Objectives: Positive rates in faecal immunochemical test (FIT)-based colorectal cancer screening programmes vary, suggesting that differences between programmes may affect test results. We examined whether demographic, pathological, behavioural, and environmental factors affected haemoglobin concentration and positive rates where samples are mailed. Methods: A retrospective cohort study; 34,298 collection devices were sent, over five years, to screening invitees (median age 60.6). Participant demographics, temperature on sample postage day, and previous screening were recorded. Outcomes from colonoscopy performed within a year following FIT were collected. Multivariate logistic regression identified significant predictors of test positivity. Results: Higher positive rate was independently associated with male gender, older age, lower socioeconomic status, and distally located neoplasia, and negatively associated with previous screening (p<0.05). Older males had higher faecal haemoglobin concentrations and were less likely to have a false positive result at colonoscopy (p<0.05). High temperature on the sample postage day was associated with reduced haemoglobin concentration and positivity rate (26–35˚C: Odds ratio 0.78, 95% confidence interval 0.66–0.93), but was not associated with missed significant neoplasia at colonoscopy (p>0.05). Conclusions: Haemoglobin concentrations, and therefore FIT positivity, were affected by factors that vary between screening programmes. Participant demographics and high temperature at postage had significant effects. The impact of temperature could be reduced by seasonal scheduling of invitations. The importance of screening, and following up positive test results, particularly in older males, should be promoted.