The acro test can have false negatives if done too early after starting insulin. From another post I did: results given in
this presentation by Dr. Vanessa Woolhead of the RVC. First of all, positive for acromegaly is greater than 1000 when measured in nmol/L, as done in England at RVC. She studied over 200 cats. 19% of the cats had IGF-1 over 1000 when tested early after starting insulin therapy. A further 9.1% changed from <1000 to >1000 after 73 days of insulin therapy. So approximately 1/3 of acrocats tested too early got false negative results. Something in starting insulin therapy temporarily lowers IGF-1 numbers.
If I was going to get testing done for IAA, I would get the acro (or IGF-1 test) done at the same time. The blood for both tests goes to Michigan State University and no need to pay for shipping costs twice. You can wait for 6.0 units, or you can test earlier if you want. We've had cats positive for acromegaly and IAA on less than 6 units. It's just that if you get to 6 units, chances are one or the other or both will be positive. Other things to make sure you've checked is whether Henry needs a dental, and also recent bloodwork to check for hyperthyroidism, though picky at food seems to counter that thought. Those and pancreatitis, kidney disease and heart disease can also cause insulin resistance.