A 65-year-old female patient had a diagnosis of non-Hodgkin lymphoma. He went to the hospital with fever, neutropenia and dyspnea after a cycle of chemotherapy.
She received treatment with empirical antibiotics and granulocyte-colony growth factor without improvement. Oxygen saturation was under 75%. A CAT scan showed a bilateral interstitial pattern but no pulmonary embolism was present. The doctor in charge started treatment with intravenous trimetoprim-cotrimoxazol with a clear clinical improvement. This “therapeutic test” made the diagnosis of Pneumocystis jiroveci pneumonia highly plausible.The term ex juvantibus (from latin, meaning “from that which helps”) refers, in medical contexts, to the process of making an inference about disease causation from an observed response of the disease to a treatment