Diagnostic pitfalls in assessment of ferritin following CAR‐T‐cell therapy: Understanding the hook effect

Abstract The hook effect is a well‐described but clinically underappreciated immunoassay interference, where a falsely lowered result is caused by analyte excess. We describe a situation in which ferritin immunoassay results from a 27‐year‐old female with immune effector cell‐associated hemophagocytic lymphohistiocytosis‐like syndrome were more than 1000 times lower at a reference laboratory than those determined in‐house after dilution. This case underscores the importance for clinical care providers to be aware of the impact of the hook effect on ferritin measurements, and to promptly communicate with the laboratory when there are discrepancies between clinical symptoms and test results.