I'm the person who made that comment. The problem here is you've been caught in a scope change. The question is rather old, dating to the earliest days of the site. When that question was asked, your answer might have been acceptable. Although references have always been required, that rule was less rigorously enforced in the past.
But earlier this year the site underwent a major revision to both the name and scope. The name was changed from Health to MedicalSciences to emphasize this change. The new focus of the group is aimed at a more scientifically rigorous, more professional audience. Requiring supporting references more stringently is part of that change in scope.
I disagree that the question is general and speculative. It's actually quite specific and practical. I think it's a pretty good question, and the high vote count indicates the community agrees. But even if it were general and speculative, that wouldn't relieve answerers of the need to support their factual claims. For example, this statement in your answer would benefit from some support:
If a doctor is considering whether to administer some treatment, they
could well use older blood tests - because those could indicate
situations you might get into, or conditions you might be prone to.
Although I think that's true, the way you've worded it is entirely speculative. This could be solved quite simply with an example. What treatments might require a review of past blood work? Find an example and give us some sort of reference for it. Answers aren't expected to be doctoral theses with a full bibliography; we only require you to demonstrate that you're not voicing pure opinion. Supply that and I'm good with your answer. That's why I only added a comment instead of deleting it -- to give you a chance to fix it. If you don't, I can almost guarantee someone will flag it, and that will lead to deletion by me or one of the other mods.