Plasma selenium level before diagnosis and the risk of prostate cancer development.

PURPOSE: Epidemiological studies and a randomized intervention trial suggest that the risk of prostate cancer may be reduced by selenium intake. We investigated whether plasma selenium level before diagnosis correlated with the risk of later developing prostate cancer. MATERIALS AND METHODS: A case control study was performed on men from the Baltimore Longitudinal Study of Aging registry, including 52 with known prostate cancer and 96 age matched controls with no detectable prostatic disease. Plasma selenium was measured at an average time plus or minus standard deviation of 3.83 +/- 1.85 years before the diagnosis of prostate cancer by graphite furnace atomic absorption spectrophotometry. Adjusted odds ratio and 95% confidence interval were computed with logistic regression. RESULTS: After correcting for years before diagnosis, body mass index, and smoking and alcohol use history, higher selenium was associated with a lower risk of prostate cancer. Compared with the lowest quartile of selenium (range 8.2 to 10.7 microg./dl.), the odds ratios of the second (10.8 to 11.8), third (11.9 to 13.2) and fourth (13.3 to 18.2) quartiles were 0.15 (95% confidence interval 0.05 to 0.50), 0.21 (0.07 to 0.68) and 0.24 (0.08 to 0.77, respectively, p =0.01). Furthermore, plasma selenium decreased significantly with patient age (p <0.001). CONCLUSIONS: Low plasma selenium is associated with a 4 to 5-fold increased risk of prostate cancer. These results support the hypothesis that supplemental selenium may reduce the risk of prostate cancer. Because plasma selenium decreases with patient age, supplementation may be particularly beneficial to older men.

The effects of modifying proficiency testing materials on thyroid function test results. A College of American Pathologists Ligand Assay Survey Study.

OBJECTIVE: To gain insight on the matrix effects, and possible clinical implications, resulting from diluting and concentrating proficiency testing survey material used for the measurement of thyroid function tests. DESIGN: To the standard set of five proficiency survey samples, three supplementary "Wildcard" samples were added. These additional samples were manufactured by overfilling and underfilling vials prior to lyophilization so as to vary the thyroxine-binding protein concentrations. Survey participants measured thyroxine, free thyroxine, and the triiodothyronine uptake and related tests on the Wildcard samples. In addition, free thyroxine indices were calculated. SETTING: The first mailing of the 1995 College of American Pathologists (CAP) Ligand Assay--Series 1 Survey. MAIN OUTCOME MEASURES: Results obtained from the regular set of survey samples and the Wildcard set were compared to values expected by the laws of conservation of matter and mass action. PARTICIPANTS: The approximately 2000 participants of the first mailing of the 1995 CAP Ligand Assay--Series 1 Survey. RESULTS: Numerous assays systems did not give the predicted results, including all of the single-step radioimmunoassays for free thyroxine and over three quarters of free thyroxine index determinations. CONCLUSIONS: Varying the dilution of proficiency survey material produced results that were not predicted by the laws of conservation of matter and of mass action. Although these observations may have been the result of matrix effects, one cannot rule out the possibility that certain thyroid assays may not work in clinical situations having abnormal thyroxine-binding protein concentrations.