Using median regression to obtain adjusted estimates of central tendency for skewed laboratory and epidemiologic data.

BACKGROUND: Laboratory studies often involve analyses of highly skewed data for which means are not an adequate measure of central tendency because they are sensitive to outliers. Attempts to transform skewed data to symmetry are not always successful, and medians are better measures of central tendency for such skewed distributions. When medians are compared across groups, confounding can be an issue, so there is a need for adjusted medians. METHODS: We illustrate the use of quantile regression to obtain adjusted medians. The method is illustrated by use of skewed nutrient data obtained from black and white men attending a prostate cancer screening. For 3 nutrients, saturated fats, caffeine, and vitamin K, we obtained medians adjusted by age, body mass index, and calories for men in each race group. RESULTS: Quantile regression, linear regression, and log-normal regression produced substantially different adjusted estimates of central tendency for saturated fats, caffeine, and vitamin K. CONCLUSIONS: Our method was useful for analysis of skewed and other nonnormally distributed continuous outcome data and for calculation of adjusted medians.

Impact of nutrients on insulin-like growth factor-I, insulin-like growth factor binding protein-3 and their ratio in African American and white males.

OBJECTIVE: Higher levels of insulin-like growth factor-I (IGF-I) and lower levels of IGF binding protein-3 (IGFBP-3) have been associated with an increased risk of prostate cancer. Nutrition is known to partially regulate IGF levels and it is possible that nutritional factors mediate the impact of IGF levels on prostate cancer risk. DESIGN: A cross-sectional analysis of the impact of nutritional factors measured by a dietary questionnaire on plasma levels of IGF-I, IGFBP-3 and their molar ratio. Multiple linear regression analysis was used to test for effects of nutrients on IGF levels. SETTING: Prostate cancer screening at the Hollings Cancer Center in Charleston, South Carolina. SUBJECTS: Ninety-five African American and 138 white males aged 33-83 years attending the screening. RESULTS: In whites, intakes of total, saturated and monounsaturated fats were positively associated with an increase in the molar ratio, while there was no association in African Americans. In African Americans, we found that increasing intake of calcium and dairy servings was positively associated with IGF-I levels. Increased vegetable intake was positively associated with IGFBP-3 in African Americans, while there was no effect in whites. A higher percentage of alcohol in the total diet was significantly associated with a decrease in the molar ratio and an increase in IGFBP-3 in both groups. CONCLUSIONS: Our results confirm previous findings of nutritional determinants of IGF levels. Additionally, we found the impact of several nutrients on IGF levels to be different in whites and African Americans, which warrants further investigation.

Clinical breast examination practices among women undergoing screening mammography.

PURPOSE: To determine the proportion of women who underwent clinical breast examination (CBE) within 12 months before undergoing screening mammography and to examine which factors, if any, were associated with having undergone CBE within 12 months before screening mammography. MATERIALS AND METHODS: A cross-sectional study of 1,135 women presenting for screening mammography was conducted with a self-administered questionnaire. Data were collected regarding sociodemographic information, health care provider information, breast self-examination practices, and CBE history. Data were analyzed with the chi(2) test of independence. RESULTS: One hundred forty (38.5%) of 364 nonwhite women and 366 (57.4%) of 638 white women reported having undergone CBE 12 months before screening mammography. This association between race and CBE was statistically significant before (P <.001) and after adjusting for education, age, and type of referring physician (P <.001). There was a statistically significant association between undergoing CBE within 12 months and age (P <.001), type of referring physician (P <.001), and education level (P <.001). Health insurance status was associated with having undergone CBE within the past 12 months, but it was not a statistically significant association after controlling for race. CONCLUSION: The proportion of women who undergo comprehensive breast cancer screening differs by race, education level, and type of referring physician.