A pre-analysis phase is that part of the analysis process encompassing the time from when the request is passed on to the laboratory to the time the sample is delivered to the laboratory. It is a vital part of this process, as it is in this period where the largest number of professionals from different disciplines intervenes. This pre-analysis phase is of vital importance to detect errors and to identify the causes that might lead to these errors. An important aspect must be to be aware of interferences that might alter the pre-analytical phase so that these interferences are recognised, controlled and avoided. A series of factors are included (physiological, endogenous and in relation to the taking of samples), the recognition and understanding of which may help prevent errors. A need to establish strategies that help improve the knowledge of these aspects in nursing professionals is emphasised, as their application in the pre-analytical phase of the process is clearly of outmost importance.