CNS disease: central nervous system disease, or the presence of leukemia cells in the cerebral spinal fluid (CSF). They do a spinal tap and test the fluid for the presence of leukemia cells.
CNS 1: no detectable leukemia cells inthe CSF
CNS 2: less than 5 white blood cells per microliter of blood and a positive "cytospin" for blasts; but the definition gets more complex, ask your oncologist
CNS 3: more than 5 white blood cells per microliter and a positive cytospin, but again, the definition gets more complex
Basically, CNS 1 means no CNS involvement, CNS 2 is a very low level, and CNS 3 is definite CNS involvement. CNS involvement is often accompanied by specific clinical signs.