In conclusion, in our simple loading test using MS/MS together with d15-OA and d31-PA, the ratio of d7-C4/d31-C16 in PBMCs, which is hardly influenced by cell number or viability, is used as a useful marker to diagnose CPT-I deficiency. This evidence concerns the gene CPT1A and hyperinsulinemic hypoglycemia, familial, 4.