Neonatal screening for isovaleric aciduria: Reducing the increasingly high false-positive rate in Germany

Newborn screening (NBS) for isovaleric acidemia (IVA) is performed by flow injection tandem mass spectrometry quantifying C5 carnitines (C5). Isovalerylcarnitine, however, is isomeric with pivaloylcarnitine which can be present in blood due to maternal use of pivaloylester-containing antibiotics, available in Germany since late 2016. During a 36-month period (January 19-December 21), all newborns screened in Hamburg with a C5 above cutoff (NeoGram®: 0.50 μmol/L or Neobase®2: 0.45 μmol/L) were included in the study. As a second-tier test, a simple ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed to differentiate the C5 isomers pivaloyl-, 2-methylbutyryl-, isovaleryl-, and valerylcarnitine. Out of 156 772 newborns tested, one turned out to have genetically proven IVA while 99 were false positive (C5: 0.5-8.2 μmol/L) due to the presence of pivaloylcarnitine. These cases have increased year by year and show local clusters. Retrospective analysis of another 39 cases from 287 206 neonates tested at the NBS center in Heidelberg with C5 elevation (0.9-10.6 μmol/L) but clinical and biochemical exclusion of IVA yielded evidence of pivaloylcarnitine in all cases. Inclusion of a second-tier test into NBS significantly reduces the high and increasing false-positive rate of IVA screening. This avoids further diagnostic steps, prevents unnecessary stress and anxiety of parents in a remarkably high number of cases. If Hamburg data of 2021 are extrapolated to all of Germany, one can assume around 800 (1‰) false-positive cases in comparison to an average of two classic IVA cases per year. Unless licensing of pivaloylester-containing drugs for use during pregnancy is reconsidered, a second-tier test for C5 determination is indispensable.

A Simple Flow Injection Analysis-Tandem Mass Spectrometry Method to Reduce False Positives of C5-Acylcarnitines Due to Pivaloylcarnitine Using Reference Ions

Flow injection analysis−tandem mass spectrometry (FIA-TMS) has been applied in a first-tier test of newborn screening (NBS). Although isovalerylcarnitine (i-C5), which is a diagnostic indicator of isovaleric acidemia (IVA), is isobaric with pivaloylcarnitine (p-C5), 2-methylbutyrylcarnitine, and n-valerylcarnitine, these isomers cannot be distinguished by the FIA-TMS. There are many reports of false positives derived from p-C5 due to the use of pivalate-conjugated antibiotics. In this study, we developed a new FIA-TMS method to distinguish i-C5 and p-C5. We found that the intensity ratio of product ions for i-C5 and p-C5 was different in a certain range even under the same analytical conditions. The product ions with the most distinct differences in ionic intensity between the isomers and the collision energies that produce them were determined to be m/z 246.2 > 187.1 and −15 V, respectively. In addition to the quantification ion, a reference ion was defined, and the similarity of the i-C5 and p-C5 reference ion ratios (i-C5 score and p-C5 score, respectively) were used to estimate which isomer (i-C5 and p-C5) was responsible for elevated C5 acylcarnitine in dried blood spots (DBSs). As a result of analyses of 11 DBS samples derived from pivalate-conjugated antibiotics and four DBS samples from IVA patients using our method, it was found that our method was able to correctly determine the type of C5-acylcarnitine (i-C5 or p-C5) in the DBS samples. Implementation of this new FIA-TMS method into the current NBS protocol will allow for a reduction in false positives in IVA.

Hypocarnitinemia Observed in an Infant Treated with Short-Term Administration of Antibiotic Containing Pivalic Acid

Carnitine is a water-soluble amino acid derivative required for β-oxidation of long-chain fatty acids. In carnitine cycle abnormalities and low carnitine states, fatty acid β-oxidation is inhibited during fasting, resulting in hypoglycemia. Pivalic acid is a substance used in prodrugs to increase absorption of parent drugs, and antibiotics containing pivalic acid are frequently used as wide spectrum antibiotics for pediatric patients in Japan. Pivalic acid released after absorption is conjugated with free carnitine to form pivaloylcarnitine, which is then excreted in urine. As a consequence, long-term administration of pivalic acid containing antibiotics has been associated with depletion of free carnitine, inhibition of energy production and subsequent hypoglycemia. Here we report a case of a 23-month-old boy treated with an antibiotic containing pivalic acid for 3 days for upper respiratory tract infection. Laboratory data at referral indicated hypoglycemia, decreased free carnitine and elevated five-carbon acylcarnitine. Isomer separation confirmed the major component of increased five-carbon acylcarnitine to be pivaloylcarnitine, thereby excluding the possibility of a genetic metabolic disorder detected with similar acylcarnitine profile. The level of carnitine was normal when the antibiotic was not administered. Our case shows that the use of antibiotics containing pivalic acid in young children requires consideration of hypocarnitinemia, even with short-term administration.