Sensitivity and specificity of free and total glutaric acid and 3-hydroxyglutaric acid measurements by stable-isotope dilution assays for the diagnosis of glutaric aciduria type I

Glutaric aciduria type I (GA I) is a recessive disorder caused by a deficie ncy of glutaryl-CoA dehydrogenase (GCDH). The biochemical hallmark of the d isease is the accumulation of glutaric acid and, to a lesser degree, of 3-h ydroxyglutaric acid and glutaconic acid in body fluids and tissues. A subst antial number of patients show only slightly, intermittently elevated or ev en normal urinary excretion of glutaric acid, which makes early diagnosis a nd treatment to prevent the severe neurological sequelae difficult. Further more, elevated urinary excretion of glutaric acid can also be found in a nu mber of other disease states, mostly related to mitochondrial dysfunction. Stable-isotope dilution assays were designed for both glutaric acid and 3-h ydroxyglutaric acid and their diagnostic sensitivity and specificity were e valuated. Control ranges of glutaric acid in urine were 1.1-9.7mmol/mol cre atinine before and 4.1-32 after hydrolysis. The respective values of 3-hydr oxyglutaric acid were 1.4-8.0 and 2.6-11.7mmol/mol creatinine. For other bo dy fluids, control ranges in mu mol/L were: for glutaric acid 0.55-2.9 (pla sma), 0.18-0.63 (cerebrospinal fluid) and 0.19-0.7 (amniotic fluid); and fo r 3-hydroxyglutaric acid, 0.2-1.36 (plasma), < 0.2 (cerebrospinal fluid) an d 0.22-0.41 (amniotic fluid). Twenty-five patients with GCDH deficiency were studied. Low excretors (12 p atients) were defined by a urinary glutaric acid below 100mmol/mol creatini ne down into the normal range, while high excretors (13 patients) had gluta ric acid excretions well above this value. With and without hydrolysis ther e was an overlap of glutaric acid values between patients and controls. Dia gnostic sensitivity and specificity of 100% could only be achieved by the q uantitative determination of 3-hydroxyglutaric acid with the newly develope d stable-isotope dilution assay, allowing an accurate diagnosis of all pati ents, regardless of the amount of glutaric acid excreted in urine.