Blood Smears 221 



The longer the washing with water, the more stain is removed from 

 the white cells. Usually only a dip or two is sufficient, but if the white 

 cells are overstained, differentiate them by longer washing. 



If the slides are overstained, the erythrocytes are too red, the white 

 cells too pale, or the stain has precipitated, the slides can be recov- 

 ered: (1) Cover the entire slide almost to excess with additional 

 Wright's stain: 15-30 miniues; (2) rinse with distilled water; (3) dry. 

 (Morrison and Samwick, 1940) 



Preparation of Thick Blood Films 



This type of film concentrates a relatively large quantity of blood in a 

 small area, thereby increasing the possibilities of finding parasites. The 

 concentration and timing of staining are adjusted so the action is 

 stopped at the point when the leukocytes have stained, some hemo- 

 globin has been dissolved and the red cell membranes have not yet 

 begun to stain. At this point the leukocytes, platelets and protozoa only 

 are stained and lie on an unstained or very lightly stained backgroinid, 

 yellowish from remaining hemoglobin. Freshly prepared films stain 

 better than films one or more days old. 



Piuicture the skin deep enough to form a large drop of blood. On a 

 slide cover a space the size of a dime with enough blood (aboiu 3-4 

 average drops) to spread easily. Too much blood will crack and peel 

 off when dry. Smear it by circling the slide imder the finger withoiu 

 making actual contact. Some find it easier to swirl the blood with a dis- 

 secting needle or the corner of a slide. Practice will determine the best 

 method and how^ much blood to use. Too thin a film has no advantage 

 over a thin smear. An ideal film is several cell layers thick in the center, 

 tapering off to one cell thickness at the periphery. 



Allow the slide to dry in a horizontal position; if tilted the blood will 

 ooze to one edge of the film. Protect from dust and flies, and do not use 

 excessive heat for drying. 



Thick Film Method 



solutions: Field (1941) 

 Solution A: 



methylene blue, C.I. 52015 0.8 gm. 



azure A, C.I. 52005 0.5 gm. 



dibasic sodium phosphate, anhydrous 5.0 gm. 



monobasic potassium phosphate 6.25 gm. 



distilled water 500.0 ml. 



