Differential Leukocyte Count Summing to More Than 100%: Technical Error Requiring Repeat Analysis
A differential leukocyte count that sums to more than 100% is mathematically impossible and represents a technical or reporting error that mandates immediate repeat analysis. By definition, differential counts express each cell type as a percentage of the total white blood cells counted, and these percentages must sum to exactly 100% 1.
Understanding the Technical Standard
The American Thoracic Society specifies that differential cell counts are performed by enumerating at least 400 cells on cytocentrifuge preparations, with each cell type expressed as a percentage of the total cells counted 1.
Manual differential counting involves classifying each leukocyte into one of five categories (neutrophils, lymphocytes, monocytes, eosinophils, basophils), and the sum of these percentages must equal 100% by mathematical necessity 2.
Normal adult differential counts typically show approximately 40-70% neutrophils, 20-40% lymphocytes, 2-10% monocytes, 1-4% eosinophils, and 0-1% basophils, with all percentages summing to 100% 2.
Common Sources of This Error
Transcription and Reporting Errors
The most likely explanation for a differential count exceeding 100% is a transcription error where absolute counts (cells/µL) were mistakenly reported in the percentage fields 3, 4.
Automated analyzers may occasionally produce flagged results that require manual verification, and errors can occur when transferring data between automated and manual systems 4.
Calculation Errors
Manual counting errors can occur when the technologist miscounts cells or incorrectly calculates percentages from the raw cell counts 3.
If a 100-cell or 200-cell differential was performed (rather than the recommended 400-cell count), calculation errors become more likely due to the smaller denominator 3.
Immediate Action Required
Request immediate repeat analysis with manual verification of the differential count, ensuring that the laboratory reports both percentages (which must sum to 100%) and absolute counts (cells/µL) for each cell type 1, 2.
Verify that the laboratory is reporting percentages rather than absolute counts—for example, an absolute neutrophil count of 7,890 cells/µL should not be reported as "78.9%" if other cell types would push the total above 100% 5, 6.
Review the complete blood count with differential report format to ensure proper labeling of percentage versus absolute count columns 1.
Clinical Interpretation Cannot Proceed
No clinical interpretation of abnormality can be made until a mathematically valid differential count is obtained, as the current result represents a laboratory error rather than a biological finding 3, 4.
Once corrected results are obtained, abnormality should be assessed based on absolute counts (e.g., absolute neutrophil count >6.7 × 10³ cells/µL indicates neutrophilia) rather than percentages alone 5, 6.
The American Thoracic Society defines abnormal differential counts as: lymphocytosis >15%, neutrophilia >3%, eosinophilia >1%, or mastocytosis >0.5% in bronchoalveolar lavage specimens, but these thresholds only apply when the total differential sums to 100% 1.