Are you a medical professional or do you have a scientific source backing up your suspicion?
The point why there is no objective blood test is mainly rooted in the fact that getting a true measurement of a neurotransmitter is prohibitively costly (measurement across intra- and inter-daily blood samples to establish a true baseline). Yet, scientists are doing this and there are genetic and neurotransmitter markers which are abnormal in people classified with ADHD. For further reading with a variety of medical sources I recommend [0].
It's a disorder, and presents with a large variety of expressions or comorbidities, but the symptoms (restlessness, jumping between hobbies) are shared and the consequences both in private as well professional lives are severe.
The inability to focus on single things lead to worse career outcomes and worse relationship outcomes. It is a serious suffering for those affected. Ritalin has a 60 year pharmacological history and has proven to help people. Modern stimulants are even better.
I am hugely grateful for the fact safe access exists and as someone affected badly, [I don't think it's appropriate to make fun of someone seeking access to this medication.] STRIKE that out, GP explained his intended meaning further below.