MONO is short for Monocytes, one of the subtypes of Leukocytes [which are the White Blood Cells which comprise you immune system.
AUTO means the % of monocytes was ESTIMATED by an Auto-mated cell counter machine.
While 4.2% is technically Low [hence the "L"] (usually Normal is between 4.5-9 or so), it is still considered in acceptable range [of 4-10].
I see no cause for concern.
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