Natural Killer Cell Testing
For evaluating NK cell deficiency or dysfunction, perform flow cytometry-based functional assays measuring NK cytotoxicity and/or IFN-γ production, along with NK cell enumeration and phenotyping, rather than relying on simple cell counts alone.
Primary Testing Approach
The evaluation of NK cell function requires functional assessment, not just enumeration:
Functional Assays (Priority Tests)
Flow cytometry-based NK cytotoxicity assays are the gold standard for assessing NK cell function 1, 2. These measure the actual killing capacity of NK cells against target cells (typically K562 cells). The test has demonstrated:
- 96% sensitivity for detecting NK cell dysfunction in conditions like hemophagocytic lymphohistiocytosis 1
- Ability to assess both natural killing and antibody-dependent cellular cytotoxicity 3
IFN-γ production assays (NKA-IFNγ) provide complementary functional assessment 1, 4. These measure NK cell activation capacity through cytokine production after stimulation. When combined with NK cytotoxicity testing, agreement reaches 88% in patients with confirmed NK deficiency 1.
Phenotypic Analysis
Flow cytometry should simultaneously assess:
- Total NK cell numbers (CD3-CD56+ cells)
- NK cell subsets: CD56dim (cytotoxic) versus CD56bright (immunoregulatory) populations 5
- Decreased CD56dim cells correlate with reduced cytotoxicity 1
Clinical Context Matters
When NK Deficiency is Suspected
Look for these clinical red flags that warrant NK testing 6:
- Severe or atypical herpesvirus infections (HSV, VZV, EBV, CMV)
- Recurrent viral infections
- Hemophagocytic lymphohistiocytosis presentation
- Specific syndromes: MCM4 deficiency presents with adrenal insufficiency, growth retardation, and markedly decreased peripheral NK cells (though CD56bright cells are preserved) 5
Avoid This Common Pitfall
Do not rely on simple NK cell enumeration by immunohistochemistry alone 7. This is inadequate because normal NK cell numbers can exist alongside severely impaired function. Functional testing is essential.
Interpretation Framework
Normal Variability
Healthy adults show substantial variation in NK function:
- Natural killing: coefficient of variation 33%
- Antibody-dependent killing: coefficient of variation 27% 3
This means borderline results require clinical correlation and potentially repeat testing.
Combined Testing Strategy
When both functional assays are available:
- High specificity approach: Combine NKA-IFNγ with ferritin levels (>10,000 µg/L) for ruling out conditions like HLH with 94.2% specificity 1
- Discordant results between cytotoxicity and IFN-γ production may indicate specific pathway defects
Additional Considerations
Genetic Testing
If functional NK deficiency is confirmed, consider genetic evaluation for the 6 known genes causing isolated NK cell deficiency 6. For reproductive contexts, KIR/HLA-C interaction testing may provide additional diagnostic value 7.
Supporting Tests
Measure serum cytokines (particularly soluble IL-2 receptor) as these correlate with NK dysfunction 1 and help distinguish primary NK deficiency from secondary causes.
The key principle: functional assessment trumps enumeration. NK cells must be tested for what they do, not just counted.