Within the Swedish National Monitoring Programme for Contaminants in Freshwater Biota, perch, pike and Arctic char are being analysed from 32 lakes distributed all over Sweden. One of the main objectives of the monitoring has been to detect temporal trends in different areas and for different contaminants. However, the measured concentrations in biota can be affected by biological factors such as age, length, trophic position (TP), fat or dry weight content. A change in one or several of these variables over time, or even among specimens taken within the same year, is likely to result in an increased variation of the measured concentrations and hence in a lower chance of detecting a trend. The purpose of this study was to evaluate if an adjustment of contaminant time trends by primarily stable isotopes, but also other confounding factors can:  A) reduce the individual and between-year variation and hence B) improve the likelihood of detecting temporal trends in time series for metals and organic contaminants. The study revealed an improvement in the likelihood of detecting a contaminant trend after adjustment of the data for almost all lakes and for all tested substances. For some lakes and substances, this improvement was substantial. Age seems to be an important confounding factor for metals and per- and polyfluoroalkyl substances. For lipophilic substances, fat content also appears to be an important factor in some lakes together with adjustments for carbon and nitrogen isotope ratios (d¹³C and d¹⁵N), which all reduced the variation in contaminant data. This suggests that there is a benefit of adjusting data on a regular basis in contaminant monitoring programmes in the future.