BLOOD PLATELETS 



44 



BLOOD PLATELETS 



better visualization of the platelets was 

 nullified. The variations in the platelet 

 count appear to be due to differences 

 in the volume of fluid contained in the 

 chamber caused by upward lifting or 

 downward bending of the coverslip. 

 Before this technique can be perfected, 

 it will be necessary to develop methods 

 for insuring scrupulous cleanliness of 

 the glassware. In addition, a radical 

 change in the design of the chamber to 

 prevent downward bending of the cover- 

 slip due to intense capillary attraction 

 will be required. 



With regard to the established direct 

 methods of platelet counting, there- 

 fore, it appears that most of the sources 

 of error can be minimized, except that 

 of accurate identification of the plate- 

 lets. This error can be made a constant 

 for a given method by an experienced 

 observer, but may lead to large in- 

 consistent variations in platelet counts 

 done by inexperienced individuals. 



Indirect Methods: One of the principal 

 advantages of indirect methods of 

 platelet counting is that microscopic 

 objectives giving higher magnification 

 and greater resolution can be employed. 

 Furthermore, the blood can be mixed 

 immediately with the diluting fluid 

 and the platelets fixed before it is neces- 

 sary for them to come in contact with 

 any foreign surface, except perhaps 

 momentarily with the skin or a veni- 

 puncture needle. In all indirect 

 methods of platelet counting unknown 

 quantities of blood and diluent are 

 mixed. The number of platelets seen 

 per 1000 erythrocytes is multiplied by 

 the number of thousands of erythro- 

 cytes found in an independent erythro- 

 cyte count done in the usual manner. 

 There are three methods of determining 

 the ratio of platelets to erythrocytes: 

 in a counting chamber, a wet slide 

 preparation or a dry smear. 



Methods, such as that of Kemp, G. T. 

 and Calhoun, H. (Brit. Med. J., 1901, 

 2, 1539) in which capillary blood is em- 

 ployed and the ratio of erythrocytes 

 to platelets is determined in the count- 

 ing chamber, are subject to the same 

 error involved in identifying the plate- 

 lets as is inherent in all direct methods 

 of platelet counting. 



Methods such as those of Aynaud, M. 

 (Compt. Rond. Soc. Biol., 1910, 68, 

 1062) and Preiss, W. (Zeitsch, Ges. 

 Exp. Med., 1932, 84, 810) referred to 

 above, in which venous blood is used 

 are sometimes referred to as indirect 

 methods. They are not, since in these 

 methods, the ratio of the erythrocyte 

 count in the platelet preparation to the 

 erythrocyte count done in the usual 



manner only serves to establish the dilu- 

 tion of blood employed in the plate- 

 let count. The number of platelets 

 counted per unit volume is an absolute 

 not a relative value. The additional 

 step of establishing the ratio of the two 

 erythrocyte counts could be avoided 

 by accurate measurement of the degree 

 of dilution of the blood in the syringe. 

 These methods are also subject to the 

 errors involved in identifying platelets 

 with the high dry lens in the standard 

 counting chamber. 



Some of the objections to the indirect 

 method of platelet counting in which 

 the ratio of platelets to erythrocytes 

 is determined in a wet preparation or 

 a dry smear have been summarized by 

 Tocantins (Arch. Path., 1937, 23, 850). 

 1) The mixture of blood and diluting 

 solution is seldom, if ever, uniform and 

 not the same each time. 2) Platelets 

 and erythrocytes are not distributed 

 evently through the preparation, since 

 no provision is made for shaking before 

 counting. 3) The method has defects 

 intrinsic in any determination done 

 indirectly, that is, in relation to another 

 equally changeable element. 4) The 

 greatest source of error, however, is 

 in the assumption that platelets and 

 erythrocytes keep an even proportion 

 in numbers toward each other between 

 the two main steps of the method. 

 The markedly different physical proper- 

 ties (adhesiveness, specific gravity, 

 size and others) of platelets and eryth- 

 rocytes lead to continuous changes in 

 this ratio. 5) The proportion of plate- 

 lets to erythrocytes varies at the same 

 time in different portions of the circula- 

 tion and this variation is even more 

 marked within short spaces of time in 

 capillaries than venules. 



The term "indirect platelet count" 

 has come to imply that the count is 

 done on capillary blood. Curiously 

 enough it seems never to have occurred 

 to anyone to do an indirect platelet 

 count on accurately diluted venous 

 blood. This would overcome some of 

 Tocantins' objections since it would 

 allow for a constant uniform dilution 

 of the blood, shaking of the diluted 

 blood before mixing and the determina- 

 tion of the erythrocyte count on the 

 same specimen as is used for the plate- 

 let count. 



Olef (J. Lab. & Clin. Med., 1935, 20, 

 416) has also raised objections to the 

 technique employed in certain indirect 

 methods of platelet counting. In those 

 in which a drop of blood is allowed to 

 fall into the diluting fluid on a glass 

 slide or contained in a special vessel 

 (Pratt, J.H.J. A. M. A., 1905, 45, 1999). 



