1. A macroscopic platelet agglutination method is described which, when properly standardized, can be used to titrate platelet-agglutinating factors in plasma, serum, or other substances and also to determine the degree of agglutinability of platelets.

2. The platelet suspension is prepared by isolating platelets from blood kept incoagulable with sodium citrate, sodium oxalate, magnesium sulfate, sodium heparin, or physiologic saline. These suspensions of isolated platelets are thoroughly washed and are free of white cells, red cells, and plasma.

3. The concentration of the platelet suspension, the time and temperature of incubation affect the intensity of the agglutination, and therefore these variables must be standardized in order to obtain comparative results.

4. Readings were performed by subjecting the tests to mild centrifugal force and reading the degree of agglutination upon resuspension.

5. Suspensions of isolated platelets are stable in saline and may be sedimented by centrifugation, and resuspended without agglutination. Upon the addition of homologous or heterologous plasma or serum, such platelets usually become unstable and agglutinate.

6. Platelets obtained from various types of plasmas from the same individual differed in agglutinability. Plasmas obtained from the same individual or dog by the use of various anticoagulants may or may not show different concentrations of the platelet-agglutinating factor.

7. Plasma obtained by the same method, or serum, from different individuals exhibit significant differences in ability to agglutinate the same platelet suspensions. Platelets obtained by the same method from different subjects vary in agglutinability when tested against one plasma or serum.

8. Sera usually agglutinated platelets more intensely than plasmas and also caused more degenerative changes in platelets.

9. The platelets of pigs are extremely susceptible to agglutination, and the platelet agglutinates disperse easily upon agitation. The pig platelets agglutinate in very high dilutions of pig or dog plasma or serum.

10. Preliminary tests indicate that the inactivation of the platelet-agglutinating factor in plasma and serum by heat or by filtration is variable and complex.

11. The serum of one case of hemophilia was able to agglutinate a suspension of isolated platelets from a normal subject.

12. Darkfield observations revealed that isolated platelets are normal in morphology, and that the granules are very obvious. In addition, isolated platelets are stable in saline but agglutinate upon the addition of certain plasmas and sera. Following agglutination, the platelets fuse or hyalinize to a greater or lesser degree, depending on the substrate. Isolated platelets do not appear to be susceptible to true lysis.

13. The mechanism of platelet agglutination and its relation to blood coagulation and thrombotic and hemorrhagic disorders is discussed. An attempt has been made to clarify thrombophilia and thrombopathy and to redefine them by applying the terms platelet hyperagglutinability and platelet hypo-agglutinability, respectively.

14. The possible application of our platelet-agglutination test to the study of certain thrombotic and hemorrhagic conditions is discussed.

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