There are a few reasons, but one in particular is to check your white count levels...if they are down, then you are more susceptible to getting an infection. If you were to receive chemo with a low white cell count, then your immune system would be even more compromised and an infection would be at the forefront. That is something you will want to avoid at all costs, because when one occurs, it's very likely you will be admitted to hospital until it clears.
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If you are a cancer patient on oral meds, then you will also need to have a blood test done on a routine basis, probably every couple of weeks or monthly to ensure your blood levels are within the norm.