More Blood!

Not done yet, this is just how to make the smear. -K

There are more than a few methods for making smears in general, and a couple for making blood smears specifically. By far the most generally useful method, and the easiest, is what I refer to as pulling a smear. In pulling a smear one may produce a large, thin, and even smear that is rather more uniform and consistent than that which might be produced by other methods. It’s very easy for beginners to read the method and then  perform the action incorrectly, so please observe the pictures and practice with a bit of fountain pen ink or food coloring before getting set to make the smear with blood.

Practice the method with ink before working with blood.

Practice the method with ink before working with blood.

In A we see a drop of blood (in this case ink for practice) placed near one end of a slip. In B a second slip has been placed on the first at an acute angle. The second slip is then slowly pushed back until it comes into contact with the blood. Once contact is made the blood will spread out along the line the edge of the second slip. Maintain firm and even contact with the slips. The blood should remain primarily on the side next to the drop; within the acute angle. As in C, the second slip is then drawn in a smooth even motion along the first so that the blood is pulled along leaving behind an suitable smear. In D care was made to produce some of the more common troubles. Midway along the smear a lighter line is observed where the slip was stopped as it was drawn along. Further along we observe two blank areas where the slip was allowed to lose contact at the lower edge.

In the above image the camera is positioned at six o’clock, and I am seated at three o’clock. Most depictions of the process provide the impression that smears are best drawn along a horizontal from left to right (or right to left). It is easiest to ensure a smooth motion (for those who are right handed) to pull the smear from from eleven o’clock towards four o’clock.  When pulling along a perfect horizontal it is more difficult to keep constant contact and pressure along the entire length of the smear. It is in some respects a matter of preference but above all do not make the common mistake of drawing over the drop or of positioning the second slip so that the drop of blood spreads out in the obtuse angle side of the arrangement.

Apart from everything else, it’s very important to form the smear on only the cleanest of slips. Oil, dust or other imperfections can prevent the smear from adhering properly once made, or from being made effectively in the first place. Always take a moment to exhale on even a clean slip prior to beginning. If the vapor of ones breath forms an even cloud upon the slip, and it dissipates rapidly it is likely to provide good results.

With the smear made it must be fixed. One may fix via heat or chemical methods but for blood smears it is often better to simply wave the slide about rapidly (with a firm grip of course) until the smear is dry. Microbiologists are apt to follow Ehrlich and place the slide on a hotplate or in a cool oven for hours on end, but such efforts are not generally practical when only a few slides are being produced. If one desires to employ chemical fixation, the film must first be adhered to the slip and the primary method for that is… waving the slide about rapidly! Because the cells in the blood smear will retain the shape when dried, it is best to forgo complex fixation and stick with that tried and true method, regardless of how indecorous it seems.

After fixation the smear may be examined immediately, with or without further treatment (even with oil immersion). Or it may be stained to improve its appearance. One my use a popular traditional stain such as Wright’s or whatever is on hand. In the next post we’ll cover the use of Wright’s, but one may wish to try the stains they are familiar with. Methylene blue is a widely available stain that will make leukocytes much more noticeable.

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