The occult blood test.

The original occult blood tests were designed for forensic purposes, Schonbein and van Deen using a chemical test with guaiacum which was more sensitive than the older Teichmann's haemin crystal test. Subsequently similar chemical methods were widely used and in I893 Weber described his version of the guaiacum test for detecting occult blood in the faeces. Boas first made extensive use of this method at the end of the last century, and in 19oI emphasized the importance of detecting occult haemorrhage from the intestine as a diagnostic measure. Types of Test The methods at present in use depend on either chemical or spectroscopic examination of a suitable extract. There are many chemical methods in addition to that described by Weber. The benzi-dine test was devised by the Adlers in i904 and modified, in 1906, by Schlesinger and Hoist, whose technique, or that evolved by Gregersen, is probably used most of all. Other methods, the relative sensitivity of which will not be considered, include Meyer's alkaline phenolphthalein test, the orthoto-lidine test of Rutton and Hardisty, and techniques using pyramidon and aloin. All such methods depend on a change in colour of the reagent after oxidation, this being facilitated by the peroxidase activity of iron-containing derivatives of haemo-globin, which-is not destroyed by boiling. The spectroscopic tests, as described by Snapper (I916) and Ryffel, are of more recent development, and depend on the detection of the typical absorb-tion bands of acid haematin or porphyrins. Concerning their Sensitivity There have been many reports on the relative sensitivity of the various tests with conflicting conclusions, but it is certain that the chemical are more sensitive than the spectroscopic tests (Ryffel and Payne, 1923). Although positive reactions with the guaiacum test may be obtained with blood in a one in eight million dilution with water, this is of no real significance since the presence of organic matter greatly decreases this sensitivity, and when blood is freshly added to faeces a concentration of one in 2,500 may be necessary. This sensitivity is further diminished by the progressive reduction in the peroxidase activity of free blood by digestion, and possibly by abnormalities of the intestinal secretions. Constipation may decrease, and diarrhoea increase, the likelihood of a positive result because of this, and examination of the faeces after a dose of castor oil has been advised (Kirschen et al., I942). Both pus and iodides in the specimen can in …