Serum Acylcarnitines and Vitamin B12 Deficiency

Measurements of acylcarnitines in plasma have several diagnostic applications, and various tandem mass spectrometry methods are more specific and accurate than earlier methods (1)(2)(3)(4). Libert et al. (5) reported that propionylcarnitine (PrCRN) and carnitine (CRN) esters of 2-methyl-branched fatty acids with chain lengths of four to nine carbons were found in the urine of patients with propionic and methylmalonic acidemia. Chace et al. (6) reported results for a high-throughput method for PrCRN analysis from dried-blood spots for the diagnosis of propionic and methylmalonic acidemias in newborns. Increased concentrations of methylmalonic acid (MMA) in urine or serum are also regarded as reliable indicators of functional vitamin B12 deficiency (7)(8)(9). The relationship between the concentrations of MMA and CRN esters in patients with increased MMA attributable to vitamin B12 deficiency has not been established. Because mass spectrometric testing is useful in newborn screening (10), with analysis of acylcarnitines, and specifically propionylcarnitine, to identify patients with methylmalonic acidemia, we evaluated the utility of acylcarnitine analysis in patients with low vitamin B12 concentrations.MMA was analyzed on a tandem mass spectrometer as described previously (11). Acylcarnitines were analyzed as follows. Aliquots (150 μL) of calibrators, controls, and patient sera were transferred into microcentrifuge tubes. Acetonitrile was added to each tube to bring the total volume to 650 μL. The tubes were vortex-mixed vigorously for 2 min and centrifuged at 10 000 g for 2 min; 50 μL of the organic phase was then transferred into an autosampler vial containing 50 μL of internal standard (IS) solution, containing d9-CRN, …