Immune deficiency in premature baby

Question:

9/26/2017
I was consulted on an ex 29 week premie, now corrected at 36 weeks, due to recurrent infections. In my reading and residency, I recall that premies are inherently immune compromised, along with active infections or other co-morbidities, will have lower immunoglobulin levels as well. But they felt it was not of the normal range they see so what work be an appropriate work-up at this stage?

Baby has had a staph aureus blood infection, then proceeded to osteomyelitis, subsequently developed candidemia from blood cultures, and now has klebsiella pneumonia in blood cultures also. Baby does have PICC line in place for antibiotics. Newborn screening was normal including TRECS. They did an immunoglobulin work-up which shows IgG at 166mg/dL, IgA <6 mg/dL, and IgM 26mg/dL.

My questions are:
1) what is a reasonable baseline work-up in a premature baby with recurrent infections given they are immune compromised and without having received vaccinations

2) what are the normal parameters for IgG, IgM IgA at this age as I was not able to find many references for this

3) When would a premie baby with recurrent infections and low IgG levels due to reason other than primary immune deficiency warrant an IVIG replacement infusion?