What indicates improper blood film preparation when observing long tails in the blood film?

The presence of long tails in a blood film typically indicates a problem with the preparation technique, and using an inappropriate spreader blade is a significant factor. A dirt spreader blade, which is not designed for this purpose, can lead to uneven pressure and result in a poorly prepared film characterized by tails or irregularities. An ideal spreader blade should provide a smooth, consistent layer of blood on the slide, which allows for appropriate observation of cellular components.

In a well-prepared blood film, the feathered edge is crucial for proper evaluation under the microscope, as it contains a monolayer of cells necessary for accurate differential counts and morphological assessment. Using a dirt spreader blade disrupts this process, leading to an unsatisfactory sample.

The other options pertain to proper techniques or conditions for preparing a blood film. For example, a proper feathering technique and utilizing a suitable glass slide contribute to achieving a quality film, while excessive anticoagulant can affect the morphology of cells but does not specifically create the 'tails' described. Thus, the choice of using a dirt spreader blade directly correlates with the observed issue of long tails in the blood film.