Smudge cells are formed when lymphocytes rupture during the preparation of a blood film. Often, the original cell type cannot be determined with certainty. It may be ruptured lymphocytes, granulocytes or blasts.
In chronic lymphocytic leukemia (CLL), smudge/shadow cells are usually increased . In this case, shadow cells are called Gumprecht cells. The term "Gumprecht cells" is reserved for CLL and should only be used if the diagnosis of CLL is established. In other diseases such as other lypmphoproliferative processes or reactive lymphocytosis (e.g. CMV infection), lymphocytes are also easily damaged. In these cases, one talks about "shadow cells".
If shadow cells account for more than 5% of all nucleated blood cells, they are counted as a group of their own in the manual differential count and integrated in the total number of leukocytes.