How this counter calculates, and what it cannot tell you
This page is for the people using the counter at the microscope and the people reading its reports. It explains where every number on screen comes from, what published standard it follows, and — just as importantly — the limits of what a manual differential count can establish. Nothing here is specific to this software; most of it is true of any manual differential, and is stated so that the numbers are read for what they are.
Want the full derivation? The Calculation Reference covers every figure in more depth — the alternatives to each method, where the professional controversy lies, the citations, and exactly which choices your laboratory can configure. This page is the summary; that one is the argument.
1. What the software does, and does not do
It counts what you tell it to count. Every cell classification is yours. The software performs no image analysis, no cell recognition, and makes no diagnostic decision. It is the digital equivalent of a mechanical tally counter, with arithmetic and report formatting attached.
If you press the wrong key, the software will faithfully record the wrong cell. Nothing in it can detect that. The accuracy of a differential produced here is the accuracy of your morphological identification, plus the counting imprecision described in §5, and nothing else.
2. How percentages are calculated
Each category is expressed as a percentage of the differential denominator — the number of cells that belong to the differential, which is not always the same as the number of cells you counted.
Bone marrow
The denominator is every cell counted. Erythroid precursors are part of the nucleated differential count, so they appear as a percentage alongside the myeloid categories. This follows ICSH 2008 §2.6.
Peripheral blood
Nucleated red cells are counted but excluded from the denominator, and reported separately as NRBC per 100 WBC. They are not leucocytes, so including them would understate every leucocyte percentage.
Worked example — 180 leucocytes and 20 NRBC
| Segmented neutrophils, 120 counted | 120 / 180 = 66.7% |
| Nucleated red cells, 20 counted | 11.1 per 100 WBC |
| Report opens | "A 180-cell differential count…" |
The counting grid shows this as 180 + 20 — the differential, then the cells counted outside it.
Rounding
By default, percentages are made to total exactly 100% using the largest-remainder method: each value is truncated, and the leftover units go to the categories with the largest remainders. No category is moved by more than one unit of the last decimal shown.
This is a reporting policy, not a mathematical necessity, and your laboratory can choose differently: the whole residual may instead be applied to the largest category, or each percentage may be rounded independently and the total left to fall where it does. The three differ materially — see the Calculation Reference §1.2. Whichever is configured is stated in the report's method note.
3. The M:E ratio
Calculated per ICSH 2008 §2.6: all granulocytes and monocytes and their precursors — myeloblasts, promyelocytes, myelocytes, metamyelocytes, bands, segmented neutrophils, eosinophils, basophils, promonocytes and monocytes — divided by erythroblasts at all stages. Lymphocytes, plasma cells and mast cells take no part.
A competing convention exists
Some laboratories exclude monocytes from the numerator. The same counts then give a different ratio. If you are comparing an M:E ratio against one from another laboratory, or from your own laboratory before a configuration change, check that both used the same convention. Every report from this counter states which convention produced it.
A ratio of two counted proportions carries the sampling error of both, so it is substantially less precise than either percentage on its own. Treat small differences between successive ratios with caution, and read the ratio alongside cellularity and the trephine biopsy.
4. What the confidence interval means
Beside each percentage the results screen shows a range, for example 15.0–26.1%. A differential is a sample: you classified 200 or 500 cells from a smear containing far more. Had you counted a different 200 cells from the same slide, you would have got a somewhat different answer. The interval is the range of true values consistent with what you observed.
It is not a measure of whether you identified the cells correctly. It is the arithmetic consequence of counting a finite number of cells, and it would apply equally to a perfect observer.
| Observed | At 100 cells | At 200 cells | At 500 cells |
|---|---|---|---|
| 5% | 2.2–11.2% | 2.7–9.0% | 3.4–7.3% |
| 20% | 13.3–28.9% | 15.0–26.1% | 16.7–23.7% |
| 0% | 0–3.7% | 0–1.9% | 0–0.8% |
95% intervals, Wilson score method. The bottom row matters: counting zero blasts in 200 cells does not exclude blasts — it places them below roughly 1.9%.
The threshold warning
When an interval spans a diagnostic threshold your laboratory has configured, the results screen says so. An observed 20% blasts at 200 cells has an interval of 15.0–26.1% — it straddles the 20% AML boundary, so the count does not establish which side of that line the true value falls on. ICSH 2008 §2.6 recommends extending the count in this situation, and "Continue Counting" adds cells to the existing tally without losing it.
Counting more cells narrows the interval, but it will not always resolve the question: a true value sitting exactly on the threshold straddles it at any count. The warning tells you what the count has and has not settled. It never blocks you from finishing — a paucicellular aspirate may make a longer count impossible, and you are the one who knows that.
5. Limitations of manual differential counting
These are limitations of the method, not of this software. They are stated because a number on a screen looks equally confident whether or not it is reliable. The quantified findings below come from a four-institution ICSH study of 616 samples comparing manual morphology against flow cytometry and haematology analysers (Hedley et al., 2026).
- Rare populations are the least reliable. Blasts correlated well between methods when present above roughly 10%, but below that level suffered "a large degree of imprecision primarily with morphology due to the small number of cells counted". Blast counts showed the largest reproducibility variation of any category, just over 15%. This is precisely the range in which diagnostic thresholds sit, which is why the counter shows an interval rather than a bare percentage.
- Band versus segmented neutrophil is observer-dependent. The study attributes variability in immature granulocytes partly to "high variation between morphologists in differentiating band and segmented neutrophils". If your laboratory does not rely on that distinction, it can be configured as a single combined category — counting what can be distinguished reliably is better than splitting what cannot.
- Immature granulocyte categories lack a traceable standard. The study notes "the lack of a traceable standard for what are arbitrary morphologic features". Two competent morphologists may legitimately disagree.
- Basophils are counted in numbers too small to be precise. Correlation between methods was poor, "in part due to the relatively small number of basophils counted by morphology".
- The smear itself is a source of error. Non-uniform cell distribution, staining quality and dysplastic features affecting identification all contribute before any counting begins. ICSH recommends counting across at least two smears where a precise abnormal percentage matters.
6. Which cells belong in the count
For bone marrow, ICSH 2008 §2.6 defines the nucleated differential count as blast cells, promyelocytes, myelocytes, metamyelocytes, band forms, segmented neutrophils, eosinophils, basophils, mast cells, promonocytes and monocytes, lymphocytes, plasma cells and erythroblasts.
Do not count these into any category
Megakaryocytes, macrophages, osteoblasts, osteoclasts, stromal cells, smudged cells, and non-haemopoietic cells such as metastatic tumour cells. ICSH excludes them from the differential. Counting one into a general category puts it into the denominator, which lowers every reported percentage — including the blast percentage compared against diagnostic thresholds.
Record such findings in the morphology comment, which is reproduced in the report. Lymphoid aggregates likewise should be commented on rather than tallied.
7. How many cells to count
The target shown on the progress bar comes from your configuration profile. The shipped defaults follow published guidance:
- Bone marrow — 500 cells. ICSH 2008 §2.6: at least 500 across at least two smears when a precise percentage of an abnormal cell type is required for diagnosis; at least 300 when the differential is not essential to it. A 2018 study of 165 cases found 300-cell counts diagnostically equivalent to 500, including 100% sensitivity at the 20% blast threshold.
- Peripheral blood — 200 cells. The CLSI H20-A2 reference method is two reviewers counting 200 cells each.
The target is advisory and never enforced. You can finish at any count above zero. Finishing below target produces a note stating the interval your count achieved, so the reduced precision is on the record rather than left to be inferred.
8. Configuration, and why reports name it
Cell categories, keyboard mappings, target counts, derived formulas, diagnostic thresholds and report wording all come from a configuration profile, editable without changing the software. This is deliberate: differential counting practice varies legitimately between institutions, and a tool that hard-codes one practice is wrong everywhere else.
Because practice varies, a percentage is not self-explanatory. Every report therefore carries the profile it was produced under and its version, and the results screen has a Method section stating the conventions in force. If two reports disagree, that is where to look first.
9. Data handling
Counting happens entirely in your browser. No patient data is transmitted anywhere. Completed counts persist only for the browser session and are lost when it closes unless exported. An in-progress count is saved locally so it can be recovered if the browser closes unexpectedly; it is cleared when the count is finished or reset.
References
- Lee S-H, Erber WN, Porwit A, Tomonaga M, Peterson LC, for the International Council for Standardization in Haematology. ICSH guidelines for the standardization of bone marrow specimens and reports. Int J Lab Hematol 2008;30(5):349–364.
- Hedley BD, Keeney M, Gambell P, Qu C, Mao J, Davis BH, Wood BL. White Blood Cell Enumeration and Differential by Flow Cytometry: The ICSH WBC Reference Method. Int J Lab Hematol 2026;48(1):93–101. Open access.
- CLSI H20-A2. Reference Leukocyte (WBC) Differential Count (Proportional) and Evaluation of Instrumental Methods, 2nd ed. Wayne PA: CLSI; 2007.
- Brown LD, Cai TT, DasGupta A. Interval estimation for a binomial proportion. Statist Sci 2001;16(2):101–133.
- Is a 500-Cell Count Necessary for Bone Marrow Differentials? Am J Clin Pathol 2018;150(1):84–89.
- WHO Classification of Haematolymphoid Tumours, 5th ed., 2022; and the International Consensus Classification, 2022.
This counter is a counting and calculation aid for trained laboratory personnel. It does not perform cell identification and does not make diagnostic decisions. Every differential it produces should be reviewed and released under your laboratory's quality management system.