Assessment of carnitine excretion and its ratio to plasma free carnitine as a biomarker for primary carnitine deficiency in newborns

In the Netherlands, newborns are referred by the newborn screening (NBS) Program when a low free carnitine (C0) concentration (<5 μmol/l) is detected in their NBS dried blood spot. This leads to ~85% false positive referrals who all need an invasive, expensive and lengthy evaluation. We investigated whether a ratio of urine C0 / plasma C0 (RatioU:P) can improve the follow-up protocol for primary carnitine deficiency (PCD). A retrospective study was performed in all Dutch metabolic centres, using samples from newborns and mothers referred by NBS due to low C0 concentration. Samples were included when C0 excretion and plasma C0 concentration were sampled on the same day. RatioU:P was calculated as (urine C0 [μmol/mmol creatinine])/(plasma C0 [μmol/l]). Data were available for 59 patients with genetically confirmed PCD and 68 individuals without PCD. The RatioU:P in PCD patients was significantly higher (p value < 0.001) than in those without PCD, median [IQR], respectively: 3.4 [1.2-9.5], 0.4 [0.3-0.8], area under the curve (AUC) 0.837. Classified for age (up to 1 month) and without carnitine suppletion (PCD; N = 12, Non-PCD; N = 40), medians were 6.20 [4.4-8.8] and 0.37 [0.24-0.56], respectively. The AUC for RatioU:P was 0.996 with a cut-off required for 100% sensitivity at 1.7 (yielding one false positive case). RatioU:P accurately discriminates between positive and false positive newborn referrals for PCD by NBS. RatioU:P is less effective as a discriminative tool for PCD in adults and for individuals that receive carnitine suppletion.

Plasma carnitine concentrations in Medium-chain acyl-CoA dehydrogenase deficiency: lessons from an observational cohort study

Our aim was to study the effect of secondary carnitine deficiency (SCD) and carnitine supplementation on important outcome measures for persons with medium-chain Acyl-CoA dehydrogenase deficiency (MCADD). We performed a large retrospective observational study using all recorded visits of persons with MCADD in the University Medical Center Groningen, the Netherlands, between October 1994 and October 2019. Frequency and duration of acute unscheduled preventive hospital visits, exercise tolerance, fatigue, and muscle pain were considered important clinical outcomes and were studied in relation to (acyl)carnitine profile and carnitine supplementation status. The study encompassed 1228 visits of 93 persons with MCADD. >60% had SCD during follow-up. This included only persons with severe MCADD. Carnitine supplementation and SCD were unrelated to the frequency and duration of the acute unscheduled preventive hospital visits (P > 0.05). The relative risk for fatigue, muscle ache, or exercise intolerance was equal between persons with and without SCD (RR 1.6, 95% CI 0.48-5.10, P = 0.4662). No episodes of metabolic crisis were recorded in non-carnitine-supplemented persons with MCADD and SCD. In some persons with MCADD, SCD resolved without carnitine supplementation. There is absence of real-world evidence in favor of routine carnitine analysis and carnitine supplementation in the follow-up of persons with MCADD.

Monitoring phenylalanine concentrations in the follow-up of phenylketonuria patients: An inventory of pre-analytical and analytical variation

BACKGROUND

Reliable measurement of phenylalanine (Phe) is a prerequisite for adequate follow-up of phenylketonuria (PKU) patients. However, previous studies have raised concerns on the intercomparability of plasma and dried blood spot (DBS) Phe results. In this study, we made an inventory of differences in (pre-)analytical methodology used for Phe determination across Dutch laboratories, and compared DBS and plasma results.

METHODS

Through an online questionnaire, we assessed (pre-)analytical Phe measurement procedures of seven Dutch metabolic laboratories. To investigate the difference between plasma and DBS Phe, participating laboratories received simultaneously collected plasma-DBS sets from 23 PKU patients. In parallel, 40 sample sets of DBS spotted from either venous blood or capillary fingerprick were analyzed.

RESULTS

Our data show that there is no consistency on standard operating procedures for Phe measurement. The association of DBS to plasma Phe concentration exhibits substantial inter-laboratory variation, ranging from a mean difference of -15.5% to +30.6% between plasma and DBS Phe concentrations. In addition, we found a mean difference of +5.8% in Phe concentration between capillary DBS and DBS prepared from venous blood.

CONCLUSIONS

The results of our study point to substantial (pre-)analytical variation in Phe measurements, implicating that bloodspot Phe results should be interpreted with caution, especially when no correction factor is applied. To minimize variation, we advocate pre-analytical standardization and analytical harmonization of Phe measurements, including consensus on application of a correction factor to adjust DBS Phe to plasma concentrations.

Effects of acute nutritional ketosis during exercise in adults with glycogen storage disease type IIIa are phenotype-specific: An investigator-initiated, randomized, crossover study

Glycogen storage disease type IIIa (GSDIIIa) is an inborn error of carbohydrate metabolism caused by a debranching enzyme deficiency. A subgroup of GSDIIIa patients develops severe myopathy. The purpose of this study was to investigate whether acute nutritional ketosis (ANK) in response to ketone-ester (KE) ingestion is effective to deliver oxidative substrate to exercising muscle in GSDIIIa patients. This was an investigator-initiated, researcher-blinded, randomized, crossover study in six adult GSDIIIa patients. Prior to exercise subjects ingested a carbohydrate drink (~66 g, CHO) or a ketone-ester (395 mg/kg, KE) + carbohydrate drink (30 g, KE + CHO). Subjects performed 15-minute cycling exercise on an upright ergometer followed by 10-minute supine cycling in a magnetic resonance (MR) scanner at two submaximal workloads (30% and 60% of individual maximum, respectively). Blood metabolites, indirect calorimetry data, and in vivo ³¹ P-MR spectra from quadriceps muscle were collected during exercise. KE + CHO induced ANK in all six subjects with median peak βHB concentration of 2.6 mmol/L (range: 1.6-3.1). Subjects remained normoglycemic in both study arms, but delta glucose concentration was 2-fold lower in the KE + CHO arm. The respiratory exchange ratio did not increase in the KE + CHO arm when workload was doubled in subjects with overt myopathy. In vivo ³¹ P MR spectra showed a favorable change in quadriceps energetic state during exercise in the KE + CHO arm compared to CHO in subjects with overt myopathy. Effects of ANK during exercise are phenotype-specific in adult GSDIIIa patients. ANK presents a promising therapy in GSDIIIa patients with a severe myopathic phenotype. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov identifier: NCT03011203.

Inter-laboratory analytical improvement of succinylacetone and nitisinone quantification from dried blood spot samples

BACKGROUND

Nitisinone is used to treat hereditary tyrosinemia type 1 (HT-1) by preventing accumulation of toxic metabolites, including succinylacetone (SA). Accurate quantification of SA during newborn screening is essential, as is quantification of both SA and nitisinone for disease monitoring and optimization of treatment. Analysis of dried blood spots (DBS) rather than plasma samples is a convenient method, but interlaboratory differences and comparability of DBS to serum/plasma may be issues to consider.

METHODS

Eight laboratories with experience in newborn screening and/or monitoring of patients with HT-1 across Europe participated in this study to assess variability and improve SA and nitisinone concentration measurements from DBS by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Quantification of nitisinone from both DBS and plasma was performed to assess sample comparability. In addition, efforts to harmonize laboratory procedures of SA and nitisinone quantifications during 5 rounds of analysis are described.

RESULTS

Nitisinone levels measured from DBS and plasma strongly correlated (R 2 = 0.93). Due to partitioning of nitisinone to the plasma, levels were higher in plasma by a factor of 2.34. In the initial assessment of laboratory performance, all had linear calibrations of SA and nitisinone although there was large inter-laboratory variability in actual concentration measurements. Subsequent analytical rounds demonstrated markedly improved spread and precision over previous rounds, an outcome confirmed in a final re-test round.

CONCLUSION

The study provides guidance for the determination of nitisinone and SA from DBS and the interpretation of results in the clinic. Inter-laboratory analytical harmonization was demonstrated through calibration improvements.