There are definitely blood tests they can do to detect markers for certain cancers but it’s impossible to tell if they did the test for leukemia if we don’t know what the test is called… what is the test called that they did?

Cancer screenings usually take longer. Depending on the type of cancer some blood tests may pick up on it. Some require specific tests. Hard to know without knowing exact testing that was done. It could be an iron deficiency or recent infection or something simple all the way up to bone cancer or leukemia etc. any diagnosis should be from a doctor directly involved in their care with all the info.

The quick test will be a point of care test (like a glucose meter but for hb) - they'll then collect more blood to check much more detailed tests - these will give good indications of why the hb is so low - blood count is pretty helpful for leukaemia as you'd find way too many of one type of blood cell. Most common is low iron, also possible low b12. Then there's other things but basically boils down to not making enough blood, or losing too much.