[Munchhausen syndrome by proxy revealed by falsely toxic methotrexate levels].

Methotrexate is an antifolate drug used intravenously at high-dose in acute lymphocytic leukemia (ALL). Therapeutic drug monitoring is required to identify patients at risk of developing toxicity and to control folinic acid rescue. We report a case of Münchausen syndrome by proxy revealed by high and persistent falsely toxic methotrexate plasmatic levels. A 12 year-old child was treated with chemotherapy including methotrexate every 70 days for an ALL. The last methotrexate plasmatic level was 0.15 μmol/L at the 72th hour of the infusion. Then, he was treated by oral rout low-dose methotrexate. Ten days after methotrexate infusion, the patient consulted for asthenia, vomiting and presented a mucositis. Methotrexate plasmatic level was 2323 μmol/L. Renal function was normal. All drugs' intake was stopped. Folinic acid rescue was instituted. Even though there was no clinical sign of toxicity, therapeutic drug monitoring showed persistent high methotrexate plasmatic levels. Investigations eliminated measurement errors and pharmacokinetic problems. A deliberate methotrexate addition in each child blood sample brought by the mother was highly suspected. We confirmed this hypothesis by measuring methotrexate plasmatic levels in three samples: one brought by the mother, the second brought by the child's doctor and the last collected in our laboratory. Methotrexate plasmatic levels were respectively over 10,000 μmol/L (first sample) and lower than 0.02 μmol/L (the two others). The diagnosis of Munchausen's syndrome by proxy revealed by falsely toxic methotrexate plasmatic levels was made and the mother was addressed to the psychiatric department.