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Aspergillus galactomannan (GM) antigenemia is an early marker of invasive aspergillosis (IA), but may yield false-positive results. A prospective study, testing GM periodically in serum samples of liver transplant recipients, was performed.An index more than or equal to 0.5 were considered positive. Positive GM in samples from patients without any other criteria of proven or probable IA was considered as false-positive. The test was performed weekly during the first month after transplantation.Three patients developed IA. In total, 414 serum samples from 85 liver transplant recipients were analyzed. Mean number of samples per patient (out of those who could be assessed) was 4.8. The number of false-positive GM samples was 40 (9.6%), corresponding to 28 patients. The frequency of false-positive results in samples obtained during the first week posttransplantation was 36% (27 of 75), significantly higher than the number of false-positive samples obtained after the first week (3.8%; 13 of 339; P<0.001). Multivariate analysis showed that antibacterial prophylaxis with ampicillin was the only independent factor associated with a false-positive result. Different vials of β-lactam antibiotics were tested for GM. We obtained a positive GM value (>0.5) in four of the six vials of ampicillin, in three of the six vials of piperacillin-tazobactam, in none of the six vials of cefotaxime, and in none of the six controls.The present study suggests that the administration of ampicillin as antibacterial prophylaxis during the first days after transplantation could be a possible cause of false-positive GM results.