For scientific purposes these very careful tests are necessary, but it seems to be clear that for clinical purposes a much rougher and quicker test is adequate. In the clinical determination of blood groups it is superfluous to carry the test to the point of watching for hæmolysis, for it is upon the presence of agglutinins in the serum and the corresponding iso-agglutinins in the corpuscles that the determination of the groups depends. Further, no error is introduced by neglecting the hæmolysis, since it has been shown that hæmolysis is invariably preceded by agglutination. It is the occurrence of agglutination therefore that is of prime clinical importance. If that is excluded, hæmolysis is necessarily excluded also, and the prolonging of the test is seen to be only of academic interest. In the methods described above the corpuscles were always tested in the form of a washed suspension. This precaution was taken on the supposition that the presence of any of the serum belonging to the corpuscles might interfere with the reaction. If, however, the amount of this serum be small relatively to the amount of the test serum, then no such interference takes place.
[Illustration: Fig. 7.--TABULATION OF SERUM REACTIONS AS SEEN IN HANGING DROPS.]
The ordinary clinical method of testing may therefore be greatly simplified, and the one commonly used at the present time is as follows: A single drop of each of the stock sera is placed on two glass slides, or, better, side by side upon a white glazed tile or plate, the numbers of the groups, II and III, being written above the respective drops. The lobe of the ear of the person to be tested is then washed with ether and pricked with a sterile surgical needle. A small quantity of the blood which exudes is taken up on the end of a blunt metal or glass rod, and is intimately mixed with the drop of serum under the number II. The end of the rod is then carefully wiped clean, and a similar small quantity of blood is mixed with the drop of serum marked III. The amount of blood to be used should not be so great as to make the drop of too deep a colour, which may interfere with observation of the reaction, but it should be enough to impart to it a very definite red tint. The slide or tile is then gently rocked, so that some slight movement is imparted to the drops, which are at the same time closely watched in a good light. The agglutinating reaction is readily seen with the naked eye, especially against the white background provided by the tile. If the serum be properly active, the agglutination of the corpuscles begins to be apparent as a definite granular appearance resembling brick dust within a minute of mixing. With a little practice this appearance is easily recognized, but it must be distinguished from the appearance produced by a mechanical gravitation of the corpuscles towards the centre of the drop. If agglutination is taking place, the granulation appears simultaneously throughout the drop, and not only in the centre. With an active serum the process may proceed rapidly, so that in less than five minutes the corpuscles have been aggregated into a few irregular masses; often it stops short of this, but the drop presents, nevertheless, a coarsely granular appearance which is quite unmistakable. If no granulation can be seen at the end of five minutes, it can be assumed that the test is negative for the serum of that group, and the group of the corpuscles may be deduced upon the principles already explained.