Usefulness of serum BUN or BUN/creatinine ratio as markers for citrin deficiency in positive cases of newborn screening

Citrin deficiency belongs to a group of urea cycle disorders that can be identified during newborn screening by measuring citrulline, phenylalanine, methionine, and galactose levels. Early diagnosis of citrin deficiency is beneficial as disease-specific interventions such as permission of food preference and/or supplementation of medium-chain triglyceride can prevent metabolic decompensation. However, there are currently no laboratory tests for the diagnosis of citrin deficiency in routine clinical practice. Our retrospective study investigated the diagnostic characteristics of citrin deficiency during infancy at secondary newborn screening. The present study included 10 patients with citrin deficiency and 35 controls without the condition. The positive likelihood ratios for serum levels of blood urea nitrogen (BUN) levels were 6.8 at the first visit, 3.2 at age ≤ 60 days, and 17.5 at age ≤ 100 days. The serum BUN/creatinine ratio also showed a high positive likelihood ratio (3.9 at first visit, 16.0 at age ≤ 60 days, and 24.5 at age ≤ 100 days). Therefore, the serum BUN or BUN/creatinine ratio may help to identify patients with citrin deficiency during newborn screening. Further studies are required to confirm its diagnostic accuracy in a larger cohort and elucidate the underlying mechanisms involved.