Green-coloured results on guaiac-based faecal occult blood testing should be considered positive

In guaiac-based faecal occult blood tests (FOBT), blue colours are considered positive. Blue-green colours should also be considered positive. Distinct green colours are said to be due to bile and it is stated that these should be interpreted as negative. The purpose of this study was to determine the clinical outcomes in individuals in whom the FOBT had difficult-to-interpret green colours that did not wash out on addition of developer.

During the examination of 134 844 FOBT received in the Scottish laboratory in the first screening round of the UK Colorectal Cancer Screening Pilot, samples with green colour that did not wash out during development were identified. The clinical outcomes were determined from the comprehensive data set collected for each participant.

A small number (77) of FOBT were recorded as green-coloured on development. These were reported as positive and the usual investigation algorithm followed. Significant pathology was present in 31 of the 77 participants (40.3%). Negative outcomes encompassed 39 of the 77 participants (50.6%). The outcome could not be determined accurately for seven of the 77 participants (9.1%). Importantly, 17 of the participants (22.1%) had polyps.

Any green colour that does not wash out to the periphery of the guaiac tape on development of FOBT should be reported as a positive result, and manufacturers should clarify their instructions on interpretation.