CBS and hyperinsulinemic hypoglycemia, familial, 4: Although CBS deficiency should be confirmed by measurement of cystathionine synthase activity in fibroblasts or plasma and/or by mutation analysis of the CBS gene, neither technique can be relied on to demonstrate abnormalities in all cases.8 In all of our patients presented here, the Hcy concentration was greater than 178 μmol/L at the time of diagnosis, therefore exceeding the consensus diagnostic threshold for CBS deficiency.8,22 Therefore, the diagnosis of HCU would have come to light had tHcy been measured at the time of first presentation.