CBC with manual differential
If suggestive of<unk> also order:
Review of peripheral smear
Chemistry, Ca, Phos, Mg, Uric acid, LFTs, LDH, coags, T+S, UCG (if applicable)
Viral titers cytomegalovirus, EBV, HIV, HBV, varicella zoster
CXRto look for mediastinal mass
Consider DIC labs, coag studies
If suggestive of<unk> also order:
Review of peripheral smear
Chemistry, Ca, Phos, Mg, Uric acid, LFTs, LDH, coags, T+S, UCG (if applicable)
Viral titers cytomegalovirus, EBV, HIV, HBV, varicella zoster
CXRto look for mediastinal mass
Consider DIC labs, coag studies
Review of peripheral smear
Chemistry, Ca, Phos, Mg, Uric acid, LFTs, LDH, coags, T+S, UCG (if applicable)
Viral titers cytomegalovirus, EBV, HIV, HBV, varicella zoster
CXRto look for mediastinal mass
Consider DIC labs, coag studies
Leukemia 
Leukemias will often involve >1 cell line; other conditions restricted to single line
Leukopenia
Aplastic anemia
Acute myeloid leukemia
Iron deficiency anemia
Myelofibrosis
Viral infection 