Counting a white cell differential under the microscope · Ceded (since 1974)

In 1974 Technicon put the Hemalog D on the market, the first instrument to produce a complete differential white cell count, classifying about 10 000 cells a minute by cytochemical staining and light scatter.

The manual differential was a person at a microscope moving across a stained film and tallying one hundred cells, a count whose statistical error is large by construction. Analysers now do this on essentially every blood count, and a film is made and read by a human only when the instrument refuses to classify what it sees, which is a minority of samples. The machine counts and sorts cells it recognises, and it says so when it cannot. Naming an abnormal cell, concluding that a leukaemia is present and ordering the marrow biopsy are still done by a haematologist at the same microscope, on the samples the machine sends up.