Hi. I had the same problem. The GAD test doesn't test for all causes of islet cell failure. It doesn't normally cover all antibody types and also viruses and pancreatitis can do damage. The c-peptide confirms low islet cell output which is all that matters. I label myself T1.5 or Late onset T1 as strictly speaking as LADA implies an auto-immune problem. Note also that ref NICE, GAD test becomes more unreliable as time passes after diagnosis. NICE also now says that someone presenting as slim with high sugars should be considered as a possible T1 which makes sense. Some would say that if you have been overweight with T2 for years then the excess insulin can damage the islet cells resulting in a low c-peptide; who knows.