docs(mimic-iv): note lab valuenum and valueuom filtering conventions

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Chessing234

mimic-iv/concepts/README.md

@@ -3,6 +3,10 @@
 This folder contains scripts to generate useful abstractions of raw MIMIC-IV data ("concepts").
 The scripts are written using the **BigQuery Standard SQL Dialect**. Concepts are categorized into folders if possible, otherwise they remain in the top-level directory. The [concepts_postgres](/mimic-iv/concepts_postgres) and [concepts_duckdb](/mimic-iv/concepts_duckdb) folders contain automatically generated PostgreSQL and DuckDB versions of these scripts; see the [top-level readme](/mimic-iv/README.md#generating-the-concepts) for how these were generated.
 
+Lab measurement concepts under `measurement/` generally exclude non-positive `valuenum` rows
+(`valuenum IS NOT NULL AND valuenum > 0`) and, where documented in the source SQL, filter
+`valueuom` to the expected unit for a given `itemid`.
+
 The concepts are organized into individual SQL scripts, with each script generating a table. The BigQuery `mimiciv_derived` dataset under `physionet-data` contains the concepts pregenerated. Access to this dataset is available to MIMIC-IV approved users: see the [cloud instructions](https://mimic.mit.edu/docs/gettingstarted/cloud/) on how to access MIMIC-IV on BigQuery (which includes the derived concepts).
 
 See the [top-level readme](/mimic-iv/README.md) for more information about generating the concepts.