After application of Congo red dye to the gastric mucosa, the change in color from red to black can be observed by endoscopy at a pH below 3.0, indicating acid production.
In 13 dogs, the time required for complete color change in the parietal cell area was compared preoperatively, after anterior or posterior vagotomy, and following total vagotomy. (1) The interval for complete color change was significantly prolonged in animals with complete vagotomy compared to that in those with no vagotomy. (2) The Congo red test correlated completely with the Hollander test in showing the completeness of vagotomy.
Identical results were found in five patients who had had incomplete vagotomy previously. Furthermore, the site of the intact vagal branch could be localized with the method.
Kusakari K, Nyhus LM, Gillison EW, Bombeck CT. An Endoscopic Test for Completeness of Vagotomy. Arch Surg. 1972;105(2):386-391. doi:10.1001/archsurg.1972.04180080230039