Cardiac ablation for wpw 3 weeks ago and now low wbc 3.9 and high monocytes 18 and extreme fatigue. what could this be?

Need attention: I think you need to see the cardiologist. Wbc of 3.9 and mono 18% are non-specific and could be due to viral infection and might go away in next blood test you do.

Not the Ablation: Or, at least not the ablation directly. Monocytosis in the setting of low WBC is most consistent with the recovery phase of a more longstanding infection or inflammatory state. If not a viral or other infection, you're recovering from, consider liver testing and repeat the labs in 2 weeks to evaluate the trend. Hope you feel better soon.