Some fecal occult blood tests insensitive for upper GI bleeding

מתוך medicontext.co.il
By Megan Rauscher

NEW YORK (Reuters Health) – There are substantial performance differences between three commonly used fecal occult blood tests (FOBTs) for the detection of occult upper GI bleeding, study results indicate.

"Aside from colorectal cancer screening, the most common application of FOBTs is for the evaluation of anemia and iron deficiency," Dr. David A. Ahlquist of the Mayo Clinic in Rochester, Minnesota, explained to Reuters Health. "More than two thirds of bleeding that leads to anemia arises from the upper GI tract, and blood from the upper GI tract is chemically altered by digestive enzymes during transit through the intestines to a much greater extent than that from the colon or rectum."

Dr. Ahlquist's team compared rates of detection of upper GI bleeding by guaiac (Hemoccult II), immunochemical (HemeSelect) and heme-porphyrin (HemoQuant) tests in 56 patients with iron deficiency and a confirmed hemorrhagic GI lesion. The tests were also conducted with 10 healthy volunteers who ingested 5 and 15 mL autologous blood to simulate upper GI bleeding.

In the iron-deficient subjects, the HemoQuant test detected 88% of upper GI bleeding compared with 26% and 2% detected by the Hemoccult and Hemeselect, respectively.

In the volunteers, all tests were negative before ingestion of autologous blood. After ingestion of 5 mL of blood, HemoQuant was positive in 60% of subjects while other two tests remained negative. After ingesting 15 mL of blood, the HemoQuant was positive in all subjects, while the Hemoccult was positive in 6 and the HemeSelect remained negative.

"Immunochemical tests, which target hemoglobin proteins, and the widely used guaiac tests, which target intact hemoglobin heme, are insensitive for upper GI bleeding due to degradation of hemoglobin target sites," Dr. Ahlquist told Reuters Health. These two tests can confound the evaluation of anemia or iron deficiency due to false-negative results, he warned.

The HemoQuant test, on the other hand, which targets hemoglobin-derived porphyrin, "is not influenced by the anatomic site of bleeding and proved to be the most sensitive test by a wide margin for detection of upper GI bleeding," Dr. Ahlquist said.

Based on their study, the Mayo Clinic team recommends HemoQuant as the fecal occult blood test of choice for the indication of anemia.

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