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Q. I had viral fever and my blood report shows lymphocytosis. What is the cause for elevation?

Answered by
Dr. Parth R Goswami
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Nov 08, 2019

Hello doctor,

I had a viral infection before two months with fever for five days and cough and increased mucus production for more than two weeks. On a follow-up visit with a PCP, I was given antibiotics for five days Azithromycin and antihistamine Levocetirizine to make sure there is no secondary infection and to help with mucus (allergies were also suspected).

I feel great by now for a few weeks and currently, I am not taking anything. I have attached my blood work done yesterday, including CBC and peripheral blood smear. As you can see, although my overall WBC count is normal, there is a relative lymphocytosis (42%, up from 40% two weeks ago). The blood smear is normal. My hs-CRP is 0.9 and ESR (denoted as VHS) is at 2 mm/hr.

What can cause this relative elevation in lymphocytes? Should I be concerned about leukemia or lymphoma?



Welcome to

I have seen your attached report. (attachment removed to protect patient identity).

The lymphocytes are slightly elevated. The lymphocytosis seems most likely to be due to previous viral infection of the upper respiratory tract.

The peripheral smear report not showing any abnormality.

So you need not worry about any leukemia or lymphoma according to provided history and reports. Do not worry.

You can repeat your CBC (complete blood count) after a week. The count of lymphocytes will fall within range after a week mostly.

I hope this helps.

Thank you doctor,

In theory, how long after a viral infection can you still have lymphocytosis?

Can you confirm that normal peripheral blood smear excludes leukemia and lymphoma as potential causes?



Welcome back to

The lymphocytosis can persist for few weeks in some patients. It can remain elevated up to six to eight weeks sometime.

In case of lymphocytosis, patient usually get feared about CLL (chronic lymphocytic leukemia) or lymphoma.

In CLL, the lymphocytosis is much high but in your case it is borderline.

Additionally in CLL peripheral smear shows smudge cells which is not seen in your case.

So, I am excluding leukemia or lymphoma especially CLL.

The myeloid cells are also not seen in the mentioned differential count, so I can exclude other leukemia as well.

Finally, based on the provided history and attached report, you may need not to worry about leukemia.

I hope this helps.

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