Carnitine/acylcarnitines screening is the most important part of the clinical laboratory diagnosis of amino acid, fatty acid or organic acid metabolism disorders. Carnitine / acylcarnitines screening allows the diagnosis of over 30 metabolic diseases from a drop of blood sample taken with a screening card. This screening test is performed by measurement of carnitine / acylcarnitines with isotope dilution tandem mass spectrometry. LC-MS/MS is a powerful tool that gives results for carnitine / acylcarnitines in 1.5 minutes. The screening can be performed by the Zivak Carnitine / Acylcarnitine LC- MS/MS Analysis Kit and Zivak NeoZin screening software. The main methods and procedures that have been selected are based on EN ISO 14971. The quantitative analysis of carnitine esters including free carnitine, C0, C2, C4, C6, C8, C10, C12, C14, C16, C16 OH, C18, C18 OH carnitine reveals disorders which are related to fatty acid oxidation. The quantitative analysis of carnitine esters including C3, C3DC, C4OH, C5, C5OH, C5DC carnitine reveals disorders which are related to organic acid metabolism.