Myeloperoxidase (MPO) Deficiency Does Not Affect True CBC Values, But Can Cause Automated Analyzer Errors
MPO deficiency itself does not alter actual blood cell counts, but it can cause significant automated analyzer artifacts that result in falsely reported pseudoneutropenia, falsely elevated monocyte counts, and markedly low mean peroxidase index (MPXI) values on certain hematology analyzers.
Understanding the Analyzer Artifact
MPO deficiency creates a technical measurement problem, not a true hematologic abnormality:
- Automated analyzers (particularly ADVIA 2120i systems) use MPO staining properties to differentiate neutrophils from other white blood cells 1
- When neutrophils lack MPO, the analyzer misclassifies them as monocytes because they don't stain appropriately 1
- This creates a characteristic pattern: falsely low neutrophil count + falsely high monocyte count + MPXI values typically below -20 1
Key Diagnostic Features on CBC
When MPO deficiency is present, you'll see:
Automated Analyzer Findings:
- Pseudoneutropenia (falsely low neutrophil count)
- Falsely elevated monocyte counts (often dramatically high) 1
- Markedly decreased MPXI values (typically below -20, can range from -6 to -7 in certain conditions) 2, 1
- Normal total white blood cell count 3
Manual Differential Findings:
- Normal neutrophil count when counted manually 1
- Normal monocyte count when counted manually 1
- Reduced neutrophil granulation on peripheral blood smear 3
- Abnormal neutrophil morphology with decreased chromatin condensation and cytoplasmic hypogranulation 3
Clinical Significance
The actual CBC values (when counted manually) are normal in MPO deficiency 4. The condition affects approximately 1 in 2,000-4,000 individuals screened, making it more common than previously recognized 4.
Critical Pitfall to Avoid:
Do not treat the automated analyzer's pseudoneutropenia as real neutropenia - this could lead to unnecessary interventions including inappropriate use of growth factors or delay of chemotherapy 1. Always perform a manual differential count when you see the characteristic pattern of high monocytes with low MPXI.
Diagnostic Algorithm
When you encounter abnormally high monocyte counts on automated CBC:
- Check the MPXI value - if markedly low (especially <-20), suspect MPO deficiency 1
- Perform manual differential count immediately - this will show normal neutrophil and monocyte counts 1
- Review peripheral blood smear - look for hypogranular neutrophils 3
- Confirm with cytochemical staining for MPO if diagnosis needs verification 4
The prevalence is approximately 1 in 2,000-4,000 patients, with most individuals being asymptomatic despite the enzyme deficiency 4. Only rare patients develop serious infections, particularly candidal infections, as MPO is more critical for fungal killing than bacterial killing 4.