Dried blood spot succinylacetone (SA) is often used as biomarker for newborn screening (NBS) for Tyrosinemia type 1 (TT1). However, false-positive SA results are often observed. Elevated SA may also be due to maleylacetoacetate isomerase deficiency (MAAI-D), which appears to be clinically insignificant. This study investigated whether urine organic acid (uOA) and quantitative urine maleic acid (Q-uMA) analyses can distinguish between TT1 and MAAI-D. We reevaluated/measured uOA (GC-MS) and/or Q-uMA (LC-MS/MS) in available urine samples of 9 referred newborns (2 TT1, 7 false-positive), 8 genetically confirmed MAAI-D children and 66 controls. Maleic acid was elevated in uOA of 5/7 false-positive newborns and in the 3 available samples of confirmed MAAI-D children, but not in TT1 patients. Q-uMA ranged from not detectable to 1.16 mmol/mol creatinine in controls (n=66), and from 0.95-192.06 mmol/mol creatinine in false-positive newborns and MAAI-D children (n=10). MAAI-D was genetically confirmed in 4/7 false-positive newborns, all with elevated Q-uMA, and rejected in the 2 newborns with normal Q-uMA. No sample was available for genetic analysis of the last false-positive infant with elevated Q-uMA. MAAI-D is a recognizable cause of false-positive TT1 NBS results. Elevated urine maleic acid excretion seems highly effective in discriminating MAAI-D from TT1. This article is protected by copyright. All rights reserved.