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Homocistein je aminokiselina koja u ljudskom organizmu nastaje u metioninskom ciklusu. Hiperhomocisteinemija, povišena plazmatska koncentracija homocisteina, uključena je u patogenezu brojnih poremećaja od kojih su najznačajnije kardiovaskularne, neurodegenerativne, neuropsihijatrijske bolesti, preeklampsija i neke kongenitalne malformacije. Klasična homocistinurija je nasljedni metabolički poremećaj kod kojeg je uslijed mutacija enzima uključenih u metioninski ciklus također prisutna hiperhomocisteinemija. Tandemska spektrometrija masa je analitička metoda koja tijekom posljednjih godina sve više ulazi u rutinsku upotrebu u medicinsko-biokemijskom laboratoriju. To je robusna metoda visoke točnosti, preciznosti, specifičnosti i osjetljivosti koja zahtijeva mali volumen uzorka, kratko je vrijeme analize i niska je cijena analize po uzorku. Homocistein se do sada određivao imunokemijskom metodom nefelometrije, no literaturni i empirijski podaci prikupljeni tijekom rada u laboratoriju upućuju na činjenicu da su rezultati dobiveni tandemskom spektrometrijom masa točniji i pouzdaniji. Tijekom provođenja kratke analitičke validacije metode tandemske spektrometrije masa za određivanje plazmatske koncentracije homocisteina utvrđeno je da su zadovoljeni kriteriji prihvatljivosti za točnost, preciznost i linearnost. Usporedbom s postojećom metodom nefelometrije Passing-Bablokovom regresijskom analizom pokazano je da postoje proporcionalno i konstantno odstupanje između metoda, tj. da one nisu usporedive. Usprkos činjenici da metode nisu usporedive, zbog mnogih prednosti koje uključuju analitičke karakteristike i nižu cijenu analize po uzorku nova je metoda uvedena u rutinski rad. Homocysteine is an amino acid which is formed during the methionine cycle in the human body. Hyperhomocysteinemia, elevated plasma homocysteine concentration, is involved in the pathogenesis of many disorders, including cardiovascular, neurodegenerative, neuropsychiatric diseases, preeclampsia and some congenital malformations. Classical homocysteurinuria is a hereditary metabolic disorder which also causes hyperhomocysteinemia due to the mutation of enzymes involved in the methionine cycle. Tandem mass spectrometry is an analytical method that increasingly entered routine use over the past few years. It is a robust method of high accuracy, precision, specificity and sensitivity that requires a small sample volume, short time of analysis and low costs of analysis per sample. Homocysteine has so far been determined by the immunochemical method nephelometry but the literature and empirical data collected during laboratory work indicate that the results obtained by tandem mass spectrometry are more accurate and reliable. During the short analytical validation of the tandem spectrometry method for determining total plasma homocysteine concentration, it was found that the acceptance criteria for accuracy, precision and linearity were fulfilled. Comparison with the existing nephelometry method with the Passing-Bablok regression analysis showed that there is a proportional and constant deviation between methods, what means that they are not comparable. Despite the fact that the methods are not comparable, due to many advantages that include analytical characteristics and lower costs of analysis per sample, a new method is introduced in routine use.