The present invention relates to a method for the diagnosis of tumoural conditions and/or of the corresponding state of advance, wherein a sample from a patient comprising at least one tumour cell is obtained. According to the invention, a purified specimen of the at least one tumour cell is obtained by individually selecting and isolating single cells in a microfluidic device the purified specimen having a purity of at least 90%. On the purified specimen thus obtained there is subsequently performed a molecular analysis such as to highlight a characteristic thereof suited to enabling diagnosis.