Diagnostic performance of the faecal immunochemical test for patients with low-risk symptoms of colorectal cancer in primary care: a service evaluation in the South West of England

Objectives To evaluate the faecal immunochemical test (FIT) for primary care clinicians to triage patients with low-risk symptoms of possible colorectal cancer, and to estimate its diagnostic performance. Design Service delivery evaluation. Setting All primary and secondary care providers in the South West of England, approximate population 4 million. Participants 3890 patients aged ≥50 years presenting in primary care with low-risk symptoms of colorectal cancer, following NICE NG12 and DG30, with a FIT (HM-JACKarc assay) analysed from 01/06/2018 to 31/12/2018. Main outcome measures Diagnosis of colorectal cancer. Results 618 (15.9%) patients tested positive at a threshold of 10μg Hb/g faeces (median 36μg Hb/g faeces (IQR 17 to 149)); 458 (74.1%) of these had an urgent referral to specialist lower gastrointestinal (GI) services within three months. 43 were diagnosed with colorectal cancer within 12 months. 3272 patients tested negative; 324 (9.9%) were referred on an urgent lower GI pathway in secondary care within three months. 8 were diagnosed with colorectal cancer within 12 months. The positive predictive value of FIT for colorectal cancer in the low-risk symptomatic population was 7.0% (95% CI 5.1% to 9.3%) and the negative predictive value was 99.8% (CI 99.5% to 99.9%). Sensitivity was 84.3% (CI 71.4% to 93.0%), and specificity 85.0% (CI 83.8% to 86.1%). The area under the ROC curve was 0.92 (CI 0.86 to 0.96). A threshold of 37μg Hb/g faeces would identify patients with an individual 3% risk of cancer. Conclusions FIT performs exceptionally well to triage patients with low-risk symptoms of colorectal cancer in primary care. The threshold value of 10μg Hb/g faeces represents a risk of cancer below 3% used in current NICE guidance; however, this lower value may be appropriate to meet the national aspiration of improving cancer diagnostics.