diff --git a/docs/interpreting_candidate_scores.md b/docs/interpreting_candidate_scores.md
index 13c5207..c8cb533 100644
--- a/docs/interpreting_candidate_scores.md
+++ b/docs/interpreting_candidate_scores.md
@@ -31,6 +31,15 @@ positive components:
 | `mapping_confidence_score` | 3 | trust in identifier harmonization (worst mapping among contributing records) | `mapping_confidence` |
 | `quality_score` | 2 | study/data quality (`1 - n_quality_flags/5`, floored at 0) | `quality_flags` |
 
+Every quality flag counts the same in `quality_score`: `batch_confounded`
+(treatment fully confounded with a sequencing run, MS acquisition batch, or
+extraction batch) removes the same 0.2 as `low_replication`, and counts
+toward the `quality_score >= 0.4` high-priority gate the same way. Adding the
+term (2026-10-16) did not change any existing score, because no registered
+study declared it yet. Once a study declares it, every candidate with a
+record from that study loses 0.2 `quality_score` (0.4 points of the 0-100
+score) unless the flag was already in its union.
+
 ### Why the unbounded components saturate instead of clipping
 
 Every component whose input has no natural upper bound uses `v / (v + half)`,
diff --git a/registry/controlled_vocabularies/quality_flags.yaml b/registry/controlled_vocabularies/quality_flags.yaml
index de93faf..fdcc28c 100644
--- a/registry/controlled_vocabularies/quality_flags.yaml
+++ b/registry/controlled_vocabularies/quality_flags.yaml
@@ -24,3 +24,5 @@ terms:
     label: Species assignment not confirmed by genotyping (required for species in a hybridizing complex, e.g. Mytilus edulis complex, unless species_identity.basis is genotyped)
   - id: reference_species_mismatch
     label: Reads were mapped to a related species' genome; gene identifiers belong to the reference species, not the study animal
+  - id: batch_confounded
+    label: Treatment contrast fully confounded with a processing batch (sequencing run, MS acquisition batch, extraction batch); treatment effect cannot be separated from batch
diff --git a/tests/test_prioritize.py b/tests/test_prioritize.py
index 1d4fe79..a74b769 100644
--- a/tests/test_prioritize.py
+++ b/tests/test_prioritize.py
@@ -44,6 +44,19 @@ def test_compute_components_bounds_are_0_to_1():
         assert 0.0 <= value <= 1.0, f"{key}={value} out of bounds"
 
 
+def test_batch_confounded_costs_at_least_as_much_quality_as_low_replication():
+    meta_row = {
+        "k_studies": 2, "total_sample_size": 24, "pooled_effect": 1.0,
+        "p_value": 1e-4, "direction_consistency": 1.0, "phenotype": "survival",
+        "distinct_tissues": 1, "distinct_life_stages": 1, "i_squared": 0.0,
+    }
+    baseline = compute_components(meta_row, ["exact"], [])["quality_score"]
+    low_rep = compute_components(meta_row, ["exact"], ["low_replication"])["quality_score"]
+    batch = compute_components(meta_row, ["exact"], ["batch_confounded"])["quality_score"]
+    assert batch < baseline
+    assert batch <= low_rep
+
+
 def test_high_priority_gate_blocks_low_direction_consistency_even_with_two_studies(isolated_reports):
     harmonize_all_demo_studies()
     meta_df = run_meta_analysis(phenotype="larval_viability", feature_type="gene")
diff --git a/tests/test_schema_validation.py b/tests/test_schema_validation.py
index f4a931b..df89e4c 100644
--- a/tests/test_schema_validation.py
+++ b/tests/test_schema_validation.py
@@ -47,6 +47,19 @@ def test_unknown_vocabulary_term_is_rejected(tmp_path):
     assert any("phenotype" in e for e in result.errors)
 
 
+def test_batch_confounded_quality_flag_validates(tmp_path):
+    with open("registry/studies/GIGAS_HEAT01.yaml") as fh:
+        study = yaml.safe_load(fh)
+    study["study_id"] = "BATCH_FLAG"
+    study["quality_flags"] = [*(study.get("quality_flags") or []), "batch_confounded"]
+    path = tmp_path / "BATCH_FLAG.yaml"
+    with open(path, "w") as fh:
+        yaml.safe_dump(study, fh)
+
+    result = validate_study_file(path)
+    assert result.valid, result.errors
+
+
 def test_resilience_classification_mismatch_is_a_warning_not_an_error(tmp_path):
     with open("registry/studies/GIGAS_HEAT01.yaml") as fh:
         study = yaml.safe_load(fh)