diff --git a/src/main/java/eu/europa/ted/eforms/sdk/analysis/fact/SchematronFileFact.java b/src/main/java/eu/europa/ted/eforms/sdk/analysis/fact/SchematronFileFact.java
index 76be4ad..97140a7 100644
--- a/src/main/java/eu/europa/ted/eforms/sdk/analysis/fact/SchematronFileFact.java
+++ b/src/main/java/eu/europa/ted/eforms/sdk/analysis/fact/SchematronFileFact.java
@@ -46,6 +46,23 @@ public List<String> getDuplicateAssertIds() {
         .collect(Collectors.toList());
   }
 
+  /**
+   * Returns the list of diagnostic identifiers that are referenced in an assert but not defined by
+   * a diagnostic element.
+   * 
+   * @return List of diagnostic identifiers missing a definition.
+   */
+  public List<String> getMissingDiagnostics() {
+    Set<String> definedDiagnosticIds = getDiagnostics().stream()
+        .map(SchematronDiagnostic::getId)
+        .collect(Collectors.toSet());
+
+    return getAsserts().stream()
+        .map(SchematronAssert::getDiagnostics)
+        .filter(id -> !definedDiagnosticIds.contains(id))
+        .collect(Collectors.toList());
+  }
+
   @Override
   public String getId() {
       return schematronFile.getPath().toString();
diff --git a/src/main/resources/eu/europa/ted/eforms/sdk/analysis/drools/schematronRules.drl b/src/main/resources/eu/europa/ted/eforms/sdk/analysis/drools/schematronRules.drl
index 900e180..47d4c54 100644
--- a/src/main/resources/eu/europa/ted/eforms/sdk/analysis/drools/schematronRules.drl
+++ b/src/main/resources/eu/europa/ted/eforms/sdk/analysis/drools/schematronRules.drl
@@ -35,8 +35,7 @@ end
 
 rule "Every assert diagnostics is defined in the schematron file"
 when
-  /schematrons[ $sch: this, $schId: id ]/asserts[ $diagnosticId: diagnostics, diagnostics != null ]
-  not (exists /schematrons[ id == $schId ]/diagnostics[ id == $diagnosticId ])
+  $missing : /schematrons[ $sch: this]/missingDiagnostics
 then
-  results.add(new ValidationResult($sch, "Schematron is missing diagnostic definitions for: " + $diagnosticId, ValidationStatusEnum.ERROR));
+  results.add(new ValidationResult($sch, "Schematron is missing diagnostic definitions for: " + $missing, ValidationStatusEnum.ERROR));
 end