Combining faecal haemoglobin, iron deficiency anaemia status and age can improve colorectal cancer risk prediction in patients attending primary care with bowel symptoms: a retrospective observational study

Background In primary care, National Institute for Health and Care Excellence suspected cancer guidelines recommend measuring faecal haemoglobin (f-Hb) if colorectal cancer (CRC) is suspected, with a referral threshold of ≥10 µg Hb/g faeces defining a 3% risk, but most have a normal colonoscopy. Objective Examine whether combining f-Hb, patient age and iron-deficient anaemia (IDA) status improves risk prediction. Design Retrospective single-centre observational study of symptomatic patients who submitted contemporaneous f-Hb and full blood count (FBC) samples between December 2015 and December 2019. f-Hb was estimated using HM-JACKarc (Hitachi Chemical Diagnostics Systems). Patients were categorised by presence/absence of IDA. Incident CRC was identified via record linkage to the Scottish Cancer Registry. Kaplan-Meier estimates determined cumulative 1-year CRC risk by patient age, f-Hb result and presence of IDA. Results Of 34 647 valid f-Hb results retrieved; 7889 (22.8%) had f-Hb≥10 µg Hb/g. Of these, 33 285 samples (96.1%) had associated FBC results of which 3000 (9.0%) had IDA. Overall, 571 incident CRC were recorded. The risk of CRC breached 3% in patients with f-Hb>99 µg Hb/g aged >40 years and reached 30% (19.4–41.0) with f-Hb>99 µg Hb/g in age >55 years plus IDA. 2029 f-Hb results (25.7%) were in the 10–19 µg Hb/g range of which 27 (1.3%) had CRC. In this subgroup, CRC risk did not exceed 3% in patients <85 years and no IDA. Conclusion Combining f-Hb, patient age and IDA status improves CRC risk prediction, identifies a low-risk group with f-Hb<20 µg Hb/g and no IDA and could inform revised referral guidance.