anyone have insight?

---

<div class="post-metadata">

**Author:** ![pitk0006](https://forum.loinc.org/letter_avatar_proxy/v4/letter/p/c89c15/32.png) [@pitk0006](https://forum.loinc.org/u/pitk0006)\
**Post date:** [June 15, 2026, 8:02pm UTC](https://forum.loinc.org/t/rdw-cv-to-rdw-sd-mapping-why/2905/2 "2026-06-15T20:02:50Z")

</div>

Hello,

Both are RDWs (although LOINC naming convention uses erythrocyte or EDW). This article may help: [Reading RDW-CV and RDW-SD When They Diverge • The Blood Project](https://www.thebloodproject.com/reading-rdw-cv-and-rdw-sd-when-they-diverge/)

Cheers,