This is a broad review of sources of error in the coagulation tests, all of which must be considered, evaluated and supervised to improve their quality. The analytical result of a laboratory examination is a scientific fact and has no medical meaning as such. It must be interpreted to become a medical finding. Recent improvement in equipment, reagents, and the fact that hemophilic and thrombotic events are considered the main causes of death are the principal reasons to prepare this article. The internal quality control program considers this fact because its beginning starts when the physician makes the request to the lab; moreover, the clinical interpretation of the results of the laboratory represents the end of the cycle. Outcome of clinical test in hemostasis is critically dependent upon the quality of the sample, the method, the instruments, the reagents and the personnel involved in the performance of the test. Use of high quality blood collection procedures and an awareness of preanalytic errors are presented to lead to quality outcomes. Today, performance of internal and external quality control programs is obligatory.