- The pre-analytical phase of laboratory testing includes all the steps taken before the actual analysis of the sample, such as patient preparation, sample collection, transport, and storage.
- Errors in this phase are the most frequent and can significantly affect laboratory results leading to misdiagnosis or inappropriate treatment.

Among the options provided:
- Use of expired reagents and incorrect calibration of analyzers are classified as analytical phase errors, occurring during the testing process.
- Data entry mistakes fall within the post-analytical phase, impacting the reporting or interpretation of results.
- Improper sample storage, on the other hand, is a classic pre-analytical error. Samples that are not stored at the correct temperature or conditions may degrade, hemolyze, or allow bacterial overgrowth, thus leading to erroneous or invalid test results.

Therefore, improper sample storage is a common and significant pre-analytical cause of laboratory test errors and must be carefully controlled to ensure reliable diagnostic outcomes.

Reference: Henry's Clinical Diagnosis and Management by Laboratory Methods, 23rd Edition, Volume 1, Chapter 2: The Pre-Analytical Phase, Page 30-35.