Abstract
It is a common experience in doing differential white blood cell counts by the supravital method 1 , 2 that the cells cannot be identified after about 2 hours in the hot-box, since by this time nearly all the cells have taken up large amounts of the dye.
It has been found that by a relatively slight modification of the usual technique, differential counts may be made with great facility as long as 12 to 24 hours after taking the blood. This has been accomplished (1) by reducing the concentration of the dye used, (2) by placing the blood smears in the refrigerator until ready for counting, and (3) by eliminating the hot-box. Through the use of this procedure, all cells have been found to be actively motile and to retain their morphological viable characteristics; a 24 hour preparation which has been kept in the icebox appears to be entirely similar to a fresh smear and cannot be distinguished from it.
Thirty drops of a saturated solution of neutral red iodide No. 2 in 10 cc. of absolute alcohol is a satisfactory concentration for rabbit's blood instead of the usual 50 to 100 drops; as much as 100 drops per 10 cc. of absolute alcohol has been used with success in the case of normal human blood. Smears left in the open laboratory were found to last about four hours or twice as long as those kept at 37° in the hot-box. Further reduction of the temperature to 5°-10° (refrigerator) effected the preservation of the cells for as long as 24 hours. When the preserved smears are examined with an ordinary electric bulb as the source of illumination, active motility of the white cells is evident.
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