The results of laboratory tests have a substantial role in the diagnosis and management of disease. However, laboratory results do not always correspond with the patient's clinical status. Unexpected results may arise from factors other than disease. The total testing process consists of 3 separate phases — pre-analytical, analytical, and post-analytical. The accuracy and reliability of a laboratory result depend on the quality of each phase. In this article, the common variables in the pre-analytical and post-analytical phases and their effect on test result quality in primary care are reviewed.