I wish you had given more information as to why the patient was hospitalized.

Now it's somewhat difficult to judge what is meant by "low" in this context. It might be that the patient earlier had a marked leukocytosis (a value of white blood cells higher than normal), and that "low" here actually menas "normalized".

However, it could also mean leukupenia ( avalue below the lower limit for normal).

I would say that "low white cells" is somewhat of a compromise, although it would probably primarily lead the thoughts of the reader towards leukopenia.

But so would the Swedish phrase. The main reason for beleiving that it might refer to a normalization is the fact that the fever is gone and the patient is being discharged.