Adsorption of CSF A1-42 during pre-analytical processing is suggested as an important confounder in testing. The aim of the present study was to assess the effect of polypropylene transfer plates (PTP) in the INNOTEST A1-42 IVD-ELISA assay on A1-42 levels. CSF samples from 26 individuals with subjective cognitive impairment (SCI) and 25 patients with suspected neurodegenerative disorders were tested using four different lots of kits. A1-42 levels in all samples that were loaded onto the PTP were significantly lower than the levels in the same samples that were analyzed without prior loading onto the PTP. We found that the PTP may adsorb A1-42 in the range 7 to 69%. The diagnosis in 20% of patients and amyloid burden assessment in 23% of SCI patients had to be modified post hoc due to initial erroneously low amyloid levels. Using a PTP prior to loading the samples onto the INNOTEST A1-42 test plate may result in erroneously low A1-42 levels.