Phlebotomy: A Blood Film or Smear
A blood film or smear is collected by spreading a thin drop of blood on a microscope slide. This type of sample is needed for a manual differential (Diff), which checks the number, type, and characteristics of blood cells. A manual differential can be used to confirm abnormal test results that are obtained from a manual-generated differential or platelet count. Two blood smears are obtained for testing.
There are only a few types of tests that require the evaluation of a blood smear from a drop of blood from a fingertip. Leukocyte alkaline phosphatase (LAP) stain is a test that requires blood smear samples from four extremities. Most often the phlebotomist prefers to obtain these samples by skin puncture collection.
Sometimes a phlebotomist must perform and collect several types of skin puncture specimens. When this occurs it is best to collect blood smears first. This helps to keep the platelets from clumping. Here are a few important steps to remember when preparing a blood smear from a capillary puncture.
1. Perform the capillary puncture on a finger or heel. 2. Wipe away the first blood drop. This is important to remove excess tissue fluid and residue that could alter or distort the cell morphology. 3. Touch a slide to the next blood drop that is ideally 1 to 2 mm in diameter and about ½ an inch to 1 inch from the end of the slide. 4. Hold a second slide, called the spreader slide at 30 degree angle and place this slide in front of the slide with the blood drop on it. Tip the spreader slide so that the blood can smear approximately ¾ of the remaining area of the slide. 5. Pull the spreader slide back to the blood drop and allow the blood to spread along the width of the slide. 6. Push the spreader slide in one smooth motion away from the drop and along the entire length of the blood drop slide.
These six steps will take some practice to master. A competent phlebotomist should be able to acquire a blood smear without having to push down on the spreader slide thereby creating lines and ridges in the blood film.