No, IMHO, IgG is not specific to "old" infections. IgG means there has been exposure - whether that's current/ongoing infection or past/old infection.
If I remember right, the body starts to convert IgM into IgG after a couple of months...at least, that's what the "ideal" is. This "ideal" assumes, though, that the immune system is functioning normally, which in Lyme Disease, we already know is at least partially messed up by the Lyme itself. So the generation of antibodies can be suppressed completely, or the ability to generate enough to show up on the test can be compromised, the ability to generate IgM may be supressed but the ability to generate other antibodies such as IgG may not be, etc.
I think a key thing to look at with the IgG in particular is whether or not there are changes in results from one test to another (from the same lab). Changing bands means active infection. So for example, say on one test, IgG is positive at bands 39 and 41 and then the next test done at the same lab shows IgG positive at 23, 31, and 41. This would give good evidence of current, active infection based on an IgG test.
And keep in mind, too, that the test is only able to detect free-floating antibodies, not antibodies that have found a target to stick to (a part of the lyme bacteria, for example). I think this may be one reason changing bands is more significant...it illustrates the changes the bacteria make in an attempt to elude the immune system.
I hope all this makes sense...and it is just my interpretation of info I've read here and elsewhere, stuff LLMD/LLND's have told me, conversations with local support group members, etc.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Pancytopenia, chronic malabsorption/malnutrition, etc.; G-Tube; Currently TPN-dependent.
Meds: Zofran, Pulmicort, Heparin (to flush PICC line), Claritin, Colloidal Silver (used topically), IV Milk Thistle, probiotics.