Peripheral blood involvement, summarized in "B category", refers to the percentage of peripheral blood lymphocytes that are atypical (Sezary) cells and whether they are "Clone negative" or "Clone positive".

Peripheral blood involvement is a prognostic factor required in AJCC 8th edition, Chapter 81 Primary Cutaneous Lymphomas, for staging of Mycosis Fungoides and Sezary Syndrome. It was previously collected as Mycosis Fungoides, CS SSF #1.

0	Absence of significant blood involvement 5% or less of peripheral blood lymphocytes are atypical (Sezary) cells Clone unknown Stated as B0
1	Absence of significant blood involvement 5% or less of peripheral blood lymphocytes are atypical (Sezary) cells Clone negative Stated as B0a
2	Absence of significant blood involvement: 5% or less of peripheral blood lymphocytes are atypical (Sezary) cells Clone positive Stated as B0b
3	Low blood tumor burden More than 5% of peripheral blood lymphocytes are atypical (Sezary) cells but does not meet the criteria of B2 Clone unknown Stated as B1
4	Low blood tumor burden More than 5% of peripheral blood lymphocytes are atypical (Sezary) cells but does not meet the criteria of B2 Clone negative Stated as B1a
5	Low blood tumor burden More than 5% of peripheral blood lymphocytes are atypical (Sezary) cells but does not meet the criteria of B2 Clone positive Stated as B1b
6	High blood tumor burden Greater than or equal to 1000 Sezary cells per microliter (uL) Clone positive Stated as B2
7	Test ordered, results not in chart
9	Not documented in medical record Peripheral Blood Involvement not assessed or unknown if assessed

Each Site-Specific Data Item (SSDI) applies only to selected primary sites, histologies, and years of diagnosis. Depending on applicability and standard-setter requirements, SSDIs may be left blank.