Urinary pyridinoline (UPD) and deoxypyridinoline (UDPD) are selective markers in kinetic studies of mature collagen degradation in connective tissue, especially in bone. In patients with renal dysfunction, the determination of UPD and UDPD is not entirely reliable, while in anuretic patients it is impossible. As renal dysfunction is considered a risk factor for bone diseases, it is essential to determine both markers directly in the serum (SPD and SDPD). Due to the high serum concentrations of proteins, which during acid hydrolysis are converted to amino acid hydrochlorides, the system butanol-water is sometimes separated into two phases during sample preparation. Should this fact not be taken into account, the usual sample processing on a cellulose sorbent could yield substantially lower false results. This calls for some preventive measures: to ensure the homogeneity of the system containing n-butanol it is recommended to add an appropriate third component, e.g. methanol.