The fecal immunochemical test (FIT) and the multi-target stool DNA (mt-sDNA) test are commonly used stool-based colorectal cancer (CRC) screening tests and have been recommended for screening by several societies [1–3]. Both FIT and mtsDNA tests are recommended for screening in the US; however, in Europe only FIT is recommended. Both tests are associated with increased polyp yield at colonoscopy and enhance CRC screening by identifying patients with higher polyp risk [4–8]. FIT and mt-sDNA tests detect different markers; past research has found differences in serrated and adenomatous polyp detection between the two tests [6, 7, 9, 10].

Differences in patient population risk factors also influence adenoma and serrated polyp yield. In the general colonoscopy screening population, for example, smoking has been linked with an increased risk for serrated polyps [11–14] and higher rates of conventional adenomas [11–16]. It is unknown, however, whether this association persists in a stool-test-positive population. There is no prior research on polyp yield in patients who smoke and have a colonoscopy after a positive stool test. Does smoking confer an additive risk beyond that of a stool-test-positive population, or is the prevalence of advanced findings in stool-test-positive patients unaffected by smoking status? Clarifying this risk could inform endoscopists’ expectations of polyp yield in stool-test-positive patients who smoke.

In this analysis, we examined the association between smoking status (current, former, and never) and prevalence of advanced findings (advanced lesions, advanced adenoma, and advanced serrated polyps) in patients with colonoscopy after a positive mt-sDNA test, patients with colonoscopy after a positive FIT, and patients with colonoscopy only, without a prior stool test.

We compared findings between smoking categories within each study cohort (mt-sDNA + , FIT + , and colonoscopy only) and also compared findings between all three study cohorts in our sub-sample of current smokers. We used data from the New Hampshire Colonoscopy Registry (NHCR), a statewide, population-based registry.

Our sample included 967 mt-sDNA + patients, 498 FIT + patients, and 58,682 colonoscopy-only patients. Demographic and behavioral characteristics are shown in Table 1.

Table 2 contains the prevalence (and 95% CI) of each polyp outcome by study cohort and smoking status. We found that mt-sDNA + patients generally had the highest prevalence of advanced lesions and advanced serrated polyps, and that within each study cohort, current smokers generally had the highest prevalence of each polyp outcome. We found less variation in advanced adenoma prevalence by smoking status in mt-sDNA + and FIT + patients in our screening-only sensitivity analysis (Supplemental Table 1).

Table 3 contains adjusted odds ratios and 95% CIs for our within-screening-cohort logistic regressions. In line with prior research demonstrating an association between smoking and an increased risk of CRC, we found that among colonoscopy-only patients, odds of all three outcomes increased significantly along smoking status categories (from never to former to current). Among positive stool test patients, however, we found no significant differences in the odds of advanced adenomas by smoking status but did find that current smokers had significantly higher odds of advanced serrated polyps compared to never-smokers in mt-sDNA + patients and compared to both never- and former smokers in FIT + patients. We found similar results in our sensitivity analysis except in the case of advanced serrated polyp odds among mt-sDNA + patients, where we did not find any significant relationships with smoking status (Supplemental Table 2).

Table 4 contains adjusted odds ratios and 95% CIs for our logistic regressions comparing polyp odds by study cohort within current smokers. We found that FIT + (OR = 1.97; 95%CI 1.11–3.50) and mt-sDNA + patients (OR = 3.27; 95%CI 2.25–4.74) had higher odds of advanced lesions, as well as of advanced adenomas and advanced serrated lesions, than the colonoscopy-only group. In our sensitivity analysis, we found that mt-sDNA + patients had significantly higher odds of all outcomes compared to colonoscopy-only patients, while FIT + patients had significantly higher odds of advanced serrated polyps only (Supplemental Table 3).

Smoking has been shown in many studies to be associated with an increased risk for conventional adenomas as well as serrated polyps [11–16]. Positive stool tests are associated with a higher yield of polyps at the first follow-up colonoscopy performed for the positive stool test compared to colonoscopies with no previous stool tests [4, 5, 7]. Our analysis included patients with a positive fecal immunochemical test (FIT) or multi-target stool DNA test (mt-sDNA). Both tests have an antibody to hemoglobin and thus can detect lesions that bleed, including cancer and large conventional adenomas [17]. The multi-target stool DNA test has additional markers for methylation mutations which are observed in serrated polyps. The goal of our study was to examine outcomes for patients with positive stool tests, as stratified by their smoking status. Patients were stratified into current, former, and never smoking groups.

As expected, we found that polyp odds increased significantly with smoking in the colonoscopy-only cohort, but we found fewer significant differences by smoking status within patients with positive stool tests. In particular, we found no significant difference in advanced adenoma odds by smoking status in either FIT + or mt-sDNA + patients. One possible explanation for this finding is that the baseline level of advanced adenoma risk in stool-test-positive never-smokers is high enough that there is no additional risk for advanced adenomas conferred by smoking status in stool-test-positive patients.

However, we did find that smokers had a higher risk for advanced serrated polyps than never-smokers in the FIT + and mt-sDNA + cohorts; the effect for the FIT + cohort persisted in our screening-only sensitivity analysis. The association between smoking and serrated polyp odds appears to persist after a positive stool test. There are several possible explanations for this result. One is that smoking may be more strongly associated with serrated polyp odds, so that smoking still confers additional polyp risk even after a positive mt-sDNA test. In FIT + patients, the explanation may be simpler. While the mt-sDNA test detects markers associated with serrated polyps, the FIT is designed to detect blood from cancers and large adenomas. The relationship between smoking and serrated polyp odds in FIT + patients therefore could be similar to that found in colonoscopy-only patients, since neither cohort has been meaningfully screened for serrated polyps prior to colonoscopy. One consequence of this relationship is that FIT + current smokers may have a substantially higher risk of advanced serrated polyps than would be typically associated with a positive FIT in a non-smoking population, given the low sensitivity of FIT in detecting serrated polyps.

We also examined the current smokers separately as a subgroup (Table 4) and observed that the odds of all three outcomes were significantly higher in patients with positive stool tests compared to the colonoscopy-only cohort; the mt-sDNA association persisted for all outcomes and the FIT + association persisted for advanced serrated polyps in our screening-only sensitivity analysis. However, we did not find significant differences in outcome odds between the two positive stool test cohorts in this subgroup analysis.

Our results have important implications for practicing physicians. When stool tests are used to screen patients, our data suggest that smokers are at increased risk for advanced serrated polyps even among patients with positive stool tests. In our analysis, 1/3 of all smokers with a positive mt-sDNA test had an advanced serrated polyp. Thus, our data demonstrate the elevated risk for patients who smoke and have positive stool tests. Although all patients with a positive stool should have a colonoscopy as follow-up, these data further underscore the importance of smokers having a colonoscopy after a positive stool test, especially for those who have a positive stool test and are declining a follow-up colonoscopy. This is an important message for frontline physicians or providers who order these tests.

Another implication for practicing endoscopists is that smokers with positive stool tests have a high prevalence of advanced lesions, both conventional adenomas and serrated polyps. Current smokers in both positive stool test cohorts had at least a twofold increase in advanced adenoma and advanced serrated polyp odds relative to current smokers without a prior stool test. We also found that current smokers in every study cohort had a higher risk for advanced serrated polyps than never-smokers. For FIT + patients especially, endoscopists need to be just as vigilant in looking for advanced serrated polyps as they are in detecting the advanced conventional adenomas which are more typically associated with FIT + patients.

An important strength of our analysis is that our data are collected from patients who had received stool tests as part of their usual clinical care. In addition, NHCR data are collected from many community and academic settings, and they may better represent real-world practice than a single institutional trial, and thus be more generalizable. However, our analysis does have certain limitations. We do not have data on time since cessation for former smokers; this population may contain patients at varied levels of risk. Although there are data to support the concept that self-reporting of smoking history can be reliable, we acknowledge that this may be a limitation [18]. Additionally, our positive stool test cohorts have smaller sample sizes than our colonoscopy-only cohort, which may result in less precision in our estimates than would be suggested by our overall sample size.

In summary, we observed that FIT + and mt-sDNA + patients who currently smoke have higher yields of advanced serrated polyps than stool-test-positive patients who never smoked. Furthermore, FIT + and mt-sDNA + patients who currently smoke have higher yields of advanced outcomes than current smokers without a prior stool test. When patients are screened with stool tests, these data underscore the importance of follow-up colonoscopies in patients with positive stool tests, especially for patients who smoke. Endoscopists need to be vigilant in detecting advanced serrated polyps in patients who smoke and have a positive stool test.

Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s10620-025-08918-y.