: Who decides the "normal" values for blood count? I'm currently researching the values for a Complete Blood Count. Obviously different sources give slightly different values, usually due to rounding, but I found that some

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Generally these decisions are "made" by work groups of specialists in the field. Your question is very general. There are so many measurements that can be done from blood samples that there is no way I can provide you with information on exactly who decides what the reference value is.

("Who decides" makes it sound like it is arbritrary - it is not.
Decisions are usually based on statistics - 2 standard deviations from the population mean, for example, are taken as a reference standard. This means that the reference range can be defined as: "what is most prevalent in a reference group taken from the general population?" Be aware: Values outside a reference range are not necessarily pathologic, and they are not necessarily abnormal in any sense other than statistically. On the other hand, values within the reference range can be pathologic even though they are "normal". This is why reference range is actually a better term than "normal" range.
Sometimes, however, the reference value is based on what values have an optimal health effect. A problem with optimal health range is a lack of a standard method of estimating the ranges.)