Tips for Eliminating Floaters

What can be done to eliminate floaters (tissue bits that are NOT from the patient’s tissue in the block) at our embedding station?

First, realize that floaters can originate at any of the following steps in the work flow to produce a slide:

  • Grossing table – not having a clean table, having contaminated forceps
  • Tissue processor – tissue bits floating out of one cassette into another
  • Embedding station – we’ll be discussing this below
  • Microtomy station – not cleaning the water bath after each block, contaminated forceps
  • Staining – tissue bits floating off one slide and onto another

So let’s concentrate on eliminating floaters at the embedding center:


  • Use plain forceps (without grooves in the tips). They are less likely to hold tissue bits, and easier to clean.
  • Clean the forceps with gauze between each cassette.
  • Change forceps between each cassette to help prevent tissue bits falling into the forcep wells.
  • Embed just one cassette at a time. That means opening only one at a time, and having only one cassette on the embedding platform at a time.


  • Don’t use any molds that don’t look absolutely clean. There might be tissue bits left over from a previous patient.
  • Store the molds upside down so no tissue bits fall into the mold well.
  • Clean the molds. There is always the question of “how often?” Some labs do it every day. Some labs do it on a routine basis, say once a week. Some labs do it only as needed. There are two main ways to clean the molds:
    • Place the embedding molds in the tissue processor chamber during the purge cycle to allow the xylene to remove the paraffin, and the alcohol to remove the xylene. Then put the molds in a chemical hood to allow the alcohol to evaporate.
    • Place the embedding molds in very hot water, almost boiling water with some soap in it, and allow them to set until the water is cool. The paraffin will float on the top.


  • The forceps need to be cleaned often.
  • The paraffin out of the forcep wells daily, in case any tissue bits floated off.
  • The paraffin out of the mold well, in case any tissue bits floated off.
  • The embedding molds, as mentioned above.


If you are having a lot of problems with epithelial cells that you think are sloughing off your hands or scalp (dandruff), it’s most likely occurring during microtomy. You can do the following at both the embedding and microtomy stations:

  • Wash hands with soap before embedding and microtoming to eliminate loose epithelial cells
  • Don’t touch the tissue with your fingers
  • Wear gloves while embedding and microtoming
  • Pull your hair back in a pony tail

If there is, what looks like, a loose piece of tissue on the outside of the cassette, do NOT embed it with the tissue. Instead, embed it in its own mold and put the patient’s number on a new cassette with a question mark, and inform the pathologist about it being a possible floater.

Scroll to Top