BLOOD PLATELETS 



45 



BLOOD PLATELETS 



the undiluted blood is allowed to come 

 into contact with the surface of the 

 skin and with the external air, thus al- 

 lowing the platelets to clump. In 

 methods in which the finger is punctured 

 through a drop of fluid (Fonio, A., 

 Deutsche Ztschr. f. Chir., 1912, 117, 

 176) the first drop of blood must be 

 used. This procedure is said not to 

 yield accurate counts because the blood 

 contains disintegrated products of 

 crushed and injured cells and is also 

 diluted by an admixture of Ij'^mph. 



With regard to methods in which 

 the blood and diluent are mixed on the 

 finger, Olef stated, "This technic in- 

 volves a number of inaccuracies. Dur- 

 ing the process of stirring the mixture 

 there is unavoidable contact of blood 

 and skin with resulting destruction and 

 clumping of platelets. If the blood is 

 flowing freely, as it should if a correct 

 count is to be obtained, the blood and 

 diluting fluid form a very large drop in 

 which it is rather difficult to obtain a 

 uniform distribution of the blood and 

 which frequently rolls off the finger, 

 especially in women in whom the surface 

 at the tip of the finger is small. Fur- 

 thermore, freely flowing blood mixed 

 with only one drop of preserving fluid 

 yields preparations too thick for ac- 

 curate platelet counting. Dameshek 

 does not stir the blood-diluent mixture 

 at all ; he places a drop of the preserving 

 fluid over the puncture wound after the 

 first drop or two of blood has been wiped 

 away, than allows some blood to escape 

 into the overljdng drop of diluent and 

 by applying a cover slip to the mixture 

 carries some of it away. This pro- 

 cedure is inaccurate because the plate- 

 lets, being very light, quickly rise to 

 the surface of the drop of diluting fluid 

 before the considerably heavier eryth- 

 rocytes have become uniformlj^ dis- 

 tributed. The fluid on the cover slip, 

 therefore, contains a relatively larger 

 number of platelets than red cells." 

 Olef has also pointed out that all glass- 

 ware must be scrupulously clean since 

 hemolysis may occur with soiled glass- 

 ware. For use in wet preparations, he 

 advocates a 1 to 5 dilution of blood, 

 since in very thin preparations both 

 the platelets and erythrocytes are 

 likely to be destroyed, whereas in thick 

 preparations the erythrocytes may ob- 

 scure some of the platelets. Olef's ob- 

 jection to dilutions of blood of greater 

 than 1 to 5 on the grounds that this 

 maj' cause dissolution of the platelets 

 is not valid if a fixative is contained 

 in the diluting fluid. 



In Olef's method, the first drop or 

 two of blood is wiped away. A drop 



of diluting fluid is then placed over 

 the puncture wound before the blood 

 reaches the surface of the skin, and the 

 hand is quickly turned over so that the 

 palmar surface is directed downward. 

 After a sufficiently large drop has es- 

 caped, the entire mixture is applied to 

 the surface of a small quantity (three 

 to four drops) of diluting fluid contained 

 in a paraffin cup. The entire drop on 

 the finger, consisting of approximately 

 equal parts of blood and diluent, drops 

 off into the cup. The contents of the 

 cup are then stirred gently with a 

 wooden applicator, the end of which 

 is coated with paraffin. The mixture 

 is allowed to stand for a minute or two, 

 stirred again, and then is transferred 

 by means of a clean paraffin-coated ap- 

 plicator to a glass slide. Usually three 

 preparations are made. A coverslip 

 is placed over each drop and after the 

 preparations have been allowed to stand 

 for ten to fifteen minutes, a relative 

 platelet-erythrocyte count is made, 

 using the oil immersion lens. While 

 this method has certain advantages over 

 other indirect methods, it does not 

 overcome the principal objection raised 

 by Tocantins, i.e., that the platelets 

 and erythrocytes may not keep an even 

 proportion in numbers toward each 

 other between the two main steps of the 

 method. That they do not can be easily 

 demonstrated by counting both the 

 erythrocytes and platelets in successive 

 microscopic fields in different parts of 

 the same preparation. It will be found 

 that the platelets maintain a fairly even 

 distribution despite large differences 

 in the erythrocyte concentration. This 

 leads to variation in the ratio of plate- 

 lets to erythrocytes in different parts 

 of the same preparation. The observer 

 must arbitrarily choose areas to be 

 counted, but because of unconscious 

 bias in selection he will tend to count 

 only those which he thinks have an 

 average distribution of erythrocytes. 

 However, in this type of preparation it 

 is impossible to determine what the 

 average distribution is. This bias in 

 sampling greatly increases the sub- 

 sampling error. The uneven distribu- 

 tion of erythrocytes occurs regardless 

 of whether the blood is capillary or 

 venous, or whether it has been thor- 

 oughly shaken in a pipette or simply 

 mixed with a stirring rod before the 

 preparation is made. This maldistribu- 

 tion of erythrocytes cannot be avoided 

 since it is caused by physical phe- 

 nomena which occur during the very 

 act of placing the coverslip on the drop 

 of blood. 

 Dry slide preparations made from a 



