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Abstract: Background: Plasma YKL-40 is a new biomarker in patients with cancer and inflammatory diseases. High plasma YKL-40 is associated with poor prognosis. Our aim was to determine reference levels in healthy subjects. Methods: Plasma YKL-40 was determined in 3130 participants aged 20–80years from the Danish general population, the Copenhagen City Heart Study. They had no known disease at time of blood sampling in 1991–1994 and remained healthy and alive during a 16-year follow-up period. In 644 participants, YKL-40 was measured again in samples taken 10years after the first. Results: The median plasma YKL-40 was 40μg/L (2.5–97.5% reference levels: 14–155) with no difference between sexes. YKL-40 increased exponentially with age. For age-adjusted reference levels, the YKL-40 percentile as a function of age in years and plasma YKL-40 in μg/L was derived: percentile=100/(1+(YKL-40^−3)*(1.062^age)*5000). In subjects with two YKL-40 measurements 10years apart, the mean increase in YKL-40 was 1.5μg/L/year (SE: 0.2), while the mean change in the calculated age percentile was minimal (−0.3; SE: 0.1). Conclusions: Plasma YKL-40 increases with age within and across healthy individuals from the general population. Age-stratified or age-adjusted reference levels are important when YKL-40 test results are evaluated. [Copyright &y& Elsevier]