Methylene blue in the evaluation of gastrointestinal tract integrity: potential limitations

The integrity of the gastrointestinal tract can be evaluated by oral or rectal application of methylene blue. In the presence of anastomotic leaks or fistulas, methylene blue can be recovered in adjacent drains. However, parts of the dye can biochemically be reduced by intestinal bacteria to its colorless form leucomethylene blue, limiting the prediction of the test.

Diluted methylene blue was added to different concentrations of bacterial suspensions of Escherichia coli and Enterococcus faecalis. The time for discoloration of the suspension was measured.

Reduction of methylene blue to leucomethylene blue was observed in both types of bacterial suspension. A 10(8) bacterial concentration discolorated the dye within 1 h in the E. faecalis suspension, respectively 2.5 h in the E. coli suspension. Longer bacterial interaction with methylene blue reduced the bacterial concentration required to achieve complete discoloration.

Methylene blue can reliably be used as routine diagnostic test for the assessment of upper gastrointestinal integrity, where bacterial load is low. In the lower gastrointestinal tract, where bacterial load is generally higher, the dye can indicate leaks, only if extended intestinal passage after oral ingestion is avoided. In all other cases, the examiner has to be aware of false-negative results by bacterial discoloration of methylene blue.