The wedge smear technique for making blood smears survives. Like handwriting, it is learned early and changed slowly. And while most colleagues make "acceptable" slides, others seem to use the heel of their shoe as the spreader.
Does it matter?
This study concludes automated preparation methods show less variability with automated counts, particularly with monocyte populations. This makes sense, as cell size affects how cells are rolled along and eventually flattened by the edge, angle and force of a spreader slide. Sample hematocrit, room temperature and humidity, and slide cleanliness can all make a difference.
- First, modern analyzers classify thousands of cells, not 100 cells from a few dozen fields. We don't need to match an automated count, just to scan a feathered edge long and wide enough to locate and identify abnormalities. How long and wide are disputable.
- Secondly, if we reduce variability in smear preparation, does it significantly increase precision or accuracy? While difficult to get techs to agree what a smear should look like, it's even harder to get them to change. Experience teaches that day-to-day, tech-to-tech variation in identifying and counting cells is greater than, and not necessarily caused by, millimeters on feathered edges.
Maybe, that's just the artist in me talking. A well-made blood smear is a pretty thing, after all.
How do you make your smears?
And if you've switched, are automated methods really any better?