I'm pretty sure they would be able to - I worked with cancer trial data (just a student job, doing data entry) but all patients were completely anonymised to anyone working with the data but readily identifiable by a specific unique ID number, so higher-level trial managers and doctors could find patient information if needed.

It is common for the data to be anonymised. Especially if there is a third party involved (e.g. a lab producing sequencing data attached to sub-sets of clinical information, according to whatever contract or legal environment they are in).

But surely, as the other poster mentions, they'd be tracking patients by some unique ID and someone has the key. Otherwise, how do you correlate blood results with any other patient data?

I mean, they could go back to the provider of the clinical information with a unique ID and request follow-up. But people are still a) busy (clinical data may come from a hospital that has little interest in research; researchers want to publish and move on); b) extremely scared of making mistakes (to the point of delusion: wanting data/results to never change, even if its an improvement - because that implies there was something wrong in the past). Getting follow-up clinical data may also be out of scope of existing agreements.