A registry-based study on universal screening for defective mismatch repair in colorectal cancer in Denmark highlights disparities in screening uptake and counselling referrals

Universal screening for defective mismatch repair (dMMR) in colorectal cancer utilizes immunohistochemical staining for MLH1, MSH2, MSH6 and PSM2. Additionally, BRAF V600E mutations status and MLH1 hypermethylation should be performed to distinguish germline and somatic dMMR alterations. A decade of Danish population-based registries has been analysed regarding screening uptake, detection rate and referral to genetic counselling. MMR testing was performed in 71·8% (N = 34,664) of newly diagnosed colorectal cancers with an increasing trend to 88·8% coverage in the study's final year. The likelihood of undergoing MMR testing was reduced in males with 2% (95% CI 0·4–2·7, p = 0·008), with 4·1% in patients above age 70 years (95% CI 1·5–6·6, p = 0·003) compared in patients below age 51 years, with 16·3% in rectal cancers (95% CI 15·1–17·6, p < 0·001) and 1·4% left-sided colon cancers (95% CI 0·1–1·7, p = 0·03) compared to right-sided colon cancers. Tumour stage II and III increased the likelihood of being tested, with 3·7% for stage II (95% CI 2·2–5·6, p < 0·001) and 3·3% for stage III tumours (95% CI 1·8–4·8, p < 0·001) compared to stage I tumours, whereas the likelihood for stage IV tumours is reduced by 35·7% (95% CI 34·2–37·2, p < 0·001). Test rates significantly differed between the Danish health care regions. dMMR was identified in 15·1% (95% CI 14·8–15·6, p < 0·001) cases with somatic MMR inactivation in 6·7% of the cases. 8·3% tumours showed hereditary dMMR expression patterns, and 20·0% of those were referred to genetic counselling. Despite the high uptake rates, we found disparities between patient groups and missed opportunities for genetic diagnostics.