Once the bone marrow cell count is completed and recorded, the M: E ratio should be assessed. This is performed by calculating the total myeloid precursors in proportion to the total erythroid precursors.
Remember that this does not use the total white blood cell tally; the myeloid cells alone are counted, excluding lymphocytes, monocytes, macrophages, plasma cells, megakaryocytes, osteoclasts, osteoblasts, and other non-myeloid cells. In most circumstances, it is quite simple to divide the myeloid total by the erythroid total to find the ratio. This is always reported as a whole number ratio, and is normally around 3:1 (reference range= 2:1 to 4:1). In some situations where the erythroid portion is increased, or the myeloid series is decreased, the M:E ratio is reversed. This would still be expressed as a whole number ratio (example: 1:2). A simple way to perform the calculation is to always divide the larger value by the smaller. Which side of the colon, the 1 is placed on, is dependent on which cell type was larger. The 1 always belongs on the side of the cell type found in lower numbers.
For example:
Myeloid total 120 : Erythroid total 40
M:E ratio = 120 ÷ 40 = 3 or 3:1
So, the M:E ratio is 3:1
Another example:
Myeloid total 30 : Erythroid total 150
Divide the larger number by the smaller (notice that the placement is reversed).
150 ÷ 30 = 5
So, the M:E ratio is 1:5