The primary cause of poor and ambiguous results obtained from the bioanalytical process is the sample pre-treatment, especially in clinical analysis because it involves dealing with complex sample matrices, such as whole blood, urine, saliva, serum, and plasma. So, the aim of this review is to focus attention on the classical and new techniques of pre-treatment for biological samples used in the bioanalytical process. We discussed the methods generally used for these types of complex samples. Undoubtedly, it is a daunting task to deal with biological samples because the analyst may encounter a substantial loss of the analytes of interest, or the overall analysis may be too time-consuming. Nowadays, we are inclined to use green solvents for the environment, but without sacrificing analytical performance and selectivity. All the characteristics mentioned above should be added to the difficulty of the withdrawal of samples like blood because it can be an invasive practice. For these reasons, now we can also find in the literature the use of saliva as alternative biological samples and new techniques that do not require substantial sample pre-treatment, such as fabric phase sorptive extraction (FPSE). The text has been divided into the following two distinct parts: firstly, we described clinical applications under different subsections, such as anticancer drugs, antibiotics, vitamins, antivirals, non-steroidal anti-inflammatory drugs, statin, imidazoles, and triazoles. The second part is dedicated to sample preparation techniques for diagnostic purposes and is divided into the following different sample preparation techniques: solid-phase microextraction (SPME), microextraction by packed sorbent (MEPS), dispersive liquid–liquid microextraction (DDLME), and fabric phase sorptive extraction (FPSE).

One of the most discussed topics concerns the sample preparation before the analysis and, therefore, all the operations necessary to eliminate the interferents, clean up the specimens, and extract the analytes of interest, reducing the matrix effect. This review highlights the fundamental steps in the treatment of postmortem samples used in forensic analysis. Through critical literature research, it was possible to choose among the countless works that could provide a general overview of the state-of-the-art in this field. Different biological matrices have been considered; blood and urine (the traditional biological fluids) are used to investigate the presence of substances that may have caused death, whilst other body fluids, such as bile and oral fluids, are still under discussion for their usability (and suitability). In the second part of the review, all the solid matrices obtained after autopsy were further divided into conventional and unconventional matrices to facilitate proper understanding. The choice of literature was also made according to the most widely used pretreatment techniques and the most representative innovative techniques.