False positivity of the Aspergillus galactomannan Platelia ELISA because of piperacillin/tazobactam treatment: does it represent a clinical problem?

OBJECTIVES False-positive results of the galactomannan (GM) ELISA caused by concurrent administration of piperacillin/tazobactam have been reported in patients with febrile neutropenia. PATIENTS AND METHODS This prospective study investigated different sampling times in 30 patients receiving piperacillin/tazobactam for febrile neutropenia. RESULTS Prior to the first piperacillin/tazobactam infusion, a median GM index of 0.2 [interquartile range (IQR) 0.1-0.3] was noted; in two patients (7%) the index was 0.5. Immediately after piperacillin/tazobactam infusion, the median index increased to 0.3 (IQR 0.2-0.4, P = 0.002) leading to 21% (7/30) false-positive results, if > or = 0.5 is assumed as the cut-off level. GM indices before the next piperacillin/tazobactam infusion were not increased (median 0.2, IQR 0.2-0.35, P > 0.05), but 10% (3/30) were still > or = 0.5. With a cut-off level of > 0.7, no false-positive results were noted at any sampling time point. CONCLUSIONS We conclude that the clinical relevance of false-positive GM results during piperacillin/tazobactam treatment is small if samples are collected prior to infusion and if a cut-off level of > 0.7 is used.