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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Clinical Immunology · Functional Natural Killer Cell Deficiency

Decoding Your Immune Lab Results

At a Glance

Functional natural killer cell deficiency is not diagnosed from one lab result. Doctors combine absolute NK cell counts, repeat tests of cell-killing function, infection history, medication review, and genetic findings; an uncertain gene variant alone does not confirm it.

Diagnosing a Natural Killer (NK) cell deficiency is a multi-step process that looks at both the quantity and the quality of your immune cells. Because NK cell numbers can fluctuate based on recent infections, age, or medications, specialists integrate repeated counts, functional assays, clinical history, and genetics to confirm a diagnosis [1][2].

Your lab report might look like an alphabet soup of markers and percentages. Understanding these terms can help you participate more actively in your care. Always ask for your absolute counts, the laboratory’s specific reference range, and the test date, rather than relying on percentages alone.

Identifying the Cells: Flow Cytometry

Flow cytometry is a technology that uses lasers to count and phenotype cells based on “markers” (proteins) on their surface. It identifies what the cells look like, but does not directly measure their killing function.

  • CD3-Negative (CD3-): Conventional NK cells do not have the CD3 marker that T cells have [3].
  • CD56 and CD16: These are common markers for NK cells, though expression can vary. Most conventional NK cells express CD56 and/or CD16 [4].
  • CD56dim vs. CD56bright: These represent useful functional and subset descriptions.
    • CD56bright cells generally focus on sending out chemical signals (cytokines) [3][4].
    • CD56dim cells are typically the mature subset that does most of the actual killing. In some NK deficiencies, this subset is reduced or abnormal [4][5].

Testing the “Kill”: Cytotoxicity Assays

To see if your NK cells can actually destroy a target, doctors order functional tests.

  • Spontaneous (Natural) Cytotoxicity: This measures how well your NK cells recognize and kill a standard “target cell” (usually a lab-grown cell called K562) without outside help [6][7].
  • Antibody-Dependent Cellular Cytotoxicity (ADCC): This test adds antibodies to the target cells to see if your NK cells can use their CD16 receptor to find and kill “flagged” targets [8][9].
  • CD107a Degranulation: When an NK cell releases its killing granules, a marker called CD107a appears on its surface. A low CD107a score indicates trouble releasing granules, but it cannot by itself distinguish whether the release mechanism is broken or if the cell is simply missing its ammunition (perforin) [10][11].

Common Elements Considered by Clinicians

Diagnosing Functional NK Cell Deficiency (fNKD) is not a simple yes/no checklist. Clinicians generally look for [2][12]:

  1. Normal Absolute Counts: You have a normal number of NK cells for your age [2].
  2. Persistent Low Function: Cytotoxicity is low on repeated tests, preferably performed when you are not fighting an acute infection [13][14].
  3. Clinical Match: You have a history of atypical or severe viral infections that fit the phenotype [13].
  4. Exclusion of Secondary Causes: Doctors must rule out temporary or secondary factors that depress NK function, such as acute viral illness, certain medications (like azathioprine or steroids), HIV, or broader primary inborn errors of immunity (like GATA2 deficiency) [15][16].

What if the Genetics are Unclear?

Many patients receive a result showing a Variant of Uncertain Significance (VUS). This means a change was found in your DNA, but the lab isn’t sure if it’s a disease-causing variant or a harmless natural variation [17].

Interpretation of a VUS generally combines your clinical phenotype, population data, and expert laboratory review. In some cases, functional confirmation tests can help by testing if your specific cells behave the way that mutation should make them behave [18]. However, functional studies are not always available or definitive, and a VUS alone does not finalize a diagnosis [1][13].

Common questions in this guide

What do my NK cell count and percentage mean?
An absolute NK cell count tells how many NK cells are present, while a percentage shows their proportion among other blood cells. Counts can vary with age, recent infection, and medicines, so compare the absolute count with the laboratory’s age-specific reference range and review the test date. A percentage alone may be misleading.
What can flow cytometry show about my NK cells?
Flow cytometry uses cell-surface markers to identify and describe NK cells, including CD3, CD56, CD16, and the CD56bright and CD56dim subsets. It shows what the cells look like and which markers they carry, but it does not directly measure their ability to kill target cells.
How do doctors test whether NK cells can kill target cells?
A spontaneous cytotoxicity test measures whether NK cells can kill standard K562 target cells without outside help. An antibody-dependent cellular cytotoxicity, or ADCC, test adds antibodies to the target cells to assess whether NK cells can use CD16 to attack flagged targets.
What does a low CD107a result mean?
CD107a appears on the cell surface when an NK cell releases its killing granules. A low result suggests a problem with granule release, but it cannot by itself show whether the release machinery is defective or the cell lacks perforin. Additional testing may be needed.
How is functional NK cell deficiency diagnosed?
Doctors generally look for an age-appropriate NK cell count, persistently low killing function on repeated tests, and a history of severe or atypical viral infections that fits the findings. They also rule out temporary or secondary causes, such as an acute infection, certain medicines, HIV, or another primary immune disorder.
Does a variant of uncertain significance confirm NK cell deficiency?
No. A variant of uncertain significance is a DNA change whose health effects are not yet clear, so it does not establish a diagnosis by itself. Specialists may consider the clinical findings, population data, expert laboratory review, and functional studies, although those studies are not always available or conclusive.
Why might my NK cell tests need to be repeated?
Recent infections and medicines such as steroids or immunosuppressants can temporarily affect NK cell counts or function. Repeating testing when you are not acutely ill and reviewing medication exposure can help show whether an abnormal result is persistent.

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References

References (18)
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This page is for informational purposes only and does not replace medical advice. An immunologist should interpret your NK cell counts, functional tests, medication history, infections, and genetic results in context.

Guide contents Autosomal recessive primary immunodeficiency with defective spontaneous natural killer cell cytotoxicity
Research & Literature
03 The Biology and Genetics of NK Cells 05 Standard of Care and Treatment Options