When there is severe red blood cell anemia, red blood cells may start reproducing outside of the spine - it can cause spinal cord compression... per one so diagnosed: "Once this happens you have a 72 hour slide period to have surgery to release the compression or a bone marrow transplant."

For exact terminology & example - check this

Turns out that as long as the bone marrow is able to compensate for early death of red blood cells, the anemia won't show up in what is normally tested for such: hemoglobin, hematocrit, even red blood cell counts (red blood cell count can be omitted from laboratory recommended list of tests needed). Retic ratio may give an indication. Are your red blood cells low functioning?

If you suspect benzene exposure (which reportedly suppresses production of new red blood cells rather than simply killing existing red cells), a more usual marker of "benzene poisoning" is an abnormally low lymphocyte count. So an absolute cell count that can be done in a regular blood test is an important number.

Either way, 

whether you suspect a chemical exposure or oil exposure, 

knowing what is going on with 

the red blood cells  

is important.

'Mother Margaret'