diff --git a/tests/api_test.py b/tests/api_test.py
index 99d143b..ba8d736 100644
--- a/tests/api_test.py
+++ b/tests/api_test.py
@@ -95,6 +95,19 @@ def subject_database(tmpdir: str, meds_dataset: str):
     return meds_reader.SubjectDatabase(str(meds_reader_dir))
 
 
+@pytest.fixture
+def threaded_subject_database(tmpdir: str, meds_dataset: str):
+
+    meds_reader_dir = os.path.join(tmpdir, "meds_reader")
+
+    subprocess.run(
+        ["meds_reader_convert", meds_dataset, meds_reader_dir, "--num_threads", "4"],
+        check=True,
+    )
+
+    return meds_reader.SubjectDatabase(str(meds_reader_dir), num_threads=4)
+
+
 def test_metadata(subject_database):
     with open(os.path.join(subject_database.path_to_database, "metadata", "dataset.json")) as f:
         loaded_metadata = json.load(f)
@@ -114,12 +127,24 @@ def test_iter(subject_database):
     assert list(subject_database) == [32, 64]
 
 
-def test_map(subject_database):
-    def h(subjects):
-        result = []
-        for p in subjects:
-            result.append(p.subject_id)
-        return result
+def h(subjects):
+    result = []
+    for p in subjects:
+        result.append(p.subject_id)
+    return result
+
+
+def h2(subjects_and_data):
+    result = []
+    for subject, rows in subjects_and_data:
+        assert len(rows) == 1
+        row = rows[0]
+        print(subject, row)
+        result.append((subject.subject_id, row.other))
+    return result
+
+
+def map_helper(subject_database):
 
     results = list(subject_database.map(h))
 
@@ -131,15 +156,6 @@ def h(subjects):
 
     table = pd.DataFrame({"subject_id": [64, 32], "other": [1, 1000]})
 
-    def h2(subjects_and_data):
-        result = []
-        for subject, rows in subjects_and_data:
-            assert len(rows) == 1
-            row = rows[0]
-            print(subject, row)
-            result.append((subject.subject_id, row.other))
-        return result
-
     results = list(subject_database.map_with_data(h2, table))
 
     final_result = {a for b in results for a in b}
@@ -149,6 +165,15 @@ def h2(subjects_and_data):
     assert final_result == {(32, 1000), (64, 1)}
 
 
+def test_map(subject_database):
+    map_helper(subject_database)
+
+
+def test_map_threaded(threaded_subject_database):
+    map_helper(threaded_subject_database)
+    threaded_subject_database.terminate()
+
+
 def test_properties(subject_database):
     print(subject_database.properties)
     assert subject_database.properties == {