False positivity of antigen immunoassays used as an early diagnostic tool to detect invasive fungal infections is known. Interpretation of the assay needs the identification of sources which could affect the specificity of the test. We focused on the influence of parenteral nutrition (PN) and piperacillin-tazobactam (TZP) on fungal immunoassays. Measurable amounts of Candida antigen mannan were detected in several compounds of PN and TZP in a previous in vitro study. In the current study, 84 patients undergoing allogeneic haematopoietic cell transplantation receiving either TZP, PN or both were monitored with Aspergillus and Candida antigen assay. Six patients were analysed closer in a kinetic analysis with more frequent blood sampling to detect mannan. PN in diverse compositions as well as TZP did not increase significantly the amount of mannan and the Aspergillus antigen in serum. We could not confirm the positive results of the in vitro study. Physicians should be aware that mannan antigenemia due to drug infusion could be a transient issue and should be considered in the interpretation of fungal immunoassays, although we could not find clinically relevant effects on mannan levels.