Here’s an excellent question that comes up from time to time in class. It has to do with units of measurement, which sounds like a boring and unimportant topic – but it can be very confusing if you’re not aware of the different systems people use.

Q. I have a very odd question about cell counts. I’m looking over my notes and saw where my Professor listed a high white count as 60000 (6.0 x 109)/L. I don’t understand what she means by 6.0 x 109/L because if that were the case the white count would be 6,000,000,000. I tried looking it up but often times I see white counts listed as 109. Shouldn’t it be 106?

A. I know what you’re saying – it is confusing because sometimes you’ll see the white count listed as the number of cells x 109/L, and other times you’ll see it listed as the number of cells/mm3. You might even see it referred to as the number of cells x 103/μL! Note that there are three different volume measurements – liters, cubic millimeters, and microliters. The 103/μL version is part of the “conventional” unit system, and the 109/L version is part of the International System of Units (the modern form of the metric system).

I tend to use either the mm3  or the L version, depending on my mood and the situation – but any of the three is ok, as long as you’re careful to do the correct conversions, and use the correct volume (L vs. mm3 vs. μL). So a normal white blood cell count could be correctly listed as any of the following:

  • 4.0 – 11.0 x 109/L
  • 4.0 – 11.0 x 103/μL
  • 4,000 – 11,000/mm3

The confusion arises when people are talking (or sometimes even writing) about the white count. Nobody says the patient’s white count is “8,200/mm3” – they just say “8,200.” So if you don’t have the context of their comment, you don’t know if that means that the patient’s white count is normal (8,200/mm3), or very high (8,200 x 109/L ).

Usually, it’s fairly obvious in real life. A WBC count of  8,200 x 109/L , for example, would be incompatible with life. Sometimes, though (especially on exams, where trick questions are unfortunately pretty common), you really need to know the units. If it’s not abundantly obvious which units are implied, then you need to ask!

Back to your question. The confusion arose because the units were not listed correctly. It would be correct to say that 60,000/mm3 is a high white count. This number would be equivalent to 60.0 x 109/L in the SI system.  The other number you listed – 6.0 x 109/L  – is within the normal range. And you’re right: 60,000 x 109/L would be a ridiculously high white count (one that would not be seen in humans!).