diff --git a/backend/pkdb_app/info_nodes/models.py b/backend/pkdb_app/info_nodes/models.py
index ba839770..31094f5a 100644
--- a/backend/pkdb_app/info_nodes/models.py
+++ b/backend/pkdb_app/info_nodes/models.py
@@ -173,6 +173,7 @@ class MeasurementType(AbstractInfoNode):
     CAN_NEGATIVE = [
         "tmax",  # tmax can be negative due to time offsets, i.e. pre-simulation with subsequent fall after intervention
         "concentration change",    # this often happens in placebo simulations
+        "concentration change absolute",
         "EHR change",
     ]
     ADDITIVE = []  # todo remove