Stage IV Rectal Adenocarcinoma After Negative Multitarget Stool DNA-Fecal Immunochemical Test in a Symptomatic Patient: Misapplication of a Screening Test in a Diagnostic Context

Multitarget stool DNA-fecal immunochemical testing (mt-sDNA-FIT) is validated for colorectal cancer (CRC) screening in asymptomatic, average-risk adults. However, its application in symptomatic patients represents a misuse of a screening tool in a diagnostic context and does not meet diagnostic standards as defined by the American Gastroenterological Association (AGA) and the American College of Gastroenterology (ACG). We present a 77-year-old man on warfarin for atrial fibrillation who developed iron deficiency anemia and alternating bowel habits over six months. A prior negative screening mt-sDNA-FIT was interpreted as reassuring one year prior, and colonoscopy was not pursued. Upon hospitalization, computed tomography (CT) revealed a large stool burden and a suspicious hepatic lesion. Colonoscopy demonstrated a circumferential, ulcerated, obstructing rectal adenocarcinoma approximately 8 cm from the anal verge. Positron emission tomography-computed tomography (PET-CT) confirmed metastatic disease involving the liver and lung. This case illustrates two critical points: (1) stool-based screening tests are not validated for diagnostic evaluation of symptomatic patients, and (2) a large distal rectal cancer was missed despite the patient being on anticoagulation, a factor that would theoretically increase fecal hemoglobin and test sensitivity. This is a single case report, and all observations should be interpreted as hypothesis-generating rather than as evidence of reduced test performance. Clinicians should maintain a high index of suspicion for CRC in symptomatic patients regardless of prior negative noninvasive screening results and should pursue a timely colonoscopy.