What is considered "significant" will vary according to an organizations process. In this scenario, the QA SysOp determined that the most significant antecedent was the review process for samples and the accompanying paperwork.

Significant Antecedent Event mismatch of information between blood sample label and request form

The most significant antecedent occurred during
          Sample Handling SH

The descriptive code for what happened is
          002 paperwork and sample do not match