Assessment and Application of Acylcarnitines Summations as Auxiliary Quantization Indicator for Primary Carnitine Deficiency

Background: Newborns are referred primary carnitine deficiency (PCD) when a low free carnitine (C0) concentration (< 10 μmol/L) is detected, leading to high false positive referrals. To improve the follow-up protocol for PCD, various acylcarnitines and the summations were comprehensively evaluated in the present study. Methods: A retrospective study was performed using samples due to low C0 concentration. Data were available for 72 patients with genetically confirmed PCD. Whereafter, C0 with the selected sum of (butyrylcarnitine (C4) + isovalerylcarnitine (C5)) was validated in an additional cohort study including about 80,000 samples. Results: In the discovery study, C4, acetylcarnitine (C2) and C5 exhibited significant discriminant power in distinguishing PCDs from NoPCDs. The area under the ROC curve (AUC) were 99.792% (C4), 98.715% (C2) and 98.620% (C5). The excellent performances in sensitivity, specificity, negative predictive value, positive predictive value (PPV) and accuracy indexes suggested that C4, C2 and C5 would be ideal auxiliary indicators in improving diagnostic performance of C0 for PCD. Multivariate ROC curve based exploratory analysis showed that C5, C4 and C2 were the most top-ranked features in differentiating PCDs from NoPCDs. AUC for C4+C5 was the highest with a cut-off required for 100% sensitivity at 0.181 μmol/L. In the validation cohort, adding C4+C5 in the NBS program could elevate PPV from 0.75% to 1.54%. Conclusions: Our work revealed that C4+C5 summation should be as the auxiliary quantization indicator to reduce false-positive results for PCD.