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Blood Test Tracker

CD19+ B Cells, percent

The share of your lymphocytes that are B cells. B cells are the white blood cells that grow into antibody-making cells.

AI-verified against cited sources. Awaiting expert reviewAlso called % CD19LOINC 8117-4

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What it is

B cells are lymphocytes, a type of white blood cell. When they meet a germ, they mature into plasma cells that make antibodies. Labs find them with flow cytometry by looking for the protein CD19 on their surface. Some labs report CD20 instead, which marks most of the same cells but is a separate test. This page covers the percent: out of every 100 lymphocytes, how many are B cells.

Why it matters

The percent shows how B cells compare with T cells and NK cells. Because it is a share of all lymphocytes, it can change when the other groups change, even if the number of B cells stays the same. Clinicians usually read it with the absolute count and with antibody (immunoglobulin) levels.

If it’s low

A low percent can mean fewer B cells than usual. It is expected during treatment that targets B cells, such as anti-CD20 antibodies. It can also happen when T cells are high, which makes B cells a smaller share.

Common causes

  • B-cell-targeted treatment such as anti-CD20 antibodies (for example rituximab)
  • Other immune-suppressing medicines or chemotherapy
  • Recent illness
  • High T-cell counts that leave B cells as a smaller share
  • Inherited antibody conditions such as X-linked agammaglobulinemia, where B cells are usually below 1% of lymphocytes

What can help

  • If you are on B-cell-targeted treatment, ask your clinician what level they expect and how they use this result
  • Ask whether antibody (immunoglobulin) levels should be checked alongside it

If it’s high

A high percent can mean there are more B cells than usual. It can also happen when T cells are low, which makes B cells a bigger share. The absolute count helps tell these apart.

Common causes

  • Normal variation between people
  • Recent or ongoing infection
  • Low T-cell counts that leave B cells as a larger share
  • Less often, a blood condition that makes too many B cells, usually found together with other blood count changes

What can help

  • Ask your clinician to read the percent together with the absolute count
  • Ask whether to repeat the test after any recent illness has passed

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 9 ranges
  • under 0.0195–22 % of lymphocytes

    Source: Labcorp

  • 0.019–-0.8334–26 % of lymphocytes

    Source: Labcorp

  • 0.167–-0.58314–39 % of lymphocytes

    Source: Labcorp

  • 0.417–-0.2513–35 % of lymphocytes

    Source: Labcorp

  • 0.75–0.2515–39 % of lymphocytes

    Source: Labcorp

  • 1.25–117–41 % of lymphocytes

    Source: Labcorp

  • 2–414–44 % of lymphocytes

    Source: Labcorp

  • 5–910–31 % of lymphocytes

    Source: Labcorp

  • 10–158–24 % of lymphocytes

    Source: Labcorp

See B cells (%) by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Recent infection or illness
  • B-cell-targeted treatment such as anti-CD20 antibodies, other immune-suppressing medicines and chemotherapy
  • Age: children's values differ from adults'
  • Time of day
  • Method: bead-based counts versus counts calculated from a separate CBC lymphocyte count
  • Whether the lab counts CD19+ or CD20+ B cells (separate tests that mark most of the same cells)
  • The sample must stay at room temperature and reach the lab quickly: Labcorp asks for arrival within 48 hours, and ARUP lists 72 hours in EDTA tubes and 48 hours in heparin tubes. Refrigerated, frozen, hemolyzed or clotted samples are rejected (Labcorp, ARUP).
  • B-cell-targeted treatment, other immune-suppressing medicines, chemotherapy and recent infection can change counts.
  • Counts calculated from a separate CBC lymphocyte count inherit that count's variation.

Counts can shift over the day, so a similar time of day helps when you compare repeat results. Counts can change over days to weeks with illness. B-cell-targeted treatment lowers them, and they can take months to return.

When to talk to a clinician

Talk with your clinician if you have repeated, severe or unusual infections, or if your B-cell count is well outside your lab's range. Get urgent care for a high fever with shaking chills, trouble breathing, confusion or an illness that is getting worse fast.

Questions you could ask

  • Why was this panel ordered for me, and what are you looking for?
  • How should I read the percent alongside the absolute counts?
  • Is my absolute B-cell count also high, or are my T cells low?
  • Should we repeat the test?
  • Could my treatment or a recent illness explain this result?
  • Should my antibody (immunoglobulin) levels be checked as well?

Questions people ask

Should I look at the percent or the absolute count?

Clinicians usually rely more on the absolute count, because the percent changes whenever other lymphocytes change. The percent is still useful for context, and some centers use it when following B-cell-targeted treatment.

My report says CD20 instead of CD19. Is that the same?

Both proteins mark B cells, and the two results are usually close, but they are separate tests. Your report shows which one your lab used.

Show the science and sources

For clinicians and researchers

The percent is the CD19+ (or CD20+) population divided by total lymphocytes (the CD45-bright, low side-scatter gate), so it moves when any other lymphocyte group changes. Interpret it with the absolute count. CD19 is a pan-B-cell marker. Some labs report CD20 instead, which marks most of the same circulating B cells but is a separate assay. Reference intervals are lab- and method-specific. Labcorp's adult interval was set by the lab, and its pediatric rows come from Comans-Bitter et al. (J Pediatr 1997). ARUP's adult interval is narrower than Labcorp's at both ends. In X-linked agammaglobulinemia, B cells are usually undetectable or below 1% of lymphocytes while T and NK counts are normal (Salzer 2019). Anti-CD20 antibodies bind CD20 itself, so some centers follow B cells by CD19 during that treatment, sometimes as a percent of lymphocytes. Specimen: whole blood at room temperature (Labcorp: lavender EDTA plus yellow ACD tubes, received within 48 hours; ARUP: EDTA, stable 72 hours, or heparin, stable 48 hours).

LOINC 8117-4 (primary), 20593-0. Specimen: whole-blood.

Sources

  1. T- and B-Lymphocyte and Natural Killer Cell Profile (test 505370), Labcorp
  2. Flow Cytometry in the Diagnosis and Follow Up of Human Primary Immunodeficiencies, EJIFCC (International Federation of Clinical Chemistry and Laboratory Medicine) (2019)
  3. Lymphocyte Subset Panel 5 - Total Lymphocyte Enumeration (test 0095892), ARUP Laboratories

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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