diff --git a/src/openflexure_microscope_server/things/stage_measure.py b/src/openflexure_microscope_server/things/stage_measure.py
index 98bae455..13761c52 100644
--- a/src/openflexure_microscope_server/things/stage_measure.py
+++ b/src/openflexure_microscope_server/things/stage_measure.py
@@ -69,7 +69,7 @@ class ParasiticMotionError(Exception):
     pass
 
 
-def parasitic_detect(delta: float, max_allowed_delta: float) -> None:
+def _parasitic_detect(delta: float, max_allowed_delta: float) -> None:
     """Compare two values and raise parasitic motion error."""
     if delta > max_allowed_delta:
         raise ParasiticMotionError(
diff --git a/tests/test_stage_measure.py b/tests/test_stage_measure.py
index a4a226a7..79c48c43 100644
--- a/tests/test_stage_measure.py
+++ b/tests/test_stage_measure.py
@@ -1,8 +1,11 @@
 """File contains unit tests for stage_measure."""
 
+import pytest
 from openflexure_microscope_server.things.stage_measure import (
     _generate_move_dicts,
     _predict_z,
+    _parasitic_detect,
+    ParasiticMotionError
 )
 
 from .mock_things.mock_csm import MockCSMThing
@@ -45,3 +48,10 @@ def test_predict_z():
     )
     expected_z_diff = 1343.1625053206606
     assert mock_z_diff == expected_z_diff
+
+def test_parasitic_detect():
+    """Check that the parasitic error is raised correctly."""
+    mock_delta = 100
+    mock_max = 50
+    with pytest.raises(ParasiticMotionError):
+        _parasitic_detect(delta=mock_delta, max_allowed_delta=mock_max)