Objective: Long-term external quality assessments suggest that the individual results of different immunoassays are often not comparable. Our goal was to assess the possible sources of these differences. Methods: The paper is based on the results of analyses using seven different immunoassays: DELFIA (PerkinElmer), Elecsys 2010 (Roche), Kryptor (B.R.A.H.M.S.), the enzyme-linked immunosorbent assay DRG and three methods based on immunoradiometric assays (DiaSorin, Immunotech and Schering-CIS). Results: The following parameters were evaluated: precision profile of individual methods, linearity on dilution, modified recovery and comparability of immunoassays. The analytical results for certain low concentration specimens correlate well while others do not (up to five – fold difference), especially in the case of controls prepared synthetically. Therefore, the current non-standardized preparation of controls seems to be questionable. In the cut-off range, the difference in the results of native samples does not exceed its double value. The variation in values higher than 100 µg/l obtained with different assays is under 40 %. All the evaluated immunoassays are efficient and highly comparable (The correlation coefficients are up to 0.994). Conclusion: Our results confirm the expected matrix interference occurring especially in the range of normal and cut-off NSE concentrations. Another source of discrepancies can be put down to different antibody affinity to αγ- and γγ-enolase isoenzymes.

        Key words: neuron-specific enolase, NSE, precision, recovery, comparability, EQA.