In daily laboratory services, there are various factors associated with errors, such as those related to the principles of measurement, blood sampling procedure, or pathological condition. If medical technologists do not understand these factors, or adopt appropriate measures, they may face pitfalls. This paper outlines blood tests to develop basic knowledge of this area, and present examples of possible daily pitfalls and coun- termeasures against them. Regarding the blood count, it explains the mechanisms of automatic blood count devices and methods to manage erythrocyte aggregation, giant platelets, schizocytes, and platelet aggregation. Focusing on blood smears, it also examines the influences of excessive EDTA on blood cells, while discussing the mechanisms of automatic blood coagulation analyzers, as well as the influences of chyle and hematocrit levels on coagulation tests. Furthermore, in relation to the erythrocyte sedimentation rate, it confirms the principles of the international standard Westergren method and capillary photometry, and the influences of hematocrit levels. In order to avoid unexpected pitfalls, it may be important for medical technologists engaged in blood tests to implement their daily duties while fully understanding the necessity of: carefully observing individual spec- imens; clarifying related items (such as the patient's age, sex, and disease); and accurately recognizing the characteristics of individual test methods and reagents used within the facility, in addition to the principles of their measurement. [Review].