Today, a clinical blood test is the most accurate method for measuring the concentration of hCG in the blood. The analysis may be unreliable due to laboratory error (which happens very rarely) or in such cases: - When the patient is constantly taking medications that affect the hormonal background, if it is impossible to refuse to take them for medical reasons, it is necessary to warn the laboratory staff about this before taking the analysis. In this case, the result can be regarded as positive. - Suppose ovulation or implantation of a fertilized egg occurred later than expected. In this case, a blood test will show too little hormone, that is, the absence of pregnancy. This happens when a sufficient amount of chorionic gonadotropin has yet to be developed. The analysis will have to be repeated to ensure that pregnancy has not occurred.