Our data suggest that FIT was not a clinically useful test in determining a patient’s need for diagnostic follow-up. While having a positive FIT was significantly associated with obtaining a diagnostic exam in multivariate analysis (RR, 1.72), having signs of overt GI bleeding was a stronger predictor of diagnostic follow-up (RR, 2.00). This salient finding is evidence that a thorough clinical history and physical exam may more strongly predict whether a patient will undergo endoscopy or other follow-up than a FIT result. These findings support other studies in the literature that have called into question the utility of FOBTs in these acute settings.13-19 Under such circumstances, FOBTs have been shown to rarely influence patient management and thus represent an unnecessary expense.13–17 Additionally, in some cases, FOBT use in these settings may negatively affect patient outcomes. Such adverse effects include delaying treatment until results are returned or obfuscating indicated management with the results (eg, a patient with indications for colonoscopy not being referred due to a negative FOBT).13,14,17
We found that, for patients who subsequently went on to have diagnostic follow-up (most commonly endoscopy), there was no difference in the likelihood of FIT-positive and FIT-negative patients to have an abnormality discovered (91.2% vs 90.6%; P = 0.86). This analysis demonstrates no post-hoc support for FIT positivity as a predictor of presence of pathology in patients who were discriminately selected for diagnostic follow-up on clinical grounds by gastroenterologists and surgeons. It does, however, further support that clinical judgment about the need for diagnostic follow-up—irrespective of FIT result—has a very high yield for discovery of pathology in the acute setting.
There are multiple reasons why FOBTs, and specifically FIT, contribute little in management decisions for patients with suspected GI blood loss. Use of FIT raises concern for both false-negatives and false-positives when used outside of its indication. Regarding false- negatives, FIT is an unreliable test for detection of blood loss from the upper GI tract. As FITs utilize antibodies to detect the presence of globin, a byproduct of red blood cell breakdown, it is expected that FIT would fail to detect many cases of upper GI bleeding, as globin is broken down in the upper GI tract.24 This fact is part of what has made FIT a more effective CRC screening test than its guaiac-based counterparts—it has greater specificity for lower GI tract blood loss compared to tests relying on detection of heme.8 While guaiac-based assays like Hemoccult have also been shown to be poor tests in acute patient care, they may more frequently, though still unreliably, detect blood of upper GI origin. We believe that part of the ongoing use of FIT in patients with a suspected upper GIB may be from lack of understanding among providers on the mechanistic difference between gFOBTs and FITs, even though gFOBTs also yield highly unreliable results.
FIT does not have the same risk of false-positive results that guaiac-based tests have, which can yield positive results with extra-intestinal blood ingestion, aspirin, or alcohol use; insignificant GI bleeding; and consumption of peroxidase-containing foods.13,17,25 However, from a clinical standpoint, there are several scenarios of insignificant bleeding that would yield a positive FIT result, such as hemorrhoids, which are common in the US population.26,27 Additionally, in the ED, where most FITs were performed in our study, it is possible that samples for FITs are being obtained via digital rectal exam (DRE) given patients’ acuity of medical conditions and time constraints. However, FIT has been validated when using a formed stool sample. Obtaining FIT via DRE may lead to microtrauma to the rectum, which could hypothetically yield a positive FIT.
Strengths of this study include its use of in-depth chart data on a large number of FIT-positive patients, which allowed us to discern indications, outcomes, and other clinical data that may have influenced clinical decision-making. Additionally, whereas other studies that address FOBT use in acute patient care have focused on guaiac-based assays, our findings regarding the lack of utility of FIT are novel and have particular relevance as FITs continue to grow in popularity. Nonetheless, there are certain limitations future research should seek to address. In this study, the diagnostic follow-up result was coded by presence or absence of pathologic findings but did not qualify findings by severity or attempt to determine whether the pathology noted on diagnostic follow-up was the definitive source of the suspected GI bleed. These variables could help determine whether there was a difference in severity of bleeding between FIT-positive and FIT-negative patients and could potentially be studied with a prospective research design. Our own study was not designed to address the question of whether FIT result informs patient management decisions. To answer this directly, interviews would have to be conducted with those making the follow-up decision (ie, endoscopists and surgeons). Additionally, this study was not adequately powered to make determinations on the efficacy of FIT in the acute care setting for detection of CRC. As mentioned, only 1 of the 4 patients (25%) who went on to be diagnosed with CRC on follow-up was initially FIT-positive. This would require further investigation.