Patients with leukemia and concomitant neutropenia are at high risk of developing invasive fungal infections (IFI) that are associated with high morbidity and mortality. As these patients typically have severe thrombocytopenia, direct diagnostic sampling with invasive procedures is often not possible due to the high peri-interventional risk. Therefore, the presumptive diagnosis of IFI is primarily based on compatible lung findings on computed tomography and serologic detection of fungal cell wall components, which, however, have limited sensitivity and specificity. With the present study, the investigators aim to determine a set of specific volatile biomarkers in leukemia patients with proven or probable IFI using secondary electrospray ionization high-resolution mass spectrometry (SESI-HRMS).