Therapeutic drug monitoring means determining the drug concentration in the patient's blood, and the obtained results are used for individual drug dosing. The so-called trough drug concentration is determined, which helps in finding out whether the concentration of the drug between doses is sufficiently high and effective. If the concentration of the drug in the blood is influenced by numerous factors such as pharmacokinetic factors, pathological conditions, general physiological factors and interactions of the drugs, it is advisable to include therapeutic drug monitoring. Antipsychotics are a group of drugs that require monitoring to prevent inadequate dosing or to reduce the risk of side effects. The method of administration of the atypical antipsychotics risperidone and paliperidone includes the oral administration and the intramuscular administration with immediate or extended release. Precise and sensitive analytical methods such as chromatography, voltammetry and immunoassays have been developed for the analysis of antipsychotics in various samples. An indispensable analytical procedure in the therapeutic drug monitoring is certainly high-performance liquid chromatography in combination with tandem mass spectrometry. The aim of this research was to examine whether there are differences in achieving the therapeutic interval of paliperidone depending on age, sex, form of administration, drug dose, kidney function and liver function. The research was conducted at the Clinical Institute of Chemistry, KBC Sestre milosrdnice. 85 subjects participated in the research, and the analysis of parameters was performed on the biochemical analyzer ALINITY c (Abbott Laboratories, Chicago, IL, USA) and the LCMS-8050 TRIPLE QUADRUPOLE LCMS/ MS analyzer (Shimadzu Corporation, Kyoto, Japan). The research results did not show a statistically significant difference in the achievement of therapeutic intervals depending on the mentioned factors, except that a weak negative correlation was found between the concentration of risperidone and the concentration of albumin, which indicates the adjustment of the dose of the drug risperidone in patients with impaired liver function.