A universal threshold of 10 μg/g for faecal haemoglobin (f‐ Hb ) in isolation does not meet the aims of NICE guidance in the urgent suspected colorectal cancer pathway

INTRODUCTION: The positive predictive value (PPV) of a test at a particular threshold depends on the prevalence of disease in the population tested. Because the prevalence of colorectal cancer rises with age and iron deficiency anaemia (IDA), applying a universal faecal haemoglobin (f-Hb) threshold of 10 μg/g would yield variable PPV across the population being referred on the urgent suspected cancer (USC) pathway. We used the number needed to scope (NNS) and the cancer conversion rate (CCR) as indicators of the PPV of f-Hb to examine this hypothesis. METHODS: Between 2021/22 and 2024/25, 192,456 patients had an f-Hb result recorded in General Practice. 84,423 USC referrals were recorded; 41,952 were linked to f-Hb results. f-Hb levels were grouped into bands and analysed against cancer diagnoses, colonoscopy activity, demographics, iron deficiency anaemia (IDA), CCR and NNS to detect one cancer. RESULTS: CCR and NNS varied markedly with age and f-Hb. In patients