Some faecal blood tests can’t detect GI bleeding

There are serious differences in performance among three commonly used faecal occult blood tests (FOBTs), often used to detect clinically undetectable upper gastrointestinal (GI) bleeding.

Apart from colorectal cancer screening, the most common use of an FOBT is in the evaluation of anaemia and iron deficiency. Say the researchers from the Mayo Clinic in Rochester, Minnesota, more than two-thirds of the bleeding that leads to anaemia arises from the upper GI tract.

Three tests – guaiac (Hemoccult II), immunochemical (HemeSelect) and heme-porphyrin (HemoQuant) – were evaluated to see how well they detected upper GI bleeding.

Fifty-six patients with iron deficiency and a confirmed haemorrhagic GI lesion were tested along with 10 healthy volunteers, who ingested their own blood to simulate upper GI bleeding.

In those with iron deficiency, the HemoQuant test detected 88 per cent of upper GI bleeding compared with 26 and 2 per cent, respectively, detected by the Hemoccult and HemeSelect tests.

In the volunteers, all tests were negative before blood ingestion. After ingestion of
5 mL of blood, the HemoQuant was positive in 60 per cent while the other two tests remained negative. After ingestion of 15 mL of blood, the HemoQuant was positive in all cases and the Hemoccult was positive in six; HemeSelect was still negative.

These results suggest that the choice of test is crucial for an accurate diagnosis of GI bleeding (Mayo Clin Proc, 2002; 77: 23-8).

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