diff --git a/src/ert/shared/storage/key_utils.py b/src/ert/shared/storage/key_utils.py
index f44e010cbaa..032a6be25b4 100644
--- a/src/ert/shared/storage/key_utils.py
+++ b/src/ert/shared/storage/key_utils.py
@@ -119,7 +119,8 @@ class SummaryVarType(Enum):
 
     @staticmethod
     def determine_var_type(var: str) -> "SummaryVarType":
-        # Default case for single characters or unknown vars
+        if var in special_vars:
+            return SummaryVarType.RD_SMSPEC_MISC_VAR
         default_case = {
             "A": SummaryVarType.RD_SMSPEC_AQUIFER_VAR,
             "B": SummaryVarType.RD_SMSPEC_BLOCK_VAR,
@@ -131,7 +132,6 @@ def determine_var_type(var: str) -> "SummaryVarType":
             "W": SummaryVarType.RD_SMSPEC_WELL_VAR,
         }
 
-        # Special case for 'L' with nested logic
         if var.startswith("L"):
             secondary = var[1] if len(var) > 1 else ""
             return {
@@ -140,7 +140,6 @@ def determine_var_type(var: str) -> "SummaryVarType":
                 "W": SummaryVarType.RD_SMSPEC_LOCAL_WELL_VAR,
             }.get(secondary, SummaryVarType.RD_SMSPEC_MISC_VAR)
 
-        # Special case for 'R' with more complex nested logic
         if var.startswith("R"):
             if len(var) == 3 and var[2] == "F":
                 return SummaryVarType.RD_SMSPEC_REGION_2_REGION_VAR
@@ -154,35 +153,22 @@ def determine_var_type(var: str) -> "SummaryVarType":
                 return SummaryVarType.RD_SMSPEC_REGION_2_REGION_VAR
             return SummaryVarType.RD_SMSPEC_REGION_VAR
 
-        # Fallback to default cases or miscellaneous if not matched
+        # default cases or miscellaneous if not matched
         return default_case.get(var[0], SummaryVarType.RD_SMSPEC_MISC_VAR)
 
 
-def identify_var_type(key: str) -> SummaryVarType:
-    if key not in special_vars:
-        return SummaryVarType.determine_var_type(key)
-    return SummaryVarType.RD_SMSPEC_MISC_VAR
-
-
 def match_keyword_vector(start: int, rate_vars: List[str], keyword: str) -> bool:
-    """
-    Check if 'keyword' matches any item in 'rate_vars' starting from
-    index 'start' onwards.
-    """
     # Get the suffix of keyword starting from 'start' index (if not out of range)
     suffix = keyword[start:] if len(keyword) > start else ""
     return any(suffix.startswith(var) for var in rate_vars)
 
 
 def match_keyword_string(start: int, rate_string: str, keyword: str) -> bool:
-    """
-    Check if 'keyword' matches 'rate_string' starting from index 'start'.
-    """
     return keyword[start:].startswith(rate_string)
 
 
 def is_rate(key: str) -> bool:
-    var_type = identify_var_type(key)
+    var_type = SummaryVarType.determine_var_type(key)
     if var_type in {
         SummaryVarType.RD_SMSPEC_WELL_VAR,
         SummaryVarType.RD_SMSPEC_GROUP_VAR,
@@ -193,7 +179,6 @@ def is_rate(key: str) -> bool:
         SummaryVarType.RD_SMSPEC_LOCAL_COMPLETION_VAR,
         SummaryVarType.RD_SMSPEC_NETWORK_VAR,
     }:
-        # Process local and network vars differently
         if var_type in {
             SummaryVarType.RD_SMSPEC_LOCAL_WELL_VAR,
             SummaryVarType.RD_SMSPEC_LOCAL_COMPLETION_VAR,
diff --git a/tests/unit_tests/shared/test_rate_keys.py b/tests/unit_tests/shared/test_rate_keys.py
new file mode 100644
index 00000000000..109ff70dc0c
--- /dev/null
+++ b/tests/unit_tests/shared/test_rate_keys.py
@@ -0,0 +1,53 @@
+import hypothesis.strategies as st
+from hypothesis import given
+from src.ert.shared.storage.key_utils import is_rate
+
+
+def nonempty_string_without_whitespace():
+    return st.text(
+        st.characters(whitelist_categories=("Lu", "Ll", "Nd", "P")), min_size=1
+    )
+
+
+@given(key=nonempty_string_without_whitespace())
+def test_is_rate_does_not_raise_error(key):
+    print(key)
+    is_rate_bool = is_rate(key)
+    assert isinstance(is_rate_bool, bool)
+
+
+examples = [
+    ("OPR", False),
+    ("WOPR:OP_4", True),
+    ("WGIR", True),
+    ("FOPT", False),
+    ("GGPT", False),
+    ("RWPT", False),
+    ("COPR", True),
+    ("LPR", False),
+    ("LWPR", False),
+    ("LCOPR", True),
+    ("RWGIR", True),
+    ("RTPR", True),
+    ("RXFR", True),
+    ("XXX", False),
+    ("YYYY", False),
+    ("ZZT", False),
+    ("SGPR", False),
+    ("AAPR", False),
+    ("JOPR", False),
+    ("ROPRT", True),
+    ("RNFT", False),
+    ("RFR", False),
+    ("RRFRT", True),
+    ("ROC", False),
+    ("BPR:123", False),
+    ("FWIR", True),
+]
+
+
+@given(st.sampled_from(examples))
+def test_is_rate_determmines_rate_key(example):
+    key, rate = example
+    is_rate_bool = is_rate(key)
+    assert is_rate_bool == rate