Direct compatibility testing.

Despite the rapid development of knowledge it still remains fundamental that mistakes within the ABO and Rhesus systems are responsible for virtually all fatal mismatched transfusions. It therefore cannot be too highly stressed that the careful preliminary grouping of the bloods of all patients for transfusion is of prime importance, irrespective of whether there is a previous record of their blood group or not. This should always be regarded as an essential part of any cross-maiching routine, and in urgent cases Stratton's (1955) " sandwich technique" is highly recommended for rapid Rh typing. The " emergency crossmatch " presents a problem, and the vital question which needs answering is: What can safely be accepted as the minimum incubation period which will not impair the efficiency of the particular compatibility tests being used ? The concept of gradual red cell sensitization progressively increasing with time is fallacious. Antigen-antibody union takes place quite rapidly, and, after a relatively short time, which varies slightly with different methods, further prolonged incubation does not increase the sensitivity or render the detection of very weak antibodies significantly more likely. This is shown by the investigations which are detailed later, and the results obtained by Lewis and Chown (1957) using a 10-minute albumin agglutination technique substantiate this view. The ideal cross-matching technique should provide a simple, easily performed procedure which is not only highly sensitive but which at the same time will yield reliable results when used routinely. Certain practical points have been investigated, and on the basis of the findings obtained a simple direct matching procedure, which is not timeconsuming but still combines the advantages of both trypsin and Coombs testing, is recommended. The Indirect Coombs Test (A.H.G.) Minimum Incubation Period for Reliable Results.Twenty-five sera were tested in triplicate, all tests being standardized so that 4 vol. of serum was used to sensitize 1 vol. 50% suspension of washed packed Group 0 Rjr cells. Incubation periods of 10, 30, and 120 minutes were used. The test sera ranged in potency from extremely weak ones, giving only a trace albumin agglutination, to strong sera with albumin titres up to 1/128. Readings were made by the usual tile method and the reactions timed with a stopwatch. If no agglutination had occurred at the end of seven minutes a negative result was recorded. All tests gave positive results although some of the " 10-minute incubation " ones were rather weak. The usual reaction in these cases was a fine granularity first appearing between two and four minutes, gradually becoming clear cut, but still tending to retain a homogeneous pink background at the end of seven minutes. The reactions of the " 30-minute incubation tests'" were all strong and clear cut. They were as good as those obtained by longer incubation and in a surprising number of instances they were somewhat better defined than the corresponding two-hour agglutinations. There seems little doubt that the optimal incubation time for complete sensitization is much shorter than is generally appreciated. Using the indirect Coombs for direct compatibility testing, it is considered that an incubation time of 30 minutes is adequate. In an emergency, a 10or 15-minute Coombs test is an extremely valuable procedure which is capable of detecting very weak sensitization.