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CD8+ T Cells, absolute

The number of CD8+ T cells in a microliter of blood. These cytotoxic cells destroy infected or damaged cells.

AI-verified against cited sources. Awaiting expert reviewAlso called CD8 abs, CTLLOINC 14135-8

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What it is

CD8+ T cells, often called cytotoxic or killer T cells, are a type of lymphocyte (a white blood cell). They find and destroy cells that are infected with a virus or that have become abnormal. A lab counts them by flow cytometry, and the result is the number of CD8+ T cells in each microliter of blood, often printed as cells/µL.

Why it matters

The CD8 count is part of a full picture of your T cells. It is usually read together with the CD4 count and the CD4/CD8 ratio. CD8 numbers often rise during viral infections and can stay higher than usual with some long-term infections, including HIV. A low or high number alone does not point to one cause.

If it’s low

A count below the lab's range means fewer CD8 cells than most healthy people have. A temporary dip is common after an illness or with certain medicines. A count that stays low is looked into with the CD4 count and the total lymphocyte count.

Common causes

  • A recent acute illness
  • Medicines that suppress the immune system, including corticosteroids and chemotherapy
  • Radiation or cancer treatment
  • Advanced HIV infection, when both CD4 and CD8 counts can fall
  • Malnutrition
  • Rare inherited immune conditions

What can help

  • Ask your clinician whether the total lymphocyte count and CD4 count are also low
  • Share every medicine and supplement you take, including steroids
  • Ask whether a repeat test after you have recovered makes sense

If it’s high

A count above the lab's range often means the immune system is responding to a virus. It can stay raised for weeks or months after the infection. In many people there is no clear cause and a high count is simply a variation.

Common causes

  • Natural variation between draws and between labs
  • Recent or ongoing viral infection, such as infectious mononucleosis (Epstein-Barr virus) or cytomegalovirus (CMV)
  • HIV infection, which often raises CD8 counts even when CD4 counts are low
  • Autoimmune or long-term inflammatory conditions
  • Smoking
  • Rarely, a blood disorder involving T cells

What can help

  • Ask your clinician to read it with the CD4 count, the ratio and your full blood count
  • Ask whether a recent illness could explain it, and whether to repeat the test after you have recovered

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 4 ranges

See CD8 count by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Age: ranges are higher in young children and drift slightly lower in older adults
  • Viral infections, including CMV and Epstein-Barr virus, which can raise counts for weeks or longer
  • Acute illness, stress, hard exercise and smoking
  • Medicines, especially corticosteroids and other immune-suppressing drugs
  • Time of day: counts vary through the day
  • Lab and method: each lab sets its own range
  • Samples must reach the lab fresh. ARUP accepts EDTA blood for up to 72 hours at room temperature (48 hours for heparin) and rejects refrigerated or frozen samples.
  • Some medicines can change the result. ARUP asks that medicines be listed on the request form. Corticosteroids, chemotherapy and other immune-suppressing drugs are known to lower lymphocyte counts.
  • Counts vary through the day, and an illness, hard exercise or stress can shift them for a short time. ARUP advises drawing at about the same time of day for serial results.

Any time, but try to draw at about the same time of day each time so results are easier to compare. Can shift within days during an illness or after steroid medicines, and usually recovers over weeks. Changes from treatment of a long-term condition build over months.

When to talk to a clinician

A CD8 result is not an emergency by itself. Talk with your clinician if it is low or high along with repeated or unusual infections, unexplained fever, night sweats or weight loss. Seek care promptly if you have trouble breathing.

Questions you could ask

  • Which of my lymphocyte results should we follow over time?
  • Could a recent or past infection explain this?
  • Do my CD4 count and CD4/CD8 ratio fit with it?
  • Could a recent illness or a medicine explain this?
  • Is my total lymphocyte count also low, and should we repeat it?

Questions people ask

Why is my CD8 count high when I feel fine?

CD8 counts can stay higher than usual for weeks or months after a viral infection, and some people simply run high. Your clinician can read it alongside your CD4 count, ratio and other results.

Is CD8 the same as CD4?

No. They are two different groups of T cells. CD4 cells help direct the immune response, and CD8 cells destroy infected or abnormal cells. They are usually read together.

Show the science and sources

Units

  • 1 cells/µL = 1 cells/µL
  • 1 cells/µL = 0.001 ×10⁹/L
  • 1 cells/µL = 0.001 ×10³/µL

Open the CD8 count unit converter

For clinicians and researchers

CD8+ T cells are CD3+CD8+ lymphocytes (most are cytotoxic T cells, though small CD8+ subsets include some NK-like cells), enumerated by flow cytometry on EDTA or heparin whole blood. ARUP reports absolute counts with an internal quantitation standard (single platform). CD8 counts expand in acute EBV and CMV infection and in HIV infection, where the CD4/CD8 ratio inverts partly because of this expansion and may remain abnormal during treatment. Counts also depend on age, CMV serostatus, and lymphocyte count. ARUP's adult interval is 210 to 1,200 cells/µL (ages 16 to 64) and 150 to 1,050 (65 and older). Other labs differ.

LOINC 14135-8 (primary), 32517-5. Specimen: whole-blood.

Sources

  1. Lymphocyte Subset Panel 4 - T-Cell Subsets Percent and Absolute, Whole Blood (test 0095950), ARUP Laboratories
  2. Reference ranges of lymphocyte subsets in healthy adults and adolescents with special mention of T cell maturation subsets in adults of South Florida, Immunobiology (University of Miami) (2014)

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

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