What are the results. Lab results need to be interpreted in the clinical context and your doctor who ordered the test is in the best position to do that.
My CBC results are WBC of 3.5 (low), rbc: normal, hemoglobin 12.3, MCH of 25.2 (low), mchc of 31.5, RDW of 18.4 (high), neutrophils 21, lymphs 74, ?
So your absolute. Neutrophil count is low -- 3500 x 0.21 = 735. If you have had infection or mouth sores it should be worked up, otherwise the test should be repeated in 7-10 days. May just be a transient finding.
Need a work-up. You might have one or more issues here. You have a low WBC count with about 75% lymphocytes, which can mean a leukemia or lymphoma. In addition you've a high RDW and low mch, with a normal rbc count which can mean that cells are being pushed out of your bone marrow by whatever is affecting your wbcs. You should see your doctor for more tests, a look at your blood smear, and a full work-up.
I suspect my pre-teen has mono. Waiting for EBV test to come back. In the meantime CBC showed normal WBC&RBC. Does this mean mono is unlikely?
No could be. Wait for the specific EB Virus antibody tests---they would tell for sure.
Maybe. The lay term mono is based on the common feature of a high proportion of monocytes on the CBC differential count. The EBV test could indicate recent (elevated IgM, or long time ago =elevated IgG mono. If negative it would likely mean little. Sometimes the test can be negative very early in the illness in kids.