According to the latest NICE guidelines, the diagnostic criteria to be followed by NHS staff is as defined by the WHO
https://www.who.int/diabetes/publications/diagnosis_diabetes2006/en/
This uses a fasting blood test (8 hr fast) and a 2 hr random blood test or OGTT test to diagnose T1D, Certainly this is how I was dx'ed many moons ago. An HbA1c test follow on and weight loss checks and family history are usually included in the second consultation to confirm.
In my area, any blood test using venous blood (i,e, HbA1c) takes 3 days or more to get a result back so is in itself not a fast process. So the NICE advice is if the IFB test is high then it needs immediate transfer to a specialist team for immediate attention. I believe that sudden unexplained weight loss is a significant factor for T1D, so going low carb could in time possibly give a false interpretation the other way due to weight loss. But LC is not a fast acting process either.
The other marker that is sometimes used is ketones above 4 mmol/l with high sugars. But my GP does not have a meter that can read ketones, so they use the urine test kit instead, But DKA can occur with any insulin dependant metabolic disorder so is only indicating an insulin insufficiency. This marker is the Blue Light switch.