Simultaneous Determination of 5-hydroxytryptophan and 3-O-methyldopa in Dried Blood Spot by UPLC-MS/MS: A Useful Tool for the Diagnosis of L-amino Acid Decarboxylase Deficiency

5-hydroxytryptophan (5HTP) and 3-O-methyldopa (3OMD) are CSF diagnostic biomarkers of the defect of aromatic L-amino acid decarboxylase (AADC), a rare inherited disorder of neurotransmitter synthesis which, if untreated, results in severely disabling neurological impairment. In the last few years, different methods to detect 3OMD in dried blood spot (DBS) were published. We developed and validated a fast and specific diagnostic tool to detect 5HTP alongside 3OMD. After extraction from DBS, 3OMD and 5HTP were separated by ultra-performance liquid chromatography (UPLC) and detected by tandem mass spectrometry (MS/MS). Instrument parameters were optimized to obtain the best sensitivity and specificity. Chromatographic separation was accomplished in 13 min. The limit of detection was 2.4 and 1.4 nmol/L of blood for 3OMD and 5HTP respectively, and response was linear over the blood range of 25-5000 nmol/L. Between-run imprecision was less than 9% for 3OMD and <13% for 5HTP. An age-specific continuous reference range was established, revealing a marked and continuous 3OMD decline with aging. The effect of age on 5HTP was less evident, showing only a slight decrease with age after the first week of life. A marked increase of both 3OMD and 5HTP was found in four patients affected by AADC deficiency (1780.6 ± 773.1 nmol/L, rv 71.0-144.9; and 94.8 ± 19.0 nmol/L, rv 15.2-42.8, respectively) while an isolated increase of 3OMD (6159.6 ± 3449.1 nmol/L, rv 73.2-192.2) was detected in three subjects affected by inherited disorders of dopamine synthesis under levodopa/carbidopa treatment (a marginal increase of 5HTP was detected in one of them). Simultaneous measurement of 5HTP and 3OMD in DBS leads to an improvement in specificity and sensitivity for the biochemical diagnosis of AADC deficiency.