Blood smears are made on untreated slides by placing 1 or 2 drops of blood on the slide depending on how thick the blood appears. The drop is smeared by placing the end of another slide at an angle and quickly pushing the drop toward the end of the specimen slide as soon as the blood starts to form a table along the edge of the slide that is held at an angle. An ideal blood smear only covers half of the slide and has a feathered appearance at the bottom edge of the smear. Two slides per specimen should be made with different concentrations of blood. Slides from a negative control specimen are also made. Slides are labeled with the specimen name, lab number, and date. Slides are left to dry in an upright position for 10-30 minutes.

The RBCs are lysed by putting the smeared slides in fresh 1:1 methanol:acetic acid for 2 minutes three times. Slides are left to dry in an upright position and then checked on phase microscopy for usable cells, cell concentration, and the best area for hybridization. Lymphocytes appear as round, bright cells on phase contrast and are the best cells to use for FISH analysis.