Tyrosine Kinase Inhibitors work by blocking the tyrosine kinases responsible of the dysregulation of intracellular signalling pathways in tumour cells. This study looked at the impact of age and sex on the levels of imatinib, dasatinib, nilotinib, and ponatinib in plasma and cerebrospinal fluid samples of patients with chronic myeloid leukaemia. Imatinib and dasatinib were used to treat the majority of the enrolled patient and most of them were paediatrics. The 82.4% of patients were men; however, sex-related differences in drugs pharmacokinetics were not found. Age and imatinib plasma concentration were found to be inversely correlated: as patients ages increased, drug levels decreased. The dasatinib concentrations in plasma were found to be substantially lower than those found in cerebrospinal fluid, particularly in paediatrics. Analysing the obtained data, we can state that Therapeutic Drug Monitoring is a useful method for adjusting a patient treatment schedule depending on drug concentrations in biological fluids. The use of Therapeutic Drug Monitoring in conjunction with Tyrosine Kinase Inhibitors for the treatment of chronic myeloid leukaemia is supported by a number of sources of evidence. As a result, as the research develops, Tyrosine Kinase Inhibitors Therapeutic Drug Monitoring classification needs to be refined in terms of factors like sexes and ages.