(1) Typhoid Fever.--Practically all cases give a positive reaction, which varies in intensity with the severity of the disease. It is so constantly present that it may be said to be "negatively pathognomonic": if negative at a stage of the disease when it should be positive, typhoid is almost certainly absent. Upon the other hand, a reaction when the urine is highly diluted (1:50 or more) has much positive diagnostic value, since this dilution prevents the reaction in most conditions which might be mistaken for typhoid; but it should be noted that mild cases of typhoid may not give it at this dilution. Ordinarily the diazo appears a little earlier than the Widal reaction--about the fourth or fifth day--but it may be delayed. In contrast to the Widal, it begins to fade about the end of the second week, and soon thereafter entirely disappears. An early disappearance is a favorable sign. It reappears during a relapse, and thus helps to distinguish between a relapse and a complication, in which it does not reappear. (2) Tuberculosis.--The diazo reaction has been obtained in many forms of the disease. It has little or no diagnostic value. Its continued presence in pulmonary tuberculosis is, however, a grave prognostic sign, even when the physical signs are slight. After it once appears it generally persists more or less intermittently until death, the average length of life after its appearance being about six months. The reaction is often temporarily present in {91} mild cases during febrile complications, and has then no significance. (3) Measles.--A positive reaction is frequently obtained in measles, and may help to distinguish this disease from German measles, in which it does not occur. * * * * * Technic.--Although the test is really a very simple one, careful attention to technic is imperative. Many of the early workers were very lax in this regard. Faulty technic and failure to record the stage of the disease in which the tests were made have probably been responsible for the bulk of the conflicting results reported. Certain drugs often given in tuberculosis and typhoid interfere with or prevent the reaction. The chief are creosote, tannic acid and its compounds, opium and its alkaloids, salol, phenol, and the iodids. The reagents are: (1) Saturated solution sulphanilic acid in 5 per cent. hydrochloric acid. (2) 0.5 per cent. aqueous solution sodium nitrite. (3) Strong ammonia.