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Simple Summary: Abomasal lesions in dairy cattle are highly prevalent, but diagnosis of the non-penetrating lesions is a challenge. We performed one experiment to estimate the amount of blood detectable in feces using two different tests, one experiment to determine if hemoglobin is degraded in the rumen to find possible false-positive tests due to blood from extra gastrointestinal sources and subsequently an observational study to estimate the diagnostic properties of the test with the observed lower detection limit. The observational study included primarily dairy cattle at slaughter, where we could observe the actual lesions postmortem. The detection limits of the tests marketed as Hemo-Fec® and Hemoccult II® SENSA® were 1–2 mL blood/kg feces and 2–4.5 mL blood/kg feces, respectively. Hemoglobin was not degraded in ruminal fluid and could possibly bypass the rumen and be detected in feces. In the observational study, the Hemo-Fec® test had no diagnostic value in dairy cattle with superficial erosions, with scarring, and with <4 acute or chronic lesions. The test had diagnostic potential in cattle with ≥4 acute or chronic lesions, where the proportion of true positives exceeded the proportion of false-positive results. However, many false-positive reactions make the use of the test a challenge. Non-perforating abomasal lesions occur with a high prevalence in slaughtered dairy cattle. Ante mortem diagnosis is a challenge, but the presence of occult blood in feces is suggested as a diagnostic criterion. The lower detection limit of Hemo-Fec® (Med-Kjemi, Asker, Norway) and Hemoccult II® SENSA® (Beckman Coulter, Brea, California, USA) for fecal occult blood were estimated. The Hemo-Fec® and Hemoccult II® SENSA® could detect 1–2 mL and 2–4.5 mL of blood in 1000 g of feces, respectively. Therefore, the Hemo-Fec® test was selected to access hemoglobin degradation in the rumen to establish if blood from outside the gastrointestinal tract could result in false-positive tests and an observational study to estimate the diagnostic sensitivity and specificity. Rumen microbiota did not degrade hemoglobin in a 1% blood concentration in vitro during 48 h of fermentation. The Hemo-Fec® test was only able to detect cattle with ≥4 acute lesions (diagnostic sensitivity: 0.40 [95% confidence interval (95% CI): 0.32–0.48] and ≥4 chronic lesions (sensitivity: 0.44 [95% CI: 0.35–0.52]). The Hemo-Fec® test had no diagnostic potential to detect superficial erosions or scar tissue in abomasa. Furthermore, the specificity was 0.71 [95% CI: 0.68–0.75%], and a positive test is thus not equivalent with abomasal lesions in cattle. [ABSTRACT FROM AUTHOR]