Reliability of plasma carotenoid biomarkers and its relation to study power

Background: The reliability of biomarkers profoundly impacts validity of their use in epidemiology and can have serious implications for study power and the ability to find true associations. We assessed reliability of plasma carotenoid levels over time and how it could influence study power through sample size and effect-size. Methods: Plasma carotenoid levels were measured in a cohort study of 1323 women participating in the control arm of the Women's Healthy Eating and Living Study. We compared mean plasma levels at baseline, year 1, and year 4 of the study for alpha-carotene, beta-carotene, lycopene, lutein, and beta-cryptoxanthin. Reliability of these levels over time was assessed by Spearman correlations and intraclass correlation. Results: We found limited variation in mean levels between any 2 time points. Variation did not exceed 8% for lycopene, lutein, and beta-cryptoxanthin, 15% for alpha-carotene, and 18% for beta-carotene. Spearman correlations for individual carotenoids over time varied between 0.50 and 0.80, with lycopene having the lowest correlation. Intraclass correlations ranged from 0.47 to 0.66 for carotenoids. Conclusion: Intraclass correlations for plasma carotenoids over a period of several years are acceptable for epidemiologic studies. However, such variation is enough to decrease statistical power and increase the sample size needed to detect a given effect.