Influence of seasonal variations in ambient temperatures on performance of immunochemical faecal occult blood test for colorectal cancer screening: observational study from the Florence district.

BACKGROUND: Faecal occult blood testing (FOBT) in population screening has proved to be effective in reducing mortality from colorectal cancer. In Italy a latex agglutination FOBT has been adopted for a single-sample screening programme. The aim of this study was to examine the performance of FOBTs in the Florence screening programme over several seasons to evaluate the impact of variations in ambient temperature on the performance of the screening test. METHODS: Measured haemoglobin (Hb) concentrations were aggregated into seasons with their average ambient temperature (AAT). Using logistic regression, the AAT over the period preceding the test measurement was analysed. This period included the time between faecal sampling and return of the test sample (mean 7days) and the time in the laboratory refrigerator before analysis (mean 4days). The AAT from days 5-11 before analysis of the test sample was considered a determinant of test positivity. The Kruskal-Wallis rank test was used to evaluate the significance of seasonal and/or AAT-related differences in Hb concentration. A logistic regression model adjusted for sex, age, season and screening episode (first or repeated examination) was constructed. RESULTS: 199 654 FOBT results were examined. Mean FOBT seasonal Hb concentrations (ng/ml) were: spring 27.6 (95% CI 26.2 to 29.1); summer 25.2 (95% CI 23.1 to 27.3); autumn 29.2 (95% CI 27.7 to 30.6); winter 29.5 (95% CI 27.9 to 31.1). Logistic regression showed that there was a 17% lower probability of the FOBT being positive in summer than in winter. The results of the logistic regression showed that an increase in temperature of 1°C produced a 0.7% reduction in probability of a FOBT being positive. In the summer the probability of detecting a cancer or an advanced adenoma was about 13% lower than in the winter. CONCLUSIONS: This study showed that there is a significant fall in Hb concentration at higher ambient temperatures. These results will have important implications for the organisation of immunochemical FOBT-based screening programmes, particularly in countries with high ambient temperatures.